A study of MicroMegas detectors with resistive anodes for muon

Master of Science Thesis

A study of MicroMegas detectors with resistive anodes for muon reconstruction in

HL-LHC

Guillaume CAUVIN

Particle Physics, Department of Physics School of Enginnering Sciences

Royal Institue of Technology, SE-106 91 Stockholm, Sweden Stockholm, Sweden 2012

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Examsarbete inom ämnet fysik för avläggande av civilingenjörsexamen

inom utbildingsprogrammet Teknisk Fysik

Graduation thesis on the subject Particle Physics for the

degree of Master of Science in Engineering from the School of Engineering Physics

TRITA‐FYS 2012:56

ISSN 0280‐316X

ISRN KTH/FYS/‐‐12/56‐SE

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Abstract

By 2018, the luminosity of the proton‐proton collisions in the High‐Luminosity Large Hadron Collider will

increase by a factor of ten. Furthermore, the energy at the center of mass will reach 14TeV. This will

imply a lot of consequences, especially concerning detectors. The detectors of the muon spectrometer

will need to be replaced.

The MicroMegas detector is a very new and promising gaseous detector technology. The MAMMA

collaboration has the aim to develop Micromegas detectors as a replacement solution. The purpose of

this diploma work is to study a new concept of Micromegas: the resistive. It is supposed to handle very

high‐rate of very high‐energy particles. In order to better understand the behavior of the resistive

Micromegas, some experimental tests such as characterization of new prototypes will be described and

analyzed in this report. Moreover, ageing tests have been performed during my thesis to prove the

capability of these detectors to operate in long data taking periods in the HL‐LHC. We irradiated a sample

with X‐rays, cold neutrons and gammas and observed the evolution of some observables such as the gain

of the detector and the generated current.

Then, the MAMMA proposal implies the construction and the integration of 128 Micromegas chamber of

2m² into ATLAS. The process of assembly of this structure deserves special attention. We need to find

out a way to assemble and align all the detectors together with accuracy better than 30µm in order to

reconstruct the particle tracks within the muon spectrometer.

Finally and since experiments need to be proved by the simulation, many 2dimensional models of

Micromegas were created with an appropriate software in order to investigate the influence of the size

of resistive anodes on the field lines and the gain of the detector.

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ACKNOWLEDGMENTS

First, I would like to thank the CEA and especially the Detector Service (SEDI) for accepting me as a

trainee and for welcoming me so warmly. The working conditions and atmosphere were great and help

me to feel good as soon as my arrival.

Then, I want to thank Philippe SCHUNE for supervising me and my work during this internship. His

passion, his wit and his 1000 ideas per second were very inspiring and motivating.

Thanks to Fabien for having been at my side and taken care of me every day, for his patience with me,

his very reassuring self‐confidence, his advices and his orders. My comprehension of the Micromegas

technology went really faster with him. I have to admit I was impressed by his general culture and his

competence in physics.

Thanks to Esther for her kindness and for having integrated me in the “Spanish physics task force”, a very

powerful diaspora here in CEA. She was able to motivate and guide me during periods of

discouragement.

Thanks to both of them for their rants, which were very funny and entertaining.

Thanks to Javier for his patience and for having dedicated lot of his time to explain me how to deal with

Micromegas detectors and for all the works I have stolen.

Thanks to Paco for his motivation, his always good mood and for having flirted with the waitresses to

have a better coffee.

Thanks to Ali for having supported and helped me and for having shown interests to my works.

Thanks to Finbarr for all the discussions about Sports and Travels and how France is better than Ireland

in Rugby. It really helped me to take a break and be more focused on my work afterwards.

Thanks to Thomas for all the weather forecasts in all the French ski resorts.

Thanks to Jacques and Ioannis for their scientific advices, their help and their expertise.

Thanks to Arnaud for his very precious technical support.

Thins long‐term internship enables me to think about my professional future and how I want my carrier

to look like. Thanks to all of them for their helps, their supports and the constructive discussions.

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TABLEOFCONTENTS

ACKNOWLEDGMENTS............................................................................................................................................4

INTRODUCTION....................................................................................................................................................12

ISCIENTIFICANDINDUSTRIALCONTEXT...................................................................................................13

I‐1 CEA .................................................................................................................................................................. 13

I‐2 Scientific Context ............................................................................................................................................ 14

I‐2‐1 CERN ................................................................................................................................................................ 14

I‐2‐2 LHC .................................................................................................................................................................. 14

I‐2‐3 ATLAS ............................................................................................................................................................... 16

I‐2‐4 Muon spectrometer ........................................................................................................................................ 18

I‐2‐4 The HL‐LHC ...................................................................................................................................................... 20

I‐2‐5 The muon spectrometer upgrade ................................................................................................................... 20

I‐2‐6 The MAMMA collaboration ............................................................................................................................. 22

I‐2‐7 The decision..................................................................................................................................................... 22

I‐3 Micromegas .................................................................................................................................................... 23

I‐3‐1 Gaseous detector ............................................................................................................................................ 23

I‐3‐2 Principle ........................................................................................................................................................... 23

I‐3‐3 Performance, Advantages and Drawbacks (Sparks) ........................................................................................ 27

IIEXPERIMENTALTESTSONMICROMEGASDETECTORS......................................................................30

II‐1 Characterization and various tests on Micromegas ........................................................................................ 30

II‐1‐1 Aim and Experimental Set‐up ......................................................................................................................... 30

II‐1‐2 Calibration and gain computation .................................................................................................................. 32

II‐1‐3 Samples studied ............................................................................................................................................. 34

II‐1‐4 Micromegas characterization ......................................................................................................................... 36

II‐1‐5 Results and conclusions .................................................................................................................................. 38

II‐1‐6 Sparks production .......................................................................................................................................... 40

II‐2 Ageing tests and background in ATLAS ........................................................................................................... 42

II‐3 Ageing studies with X‐rays ............................................................................................................................. 43

II‐2‐1 X‐rays tests set‐up .......................................................................................................................................... 44

II‐2‐1 Tests and results ............................................................................................................................................. 46

II‐4 Ageing studies with Neutrons ........................................................................................................................ 49

II‐4‐1 Set‐up and installation ................................................................................................................................... 49

II‐4‐2 Calibration and activation of the detectors.................................................................................................... 51

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II‐4‐3 Results and conclusions .................................................................................................................................. 52

II‐4‐4 Post characterization ...................................................................................................................................... 53

II‐5 Ageing studies with Gamma Sources .............................................................................................................. 54

II‐6 Conclusion ...................................................................................................................................................... 56

IIIALIGNMENTANDASSEMBLYPROCESSOFTHEMICROMEGASSMALL‐WHEEL........................57

III‐1 A strong need of precision ............................................................................................................................. 57

III‐2 Mechanical measurement of the New Small Wheel ...................................................................................... 58

III‐2‐1 Detector components ................................................................................................................................... 58

III‐2‐2 Chamber precision ........................................................................................................................................ 60

III‐2‐3 The New‐Small‐Wheel ................................................................................................................................... 61

III‐3 Alignment and Assembly process .................................................................................................................. 63

III‐3‐1 Preliminaries .................................................................................................................................................. 63

III‐3‐2 Multilayer Assembly and alignment .............................................................................................................. 64

III‐3‐3 Quality and control ........................................................................................................................................ 65

III‐4 Mechanical analysis of Micromegas chamber ............................................................................................... 70

III‐4‐1 The models .................................................................................................................................................... 70

III‐4‐2 First study: displacements under thermal conditions ................................................................................... 71

III‐4‐3 Second study: displacement under its own weight ....................................................................................... 74

III‐4‐4 Conclusion ..................................................................................................................................................... 75

IV3D‐MICROMEGASBEHAVIOR’SSIMULATIONWITHLORENTZ‐3D................................................76

IV‐1 The software ................................................................................................................................................. 76

IV‐2 3D‐models ..................................................................................................................................................... 77

IV‐3 2D‐models ..................................................................................................................................................... 78

IV‐4 Results and comparisons ............................................................................................................................... 79

IV‐4‐1 Streamlines ................................................................................................................................................... 80

IV‐4‐2 Equipotential ................................................................................................................................................. 83

CONCLUSION..........................................................................................................................................................84

REFERENCES.......................................................................................................................................................863

APPENDIX...............................................................................................................................................................88

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TABLEOFILLUSTRATIONSFigure 1: CEA‐Saclay .................................................................................................................................... 13

Figure 2 : Map of LHC‐CERN, Geneva .......................................................................................................... 14

Figure 3: ATLAS description ......................................................................................................................... 16

Figure 4: Positions of different technologies in the muon spectrometer (from CERN‐OPEN‐2008 Atlas) . 19

Figure 5: Position of the Small‐Wheel in ATLAS .......................................................................................... 21

Figure 6: Principle of a MicroMegas detector ............................................................................................. 24

Figure 7: Simulation of an avalanche with GARFIELD ................................................................................. 25

Figure 8: Streamlines in a MicroMegas detector with LORENTZ ................................................................ 25

Figure 9: Principle and scheme of a MicroMegas in 3D .............................................................................. 26

Figure 10: Different materials of the mesh ................................................................................................. 26

Figure 11: Scheme of a resistive Micromegas (top: face view, bottom: side view) .................................... 29

Figure 12: Electrical equivalent circuit of a resistive Micromegas (by Rui de Oliveira, CERN) .................... 29

Figure 13: Electrical set‐up for any experimental test with a Micromegas ................................................ 31

Figure 14: Electronics crate ‐ with power‐supply and amplifiers ................................................................ 31

Figure 15: Schemes of Micromegas components (with Gerber software) ................................................. 34

Figure 16: Scheme of prototype 1 ............................................................................................................... 35

Figure 17: photo of prototype 2 with its mask, the pre‐amplifier and all connectors ................................ 35

Figure 18: A typical transparency curve (Mesh voltage is fixed) ................................................................. 36

Figure 19: A typical histogram with Amptek MCA software. ...................................................................... 37

Figure 20: schematic of prototype 2 ........................................................................................................... 38

Figure 21: Transparency curves of all zones and holes of Prototype 2 ....................................................... 39

Figure 22: Gain curves of all zones of Prototype 2 ...................................................................................... 40

Figure 23: Sparks production by zone ......................................................................................................... 41

Figure 24: scheme of 2D (X and Y readouts) Micromegas .......................................................................... 44

Figure 25: X‐rays test set‐up ........................................................................................................................ 45

Figure 26: Photo of detector R17a with its mask before irradiation .......................................................... 45

Figure 27: Evolution of the mesh current during the X‐rays exposure ....................................................... 46

Figure 28: Gain curves comparison before and after the irradiation.......................................................... 47

Figure 29: Evolution of the mesh current during the second exposure ..................................................... 47

Figure 30: Gain comparison on irradiated detector R17a (left) and non‐irradiated R17b (right)............... 48

Figure 31: Detector's emplacement at Orphee reactor neutron guide ...................................................... 49

Figure 32: Photo of the detector R17a in the neutron guide ...................................................................... 50

Figure 33: Spectrum from the MCA after 5 minutes neutron exposure ..................................................... 51

Figure 34: Spectrum from the MCA after 2 hours exposure ....................................................................... 52

Figure 35: Evolution of the mesh current during neutrons exposure ......................................................... 52

Figure 36: photo of R17b, ready for a gain measurement .......................................................................... 53

Figure 37: Gains comparison before and after neutrons irradiation of detector R17a .............................. 54

Figure 38: Detector R17a, the Cobalt source and its shielding (in Orange) ................................................ 55

Figure 39: Evolution of the mesh current in COCASE for the first four days (bottom: zoom of the top) ... 55

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Figure 40: Current muon spectrometer's scheme. In blue, the three wheels of MDT chambers. ............. 57

Figure 41: Inspection of a Micromegas with a 3D‐microscope ................................................................... 58

Figure 42: Geometrical properties of the Micromegas detector ................................................................ 59

Figure 43: Scheme of a Micromegas multilayer .......................................................................................... 61

Figure 44: schematic view of a Micromegas chamber ................................................................................ 61

Figure 45: Scheme of the New‐Small‐Wheel (from Saclay's engineering department) .............................. 62

Figure 46: Typical dimensions and lengths of micromegas chambers ........................................................ 62

Figure 47: Layout of the chambers to avoid dead‐zones and detect all particles ...................................... 63

Figure 48: Schematic view of a PCB and copper strips ............................................................................... 65

Figure 49: Scheme of the potential assembly device .................................................................................. 65

Figure 52: Schematic view of a possible device from optical control during the assembly ........................ 66

Figure 53: Scheme of the optical control system with lenses, masks and cameras ................................... 67

Figure 54: Possibility to design slots for a camera and masks on the PCB .................................................. 68

Figure 55: The current Small‐wheel with eight alignment bars full of optical devices ............................... 68

Figure 56: The full optical control system ................................................................................................... 69

Figure 57: The In‐plane system mounted on a chamber ............................................................................ 69

Figure 58: Models designed for the study ................................................................................................... 70

Figure 59: Zoom on a multilayer from model 2 .......................................................................................... 71

Figure 60: Thermal conditions applied on model 4 ..................................................................................... 72

Figure 61: Displacement module (in millimeters) ....................................................................................... 72

Figure 62: Longitudinal distortion (along the y‐axis) causing a misalignment of strips along z‐axis .......... 73

Figure 63: Von Mises stresses over the frame (in MPa) .............................................................................. 73

Figure 64: Conditions for study 2 ................................................................................................................ 74

Figure 65: Displacement module of model 1 for study 2 ............................................................................ 75

Figure 66: Logo of Lorentz ........................................................................................................................... 76

Figure 67: Schematic views of Micromegas in 3D with LORENTZ ............................................................... 77

Figure 68: Geometrical properties of the 2D model ................................................................................... 78

Figure 69: Materials used for 2D model ...................................................................................................... 78

Figure 70: Boundaries conditions and voltages of 2D model ...................................................................... 79

Figure 71: Streamlines for Micromegas detector. ...................................................................................... 80



Figure 74: Equipotential lines near the strips (only from 0V to ‐500V) for the three configurations ......... 83

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IntroductionThe Large Hadron Collider [1] in CERN [2] at Geneva is certainly one of the biggest and most impressive

experiments which have ever been built. Its size, its power, its costs and all the expectations associated

are tremendous. The LHC raised a new hope for physicists to discover a new physics and better

understand the existing one. It is a huge breakthrough for particle physics.

However, scientists need and deserve always more and more to deeper investigate, find new particles

such as Higgs boson [3] or prove new theories like Supersymmetry .This is the reason why the CERN gave

birth to the HL‐LHC project which is simply an enormous upgrade of the existing LHC. The luminosity will

be increased by a factor of 10 and the energy in the center of mass of proton‐proton collision will reach

14 TeV by 2018‐2020.

Of course, this kind of project has a lot of fallouts. Since the number of collision and their energy will get

bigger, the background and the numbers of particles created will reach a tremendous level.

Consequently, many evolutions have to be foreseen. All detectors very near the impact point or the

vacuum tube will have to be upgraded in order to handle the new conditions and particle fluxes.

The Small Wheel of the muons spectrometer is part of the devices that have to be redesigned. This is a

10 m diameter wheel, full of gaseous detectors, which plays a significant role in the study of muons.

The CEA, within an international community called MAMMA1, is involved in the “competition” for the

Small wheel upgrade. The issue is to replace all detectors in the wheel by up to date detectors, more

robust and precise. The MAMMA collaboration proposes to install a new kind of micro pattern gaseous

detector called MicroMegaS (MICRO MEsh GAseous Structure). This very recent and innovative

technology is a real breakthrough in the field of physics of detectors.

The CEA‐Saclay on behalf of the MAMMA collaboration is in charge of some parts of the proposition.

First, R&D researches are conducted to better understand the detector and its specificities. Some

evolutions and new techniques had to be implemented on a classical Micromegas to enable it to cope

with extreme operations conditions.

Then, the HL‐LHC research team has to study the ageing of the detector. The principle is to prove the

capability of detectors made of new Micromegas technology to operate in long data taking periods.

Finally, the CEA has to work on the integration, the alignment and the installation of Micromegas

detectors chambers on the New Small Wheel. The two wheel of spectrometer represents more than

1000m² of detectors that have to be placed and aligned with an accuracy of 30µm.

As a long‐term trainee I have worked on the three tasks and had the possibility to see all the aspects of

the projects. After a brief presentation of the scientific and industrial context, I will present results of

experimental and ageing tests I conducted. Then, I will focus on the assembly process of the Small wheel.

Finally, I will introduce some preliminary simulations of Micromegas behavior I have done.

1 Muon Atlas MicroMegas Activity

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IScientificandIndustrialContext

I‐1CEA

The CEA is the French Alternative Energies and Atomic Energy Commission (Commissariat à l'énergie

atomique et aux énergies alternatives). It is a public body established in October 1945 by General de

Gaulle. A leader in research, development and innovation, the CEA mission statement has two main

objectives: To become the leading technological research organization in Europe and to ensure that the

nuclear deterrent remains effective in the future.

The CEA is active in four main areas: low‐carbon energies, defense and security, information

technologies and health technologies. In each of these fields, the CEA maintains a cross‐disciplinary

culture of engineers and researchers, building on the synergies between fundamental and technological

research.

In 2009, the total CEA workforce consisted of 15 718 employees. Across the whole of the CEA (including

both civilian and military research), there were 1,360 PhD students and 289 post‐docs. In 2009, the

civilian programs of the CEA received 45 % of their funding from the French government, and 34 % from

external sources (partner companies and the European Union). In 2009, the CEA had a budget of 3.9

billion euros.

The CEA is based in ten research centers in France, each specializing in specific fields. The laboratories

are located in the Paris region, the Rhône‐Alpes, the Rhône valley, the Provence‐Alpes‐Côte d'Azur

region, Aquitaine, Central France and Burgundy. The CEA benefits from the strong regional identities of

these laboratories and the partnerships forged with other research centers, local authorities and

universities.

Figure 1: CEA‐Saclay

The SEDI (Service d’Electronique, des Détecteurs et d’Informatique is involved in many international

experiments and I was integrated to a team working on HL‐LHC, the upgrade of LHC in CERN. The team is

composed by five physicists, a technician, consultants and students, who also work on other projects.

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I‐2ScientificContext

I‐2‐1CERN

Figure 2 : Map of LHC‐CERN, Geneva

It is almost unnecessary to introduce CERN, since the European laboratory is very well‐known plays a

major role in Physics. CERN, the European Organization for Nuclear Research, is one of the world’s

largest and most respected centers for scientific research. At CERN, the world’s largest and most

complex scientific instruments are used to study the basic constituents of matter in order to find out

what the universe is made of and how it works.

Founded in 1954, the CERN Laboratory has its site astride the Franco–Swiss border near Geneva. It was

one of Europe’s first joint ventures and now has 20 Member States.

I‐2‐2LHC

a) TheprincipleThe Large Hadron Colliger is a two ring superconducting hadron accelerator and collider constructed at

CERN. It has been designed to collide protons with a center‐of‐mass energy of 14 TeV. These conditions

have never been achieved before in any experiment.

Before been injected into the LHC, protons are progressively accelerated through a set of linear and

circular accelerators. Protons are injected into the two main LHC rings, such that they are assembled in

trains of bunches with around 1011 protons per bunch in both directions, clockwise and anti‐clockwise.

Once there, these proton bunches will be accelerated up to 7 TeV (energy per proton) and finally collided

at four different points where detectors have been constructed to probe the physics laws at thse

energies.

The bunch crossing rate, at each of these points, will be about 40 MHz (25 ns). There are six detectors

installed at the LHC: Atlas, CMS, Alice, LHCb, Totem and LHCf. Atlas and CMS [4] are designed to cover

the widest possible range of physics in proton‐proton collisions, while LHCb and Alice are designed to

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study specific phenomena, LHCb for B‐physics and Alice for the interactions in heavy ions collisions. The

detectors used by the Totem and LHCf experiments are positioned near CMS and Atlas respectively.

Totem and LHCf are designed to focus on particles which are scattered at small angles compared to the

beams.

On November 2009, the proton beams were successfully circulated, and the first proton‐proton collisions

recorded at the injection energy of 450 GeV per beam. The LHC became the world’s highest‐energy

particle accelerator on 30 November 2009, achieving 1.18 TeV per beam. After the 2010 winter

shutdown, the LHC was restarted and the beam was ramped up to 3.5 TeV per beam. We reached 7 TeV

in 2011.

b) Up‐gradeandmotivationThe main motivation for a luminosity upgrade is to provide more statistics to improve physics studies

beyond those possible at the original LHC design. The HL‐LHC, with a tenfold increase in luminosity, will

extend the discovery reach of the LHC for new particles such as those arising from Supersymmetry, and

will allow for detailed measurements of Standard Model processes and any new phenomena discovered

during LHC operations. Some of the possibilities that can benefit from the increased luminosity of the HL‐

LHC are:

The precision measurement of the electroweak parameters is a tool to look indirectly for physics

beyond the Standard Model (SM).

Most of the top quark studies at the LHC will have been done before HL‐LHC comes into

operation. An important exception is the search for rare top decays.

If Supersymmetry (SUSY) has not yet been discovered in data samples collected during LHC running, inclusive searches may continue with the larger integrated luminosity of the HL‐LHC. If evidence for SUSY is discovered, it will be important to measure: i) more exclusive final states in order to measure the particle mass spectrum. ii) to determine the spin of the new particles in order to understand whether counterparts to the SM particles are observed with opposite spin statistics, or whether some other new phenomenon is observed.

The increase in luminosity at the HL‐LHC will give access to jets with energy around 4.5 TeV. This offers the opportunity to extend the search for quark substructure.

The Standard Model Higgs, if it exists, might have been discovered by the time the HL‐LHC starts

its operation. It will however remain important to measure its properties more precisely. If no

Higgs is discovered then it is expected that the high energy scattering of electroweak gauge

bosons will show structure beyond that expected in the Standard Model at WW and ZZ masses

of order of 1 TeV . The discovery of such effects may be very difficult at the LHC.

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I‐2‐3ATLAS

Atlas is one of the experiments built at the Large Hadron Collider. It is a general purpose detector

designed to explore physics at the TeV energy scale. Its dimensions are roughly: 44 m long, 25 m high

and 25 m wide, with a weight of 7000 tones. The main feature of this detector is its enormous toroidal‐

shape magnet system, and that is why it is called ATLAS, A Toroidal LHC ApparatuS [5]. The toroidal

magnet consists of eight 25 m long by 5 m wide superconducting magnet coils, arranged to form a

cylindrical toroid surrounding the beam pipe.

Figure 3: ATLAS description

This experiment is the result of an international collaboration, where over 2900 physicists and engineers,

from 184 institutes, from 37 countries participate.

The detector is made up of four sub‐detectors. These are:

Inner Detector: The task of the Inner Detector (ID) is to measure the track and momentum of charged particles. The position of the charged particles is measured in different sets of layers as they pass throughout. Beginning from innermost part, the inner detector has three layers of silicon pixels detectors, four double layers of semiconductor trackers (SCT), and a transition radiation tracker (TRT). The TRT also identifies high energy electrons w.r.t other charged particle like pions, muons etc. The tracking system sits inside a solenoidal magnet that produces a magnetic field of 2 T; thus, charged particles are bent permitting to determine their charge and momentum.

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Calorimeters: High energy particles initiate hadronic or electromagnetic showers, as they

encounter the detector material. The Atlas experiment has electromagnetic and hadronic

calorimeterswere the energy of particles is measured by stopping them with dense materials.

The particles interact generating showers of secondary before being stopped (with the exception

of muons). The calorimeters are the primary shield protecting the muon system.

The electromagnetic calorimeter system: detects and identifies electrons and photons, and

measures their energy. It is divided into a barrel and two end‐cap calorimeters, working in a

similar way. These calorimeters have an accordion‐shape structures that consist of many layers

of lead absorbers and liquid argon (LAr). A copper grid immersed in each liquid argon layer acts

as an electrode. In these calorimeters, the particles interact with the lead plates generate

electromagnetic showers. Then, the secondary particles ionize the argon as they pass through.

The electrons resulting from the ionization are drifted to the copper electrodes and the electric

current is measured. The greater the energy of a particle entering in the EM calorimeter, the

greater will be the number of secondary particles generated in the shower, and in consequence

the current. The accordion geometry of this calorimeter provides complete φ symmetry without

azimuthal cracks and good trigger capabilities. In front of the barrel calorimeter, there is also a

LAr layer with active electrodes. The information that it provides is utilized to correct the energy

lost by electrons and photons when they go through the matter in front of the calorimeter.

The hadronic calorimeter: measures the energy of particles where only part of the energy is

deposited when they traverse the EM calorimeter; these are primarily hadrons. This calorimeter

is divided into a barrel part and two end‐cap components as well. But in this case, these parts

work differently; the tile barrel calorimeters utilize scintillating plates and the end‐caps are liquid

argon calorimeters in the same cryostat as EM calorimeter. That is because the radiation

emanating from the collision point is more intense at large values of , and the scintillating tiles are damaged by excessive exposure to radiation. The tile barrel calorimeter utilizes steel sheets

in order to generate the hadronic shower and scintillating sheets as the active material. They are

placed in planes perpendicular to the beam, forming layers of steel including scintillating

material. When the shower particles pass through the scintillating tiles, they emit light in an

amount proportional to the incident energy. Then fibers carry the light to devices where the light

intensity is measured. The liquid argon end‐cap hadronic calorimeter is very similar to the EM

calorimeter. The difference is that it uses copper planar plates instead of lead accordion plates,

which are more appropriate to the hadronic showering process, and the argon gaps are twice

larger as well.

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I‐2‐4Muonspectrometer

High momentum final‐state muons, when they occur, are amongst the most promising and robust

signatures of physics in the LHC. To exploit this potential, a high‐resolution muon spectrometer has been

installed. This sub‐detector measures the particles tracks and their momentum using the deflection

caused by the superconducting toroidal magnets. There are three toroidal magnets: the large barrel

toroid and two smaller end‐cap magnets, which are inserted into both ends of the large one. The muon

spectrometer has been designed to have a good momentum resolution of ∆ 10% even at pT = 1 TeV.

The muon chambers are complemented by fast trigger chambers with a time resolution of the order of 2

ns. The chambers are arranged such that particles from the interaction point traverse three stations of

the chambers. The position for these three stations is the result of the compromise between the

optimum momentum resolution and the place avalaible inside the detector (supports for the magnets,

electronics and services). Precision momentum measurement and triggering are done by four chamber

technologies that will be described in the following sections.

a) Trackandmomentummeasurement:

To make precision measurements of the track coordinate two types of chambers are used:

The Monitored Drift Tubes (MDT) [6]: They are used in two regions, the barrel and the end cap.

The basic detection element is a cylindrical aluminum drift tube of 30mm diameter with a central

wire of 50 μm diameter. The tube is filled with non‐flammable gas composed of Argon (91%) and

CO2 9% at 3 bars absolute pressure. When a ionizing particle passes through the tube, it will

ionize the surrounding gas. The resulting ions and electrons drift in the electric field due to the

potential on the wire (3270 V). Close to the wire, the field is high enough to cause an avalanche

resulting in a measurable electric current. The relation between the drift time and the drift

distance can be calculated giving the local position of the muon track.

An MDT Chamber is an assembly of six parallel layers of drift tubes on a support frame, three

layers on each side. The tubes are closely spaced so that each ’triple layer’ or ’multilayer’ has a

thickness of about 82 mm. This results in a measurement of effectively one coordinate with 40

μm precision and one angle with 3.10−4 precision.

Cathode Strips Chamber (CSC) [7]: are used in the internal part of the end‐caps, where high

(>200Hz/cm2) counting rates are expected, due to their higher rate capability and time

resolution. The CSC’s are multiwire proportional chambers with cathode planes segmented into

strips in orthogonal directions. This allows both coordinates to be measured from the induced

charge distribution. The chamber is filled with a gas mixture of Ar 80%, and CO2 20%. The

resolution of a chamber is 40 μm in the bending plane and about 5 mm in the transverse plane.

The difference in resolution between the bending and non‐bending planes is due to the different

readout pitch, and to the fact that the azimuthal readout runs parallel to the anode wires.

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To achieve the required resolution, the location of MDT wires and CSC strips along a muon trajectory

must be known to better than 30 μm. For these reason, a high‐precision optical alignment system

monitors positions and internal deformations of MDT chambers.

b) Triggersystem:

The precision tracking chambers have been complemented by a system of fast trigger chambers capable

of delivering track information within a few tens of nanoseconds after the passage of the particle. The

purposes of these trigger chambers are to provide:

‐ the second coordinate measurement

‐ timing information to relate muons tracks to the correct bunch crossings

‐ and define sharp pT thresholds for the trigger.

For this purpose two types of detectors were used:

Resistive Plate Chamber (RPC) [8]: are placed in the barrel region. The RPC is a gaseous detector

(Tetrafluoretane C2H2F4 94.7% + C4H10 5% + SF6 0.3%) formed by parallel electrode plates with a 2

mm gap. A uniform electric field produces the avalanche multiplication of the primary electron.

The trigger function is provided by three planes of RPC stations, located on both sides of the

middle MDT, and either directly above or directly below the outer MDT station. After matching

of the MDT and trigger chamber hits in the bending plane, the trigger chamber’s coordinate in

the non‐bending plane is adopted as the second coordinate of the MDT measurement.

Thin gap Chambers (TGC) [9]: are used in the end‐cap regions. TGCs are MWPC filled with a gas

mixture of 55% CO2 + 45% C5H12.The gap is also around 2mm. The inner tracking layer is

complemented by two layers of TGC, providing second coordinate whit out participate in the

trigger. The traverse momentum selection is done with a fast coincidence between strips on

different planes. The number of trigger planes is defined by the need to minimize the rate of

accidental coincidences and optimize the efficiency.

Figure 4: Positions of different technologies in the muon spectrometer (from CERN‐OPEN‐2008 Atlas)

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I‐2‐4TheHL‐LHC

Since the end of 2009, the LHC has worked successfully. Many proton‐proton collisions are already

recorded. On october 30th , the end of the 2011 proton‐proton luminosity was around 3.1033 cm‐2s‐1 with

beam crossing occuring almost every 25 ns. The road map and the schedule of the LHC plans to increase

the luminosity by a factor of ten by 2018.

As mentioned before, the main motivation for an upgrade is to provide more statistics to improve

physics studies beyond those possible at the LHC. The HL‐LHC with its tenfold increase in luminosity, will

extend the discover reached by the LHC for new particles such as those arising from Supersymmetry and

will allow for detailed measurements of Standard Models processes and any new phenomena discovered

during LHC operations.

The completion of the program defines the phase‐1 upgrade which will be achieved after a shut‐down

currently scheduled for 2017, and will allow a peak luminosity of 3.1034 cm−2s−1, a factor of three higher

than the nominal luminosity of LHC. At this rate, the number of interactions per beam crossing (40 MHz)

in Atlas or CMS is equal to about 70. A second upgrade, called phase‐2, is being designed with the aim at

reaching a peak luminosity of 1.1035 cm−2s−1.

This upgrade requires lots of changes. The LHC detectors must be adapted to these new conditions. Atlas

is already studying the upgrade solutions of phase‐1 and phase‐2.

With the HL‐LHC luminosity, the radiation (i.e. thermal neutrons, photons…) and the event pile‐up are

expected to increase considerably, especially in the muon spectrometer. The background will degrade

performance and damage detectors and electronics and induce more data corruption. Therefore, the

unprecedented level of radiation is going to have a major impact on the design of detectors.

I‐2‐5Themuonspectrometerupgrade

The muon spectrometer will have to be modified progressively. The first step is to upgrade detectors in

the Small Wheel. This wheel is the first and the smallest of the three wheels of the muon spectrometer.

Its diameter is around 10 meters. There are two Small‐Wheels, one on each side of the collision point.

Page21

Figure 5: Position of the Small‐Wheel in ATLAS

The ATLAS commission stated that the future muons chambers in the Small‐Wheel will have to meet the

following specifications:

‐ High rate capability (<10 kHz.cm‐2)

‐ Detector efficiency : 99% (for pT > 3GeV/c)

‐ Spatial resolution : <100 µm

‐ Time resolution : 5 ns

‐ Level‐1 trigger capability

‐ Radiation and ageing hardness

Different technologies in competition are being considered for the upgrade of the muon spectrometer.

Two of them are just an upgrade or an evolution of detectors that are already in the spectrometer and

have been introduced previously in this report:

Thin Gap Chamber (TGC): An upgraded version of this technology can henceforth provide:

‐ precision tracking by analogue read‐out of strips orthogonal to the anode wires

‐ second coordinate by grouping of anode wires or pad read‐out

‐ Higher rate capability

‐ Triggering devices

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Small Monitored Drift Tube (sMDT): A version with smaller diameter tubes (15 mm rather than

30 mm) is proposed for the upgrade of the Muon spectrometer. Compared to the current design,

the smaller radius provides an improvement of rate capabilities thanks to a reduced drift time,

thus reducing the sensitive time for background hits, by a factor of 3.5. A further reduction of the

background hit probability comes from the shorter track segment crossing the tube, which leads

to shorter pulses. Another reduction comes from the two times smaller area exposed to

gammas. The small tubes also allow more tube layers to be installed in the available space,

leading to improved position resolution and robust tracking in the presence of tube

inefficiencies.

Resistive Micromegas detector: This new technology was initially developed at CEA‐Saclay. This

is a totally innovative kind of detector which can play a role in the trigger as well. I was involved

in the study and the development of Micromegas detectors during my internship. They are

described in detail in section I.3

I‐2‐6TheMAMMAcollaboration

The MAMMA (Muon Atlas MicroMegas Activity) collaboration is a group of laboratories around the

world, led by the CERN, aiming to develop Micromegas detectors as a replacement solution for the muon

spectrometer of HL‐LHC. CEA has joined this international community in 2007. 21 institutes such as

Arizona, Athens, Brandeis (USA), and Naples belong to MAMMA.

Micromegas is not the favored technology to replace the current detectors. Indeed, this is a new kind of

detectors which needs to be tested more deeply. Moreover, the other candidates are already used in

ATLAS‐LHC and their lobbying is powerful and efficient.

I‐2‐7Thedecision

At the time of writing this report, the decision about the new replacement technology is discussed.

However, the referees have proposed two consensual solutions :

The homogeneous solution : sTGC (trigger) + Micromegas (tracking: 1200 m²)

The split solution : sTGC(trigger) + sMDT(tracking in the outter part) + Micromegas(tracking in

the central part: 300 m²)

The description made by the referees of the two proposals can be found in the Appendix. The proposals

are highly political because they foresee a New Small Wheel composed by a mixture of different

technologies. It will make the construction process and the usage more complex. There will also be less

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electronics channels and problems of alignment. Finally, it will satisfy more laboratories and

communities.

The good news is that Micromegas is involved in both propositions. That justifies and rewards all the

work that has been and will be done.

The final decision meeting was supposed to take place in CERN on March the 23rd. During the meeting, it

was decided to chose the homogeneous solution temporarly. Indeed, this solution is highly risky because

lot of works is still needed for Micromegas. Consequently, the deciding committee has put some

milestones during the present year. If technologies involved in the homogeneous solution fulfill all the

milestones, then the solution will be finally approved. Otherwise, the split solution will be chosen as a

backup solution.

I‐3Micromegas

I‐3‐1Gaseousdetector

In particle physics there are a large variety of detectors using different materials and based on different

technologies. For example, one can mention solid detectors, semiconductor detectors (silicon or

germanium) or scintillation detectors.

MicroMegas detectors belong to a particular sort of particle gaseous detectors. They are all based on the

same principle: the particle will pass through a certain gas, and will ionize atoms in it, and then create

ions and electrons. This ionization is amplified and converted into an electrical signal. Then, MicroMegas

can be classified as a Micro‐pattern gaseous detector [10].

To produce an electrical signal they all have the same basic design: the gas is embedded between two

electrodes, on which the ionization signal is collected

Then, the method to count, detect, measure or amplify the signal differs from a gaseous detector’s

technology to another. I will only focus on the one I worked with: The MicroMegaS Technology

I‐3‐2Principle

MicroMegas detector stands for “MICRO Mesh Gaseous Structure”. This technology was created to be

used in accelerators and particle physics, especially for high‐rate applications. It was developed here in

CEA‐Saclay by Ioannis Giomataris [11] in the nineties. However, some predecessors thought about this

idea before: A.Oed [12] first and G.Charpak & F.Sauli [13] imagined the first MicroStrip Gaseous

Chambers.

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The principle is very simple: the gas volume split into two regions is separated by thin micromesh

(typically of thickness of 30 μm), one where the conversion and drift of the ionization electrons occurs

and the other which is only 50 − 100 μm thick where the amplification takes place. In the amplification

region, a very high field (40 to 70 kV/cm) is created by applying a voltage of a few hundred volts between

the mesh and the anode plane, which collects the charge. The anode can be segmented into strips or

pads. A schematic view can be seen in figure 7. Thus, a charged particle ionizes atoms in the conversion

region. Thanks to a high electrical field in the amplification gap, the electrons created in the conversion

gap will form an avalanche. Indeed, primary electrons will gain enough energy in the amplification gap to

ionize other gas molecules. The newly created electrons will accelerate and cause

new ionizations, and so on to form this avalanche.

Figure 6: Principle of a MicroMegas detector

The electron multiplication, taking place between the anode and the mesh is up to 105 or more. It means

that a primary electron (created by ionization) will cause another 105 ionizations Consequently, charge

amplification occurs near the copper strips. Electrons are then collected by the strips. Ions also created

via the ionization, slowly go up to the anode. The electrons move with typically 5 cm/µs (or 20 ns/mm) to

and through the mesh.

The signal is generated thanks to the movement of the charged particles. Electrons travel only through a

tiny portion of the detector. The signal is mainly due to ions. These moves tend to create an electrical

field and thus a current on the strips. This can be explained by the Ramo’s theorem [14].

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Figure 7: Simulation of an avalanche with GARFIELD

One difficulty is to make the mesh transparent for electrons. One has to choose an appropriate ratio

between the amplification field and the conversion’s one. The aim is to obtain a “funnel effect” with the

E‐field lines. This effect can be seen here:

Figure 8: Streamlines in a MicroMegas detector with LORENTZ

On a technical point of view, the mesh is supported by pillars. You can find one of them every 2 or 5mm

depending on the prototype. They are here to avoid the sagging of the mesh and keep the distance

between the mesh and the strips constant.

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Figure 9: Principle and scheme of a MicroMegas in 3D

Many different technologies have been developed for making meshes. A mesh can be built in many

metals: Nickel, copper, stainless steel, aluminum… but also gold and titanium are also possible. I will not

get into the existing technologies, but here is a non‐exhaustive panel of what can be done:

Figure 10: Different materials of the mesh

The gas used is actually a mixture. The main component is a noble gas because the energy of the inner

gas has to be dissipated by ionization. Noble gas molecules have no rotation or vibration excited states.

However, this kind of gas emits UV‐photons in the avalanche (11.6 eV for Ar). These photons are likely to

extract an electron from the surrounding environment (i.e. from the copper cathode). The risk is to

generate a permanent avalanche and make the phenomena instable. One has to add a “quencher”: a

heavier or polyatomic gas which is able to absorb these photons. It can be CH4, BF3, or CO2.

Typically, the gas‐mixture can be: 90% of Argon with 10% carbon dioxide. One can found some

percentages of isobutane also (but not in ATLAS, since it is flammable and not suitable for ageing tests).

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I‐3‐3Performance,AdvantagesandDrawbacks(Sparks)

a) Previousexperience The advantages of Micromegas follow from the thin size of the amplification gap and the particular

configuration of the electric field on the two sides of the mesh, itself depending on the mesh pitch. The

gap being very small, the size of the avalanche and hence the signal rise time are very small, leading to

an excellent spatial and time resolution: 12 μm accuracy has been already reached while resolutions in

the sub nanosecond range have being measured by several experiments. Starting from the avalanche

concentrated in the last few microns of the gap, the ions flow back to the mesh in the amplification field.

Such a fast signal and ion collection allows high rates to be sustained.

Micromegas’ properties and advantages have led many experiments to choose this technology. Also,

Micromegas has already proven its utility and efficiency in various fields of particle physics. The most

relevant for this purpose are:

COMPASS [15]which is fixed target experiment at CERN that has pioneered the use of large 40 ×

40 cm2 [16] Micromegas detectors for tracking close to the beam line with particle rates of 25

kHz/mm2. All detectors performance was conserved in the COMPASS detectors after several

years of operation with an accumulated charge of a 2 mC/cm2.

T2K(Tokai to Kamioka) [17] is a neutrino experiment with an intense beam of muon neutrinos from J‐PARC to Kamioka that is able to measure the momenta of muons produced by charged current reactions in the detector. In order to do that, T2K has the largest Micromegas detector ever constructed (9 m²). The capability to pave a large surface with a simple mounting solution and small dead space has been demonstrated. This is of particular interest for applications in the HL‐LHC context.

Micromegas are also in use or under development for low energy neutrino experiments including

neutrino oscillations, neutrino magnetic moment, coherent neutrino scattering and searches on solar

axions or dark matter WIMPs.

b) LimitationsinHL‐LHCenvironment As mentioned before, our research group is focused on Micromegas detector studies as a replacement

solution in the muon‐spectrometer of ATLAS. Previous uses in international experiments such as

COMPASS and T2K have shown that MicroMegas could be a good alternative because of its good spatial

and time resolutions, its high‐rate capability, radiation hardness, robustness and the possibility to build

large areas. But in the HL‐LHC environment, the rate will be so high, that we will have to cope with other

obstacles.

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The limitation for the Micromegas appears in the case of high‐energy deposition events which can occur

with a high frequency in ATLAS. Indeed, these events can produce accidental discharges or "Sparks" [18]

and then limit the rate capability. The sparks develop when the local electron charge concentrations

exceed the Raether limit [19] (G × n0 < 108 electrons, where n0 is the number of primary electrons and G

the gain of the detector). Sparks are a major concern. When a sparks occur it leads to the discharge of

the micro‐mesh. The consequences for a running experiment can be translated into dead time; which is

mainly due to the readjusting of the micro‐mesh to its working voltage, and can take around 1 ms

depending on the power supply. Also if the charge released in a spark is large enough it can damage the

electronics, therefore a protection is needed. The material of the detector must be chosen to handle the

radiation environment and also the energy released in a spark that can sometimes be destructive for

very thin strips or thin micro‐mesh.

So, there are two ways to approach the problem: Avoid high concentrations of charge e.g by spreading

the charge or live with it and make the detector insensitive to sparks.

The problem was detected and solved since quite sometimes. Many options have been studied [20]. The

two main ideas were:

Micro‐mesh segmentation: If the mesh is segmented, the electrical capacity of the detector is

segmented and the charge stored will be reduced in each segment. So in case of a spark the charge

discharged is smaller, and that can be translated in a reduced and local dead time and reduce risk of

electronics damages. Moreover, the dead zone will be localized to the segment that undergoes the

spark. Segmentation is thus favorable for two reasons: reduction and localization of the dead time. The

problem comes with the multiplication of insensitive zones in the detector due to the segmentation

process.

Resistive anodes strips: A way to avoid high concentrations of charge is by spreading the charge. A

possibility to make charge sharing is to make a resistive anode by adding a continuous RC circuit on the

top of the pad plane. A resistive anode will slow down the spark development, then reduce the drop in

voltage and then the dead time. There are different techniques of resistive anodes. The one I focused

one is to implement resistive pads or resistive strips. All the detectors described in this work, were

equipped with that technology.

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Figure 11: Scheme of a resistive Micromegas (top: face view, bottom: side view)

The idea is thus to spread out the charge thanks to resistive strips parallel to the standard copper ones

[21]. The resistive strips are connected to the ground through a resistor. They are not directly above the

standard strips: a thin insulating layer is between the resistive and the readout strips (figure [13]).

In this configuration, sparks are neutralized through the resistive strips to the ground.

The principle of operation of Micromegas is thus slightly modified. The signal is not read directly by the

copper strip anymore. Indeed, the electrons (and thus charges) are collected on the resistive strips and

the electrical signal is generated via a capacitive coupling between the resistive strips and the readout

strip. The layer in between plays the role of the insulator in a standard capacitor.

One can thus sketch an equivalent circuit [22]of this kind of Micromegas (figure [14]):

Figure 12: Electrical equivalent circuit of a resistive Micromegas (by Rui de Oliveira, CERN)

This technology was a real breakthrough and enabled to use MicroMegas in high‐rate conditions. Many laboratory tests have been conducted and have demonstrated the performances.

Page30

IIExperimentaltestsonMicromegasdetectors

In order to better understand the MicroMegas technology and to build a detector meeting all the

requirements use at the HL‐LHC, one has to thoroughly investigate the behavior of different MicroMegas

prototypes under different experimental conditions.

In order to obtain a detector able to deal with the HL‐LHC conditions, the MAMMA collaboration ordered

several prototypes of Micromegas detectors with different intrinsic properties and asked CEA‐Saclay to

study some of them. One of my tasks was to characterize them.

II‐1CharacterizationandvarioustestsonMicromegas

“To characterize a detector “ implies to gather various information regarding behavior, performance and

limits of operation of a detector. It is a mandatory and essential step before experimentally testing the

sample.

Two values are really significant: The gain and the energy resolution. The gain of a detector is the ratio

between the number of electrons collected and the number of electrons released by ionization. The

value strongly depends on the gas‐mixture, the amplification field and the gap. The energy resolution

reflects the fact that, for the same deposited energy, there are fluctuations in the number of avalanche

of electrons created. This value is useful to determine how precisely the detector can evaluate the

energy deposited by a particle.

Thus, a characterization means gain measurements under different conditions (gas mixture, pressure,

voltages) as function of position in the detector.

II‐1‐1AimandExperimentalSet‐up

Before introducing the samples I studied, the experimental set‐up is described.

The gas was the one generally used by the MAMMA community: 90%Ar + 10%CO2. MicroMegas

detectors have previously been successfully tested with this gas. The gas is enclosed in a box.

A 55Fe radioactive source is used to create the incoming particle. This source emits photons with an

energy of 5.9keV, and is placed above the detector. The window is around 1.5cm above the conversion

gap. The source is collimated to only irradiate a small portion of the detector.

Page31

Here is a scheme of the electrical set‐up:

Figure 13: Electrical set‐up for any experimental test with a Micromegas

Figure 14: Electronics crate ‐ with power‐supply and amplifiers

The drift of the Micromegas detector is connected to the power supply through a RC‐filter. This filter is

low‐pass filter and is used to reduce the noise and the background.

As we will see later, we will extract interesting information via the mesh. To read out the mesh, we first

go through a pre‐amplifier and an amplifier. They integrate and shape the signal . Then, one can read the

signal via an oscilloscope or a Multi‐Channel Analyzer linked to a computer. The MCA is a device that is

Page32

able to read a voltage level and with its software generate a histogram. We will be able to see the iron

peak thanks to the MCA. We just have to calibrate the whole chain to know the relation between the

MCA’s channel number and the charge at the entrance of the amplification chain (see II‐1‐2 Calibration

and gain computation) which is directly related to the gain of the detector.

A major problem is to ground the installation. The ground, provided by the power supply, is connected to

almost all wires and connectors of the detector, via a big copper wire. It is a step that one should not

neglect because it is necessary to reduce the noise in the future measurements. Sometimes, a Faraday

cage has to be used to protect the detector from parasite signals.

II‐1‐2Calibrationandgaincomputation

One of the main tools to proceed to a characterization or any experimental tests on detectors is to

measure and compute the detector’s gain. In order to perfectly evaluate this gain, one has to know the

transfer function of the electronics, thus to calibrate all the electronics chain.

To accomplish a calibration, we inject a signal, provided by a pulse generator through a capacitance. The

signal passes by all electronic devices (preamplifier and amplifier) and is read by an oscilloscope. The

capacity C is then known and the voltage V delivered by the pulse generator can be read. Then, we can

know the charge that we put in thanks to:

.

One can establish a relation between the amplitude (in Volt) of the measured signal at the end of

the chain and the charge Q we inject by reading the with an oscilloscope. It then yields the relation

between and the number n of the MCA’s channel corresponding to the position of the peak. The

amplification is linear and the plot V(n) is a straight line.

Here, the pulse generates an input voltage of 600mV and the capacitance is 4.9pF. Moreover, we get

4.8 .

It yields that: 2.94 and the coefficient is 1.5 . . One just has to determine

with channel n of the MCA corresponds to and deduce the coefficient K, the slope of V(n).

Once the chain is calibrated, the same devices have to be kept for any tests. Hence, we know the relation

between V and n, we can determine the gain G of the detector. The gain is defined as the ratio between

the total number of electrons in the avalanche and the number of ionized electrons

Indeed, the incoming particle will produce a certain amount of primary ionizations and thus create pair

of electron‐ion. The newly created electrons will get enough energy to induce new ionizations. The sum

of these two phenomena constitutes the quantity of ionized electrons. It is directly related to the nature

Page33

of the gas and the energy of the incoming particle (here equals to 5.9keV) and can be expressed like

this,

Where is ionization potential of the gas and corresponds to the necessary energy to produce a

electron‐ion pair.

Here, the noble gas is Argon and

26 /

The carbon dioxide can also play a role, so that

33 /

and

0,9. 0,1.

It finally yields:

221

Then, it remains to compute the total number of electrons. It depends on the total charge collected by

the anode, the electron charge and the calibration constant K :

...

And finally, the gain formula is given by:

.221. .

Consequently, in order to compute the gain, we just have to read the channel of the MCA corresponding

to the Iron peak, to find the voltage V corresponding to the channel thanks to the calibration made

before.

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II‐1‐3Samplesstudied

All the samples are Resistive Micromegas detectors. It means that they have standard copper strips and

resistive strips as previously detailed. They were all made at CERN in the laboratory of Rui De Oliveira.

Three different prototypes with different properties were tested: Different resistive strips width;

Different conductive strips width; Different conductive and resistive strips width.

Printed Circuit Board (PCB) and strips are first designed as gerber files using the GerbV software. The

software produces pattern and drills masks on the PCB. We received the 3 with associated Ferber files to

perfectly know the design of all prototypes. One can easily, for example, measure the width of the

standard and resistive strips.

Prototype 1 Prototype 2 Prototype 3

Pitch (µm) 500 500 500

Copper Strips

Zone1 Zone2 Zone3 Zone4

400 300 200 100

400 for all

400 300 200 100

Resistive Strips


350 for all

360 270 180 90

360 270 180 90

Table 1: characteristics of all prototypes from CERN (in µm)

Figure 15: Schemes of Micromegas components (with Gerber software) Top‐left: the whole detector, Top‐right: copper strips, bottom: resistive strips

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Figure 16: Scheme of prototype 1

On the same prototype, one can find different widths of the resistive strips depending on the region

(figure [18]). A mask with small holes was made in order to only irradiate the selected region (figure

[19]).

Figure 17: photo of prototype 2 with its mask, the pre‐amplifier and all connectors

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II‐1‐4Micromegascharacterization

The process is not complex but repetitive. The first step is, of course, to connect the detector. One has to

wait for few hours to let the gas circulate in the detector. The gas flow is about 1 renewal per hour. The

volume of gas for a detector is around 1L.

The next step is to establish the “transparency plateau”. As mentioned in the previous part, the

transparency of the mesh to electrons (electrons have to pass through the mess to reach the

amplification gap) depends on the ratio between the amplification and the conversion field (or the ratio

between the drift voltage and the mesh voltage). Previous studies [23] have shown that the electronic

transparency of the mesh is maximal for a certain range of voltages: it is called the “transparency

plateau”. It also depends on MicroMegas characteristics such as mesh’s materials, geometry, etc.

To find this plateau, one fixes the mesh voltage (350V for example) and changes progressively the drift

voltage and thus the ratio. For all iterations, one measures the gain of the detector (calculated thanks to

the central position of the 5.9keV peak of the Fe‐source given by the MCA) and plots gain as function of

the ratio between electrical fields. This is called a “transparency curve”.

The transparency curve looks typically like this:

Figure 18: A typical transparency curve (Mesh voltage is fixed)

One can now distinguish 3 different parts, which correspond to 3 different states of operation:

‐ In zone 1, the electrical field in the conversion gap is not high enough and the electrons

recombine with the ions

‐ The zone 2 corresponds to the plateau. The ratio of the fields is optimal. The field lines (funnel

effect) lead most of electrons to the amplification gap and the anode

‐ In zone 3, the drift voltage is too high. The field lines are too straight and an increasing part of

the electrons are collected on the mesh instead of entering the amplification gap.

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Henceforth, since one knows where the plateau is, the perfect ratio can be computed.

.

Any and can be applied if they respect theratio. The gain measurements can thus start.

The best is to start with a relative low value of , compute the suitable value of respecting the

ratio, and do the same process again and again while increasing step by step. For all iterations,

one measures the gain via the MCA which can draw the spectrum of the 55Fe‐source.

The histogram is shown on the following figure [21]:

Figure 19: A typical histogram with Amptek MCA software.

Two peaks appear. The main one is the interesting one: it comes from the photon emitted by the 55Fe.

The weakest one is due to Argon escape peak. Indeed, this noble gas loses a photon from its K‐shell, by

fluorescence. The energy measured is the difference between the 55Fe photon energy (5.9keV) and the

binding energy of the electron in K‐shell (3.2keV).

The main peak has a Gaussian shape. The center of the gaussian is used to compute the detector gain.

The full width at half maximum (FWHM) of the peak gives us the energy resolution.

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II‐1‐5Resultsandconclusions

The first prototype received and tested was the number “2”. As said before, a mask was built to only

irradiate specific areas. Each zone corresponds to a specific standard strips’ width. Hole 1 is nearer to the

readout electronics than hole 3 as shown on figure [19].

Copper Strips


width : 400 for all

Resistive Strips


380 270 180 90

Table 2: Characteristics of Prototype 2

Figure 20: schematic of prototype 2

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a) TransparencyCurvesThe transparency curves for each zone and each hole are shown below.

Figure 21: Transparency curves of all zones and holes of Prototype 2

For the two top graphs, the plateau is wide and covers a large range. The detector, in these

configurations, can operate with many different voltages. One can also state that the behavior is the

same no matter where you are along the strips (holes 1, 2 or 3).

On the contrary, the curves for zone 3 and 4 are not typical. One can observe that the end of the two

curves is very similar to the end of the two previous ones. However, there is a gain peak for very low

ratios. There is thus no transparency plateau but just a peak. Moreover, an enigmatic raise of gain, like a

bump, appears for high ratios. This is something not appropriate for “standard” Micromegas and it was

not expected.

We can conclude that as soon as the resistive strips reach a too small width compared to the standard

ones, the behavior changes radically. Consequently, we have learned that the standard strips width must

be at least twice bigger than resistive ones if we want the detector to work as we expect. Nevertheless, it

does not mean that configurations in zone 3 and 4 are not acceptable: it can perhaps have some utilities

for future experiments and it helps us to better understand the subtleties of resistive micromegas.

The explanation of this effect is still under investigation but it might come from a difference of the

electrical field for the different configurations as we will see in the simulation in the last section.

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In addition to that, the four graphs tell us something more: the gain seems to be the lowest when the

source is near the readout electronics (hole 1) and the highest when the source is at the middle of a

strip (hole 3). The sagging of the mesh could be an explanation but pillars are here to avoid this effect.

b) Gaincurves

Figure 22: Gain curves of all zones of Prototype 2

The gain is computed as shown before via the MCA channel of the Fe‐peak. The purpose of this plot is to

compare gain of the different zones. The results are coherent with what we saw just before. Indeed, we

made the gain measurements for the setting so that the gain is maximal in the transparency curves

(either in the plateau for zone 1 and 2 or in the peak for 3 and 4). Consequently, we can once again

observe that gain for zone 3 and 4 is higher than for zone 1 and 2.

II‐1‐6Sparksproduction

Another task was to count and measure sparks to determine if resistive strips effectively eliminate (or at

least reduce) the sparks effect. The set‐up is almost the same as before. We just remove the Fe‐source

and put a 241Am‐source, which produces Alpha particles. These particles are much more ionizing and

would create more sparks.

The purpose of this test is also to displace the source over the mask and count the sparks in each case.

Then, we would be able to determine which configuration (width of standard and resistive strips) is the

best to protect the device from sparks.

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In order to count sparks, we measure the current provided by the power supply. Indeed, when a spark

occurs, the voltage in the mesh goes down and then the power supply has to compensate this drop. It

appears thus a peak in the power supply’s current and we can count sparks. One just has to monitor the

current delivered. The current is measured every second (sample time), and if there is a peak above a

certain threshold, it is considered to be a spark. The number of sparks detected depends on the

threshold selected. It is not a problem if the threshold is chosen to be the same for all cases since it is

just a relative count.

The results over all zones are shown below.

Figure 23: Sparks production by zone

One can see that the narrower the resistive strips are, the more intense, powerful and numerous the

sparks are. This is reasonable because the resistive protection is less efficient when resistive strips are

narrow compared to wider ones and because the field lines are concentrated on a small surface.

Tests are being continued for further understanding.

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II‐2AgeingtestsandbackgroundinATLAS

In order to understand and optimize the detector behavior as function of time, “ageing tests” are

conducted. The principle is to recreate conditions of long term operation in the real environment to

prove the capability of detectors made of resistive MicroMegas technology to operate in long data taking

periods. The CEA‐Saclay is in charge of all ageing tests on behalf of MAMMA collaboration.

In order to measure or detect the ageing of a detector, some observables are needed. Three different

ones are used:

The gain: one can compare the gain before and after an ageing test and analyze any differences

The current in the mesh: one can monitor and analyze this evolution of the current in the mesh

generated by the charged particles during the ageing test

The material: one can inspect the materials of a detector, with a microscope for example, and

search for any defaults or degradations.

In our case, one has to know what kinds of particles are produced in ATLAS and would reach the

detectors. Then, one must reproduce these conditions but with a very high rate (higher than in Atlas) to

accelerate the ageing and emulate many years of ageing in a short amount of time.

There are three major sources of radiation at the LHC: particle production at the interaction zone, local

beam losses and beam‐gas interactions. The total beam loss around the LHC’s ring should not exceed 107

protons per second. This is small compared to the proton‐proton collision rate: 109 per second per

collision point. Beam‐gas interactions are estimated to be 102m‐1s‐1 in the interaction area. Therefore,

the dominant radiation source will come from particles produced in the proton‐proton collisions.

A huge effort is dedicated to a strong and efficient shielding strategy. In case of muon detectors most of

the collision products are absorbed in calorimeters and in the shielding. However, it is not enough and

there are remaining neutrons, traveling long distances, losing their energy gradually and created new

particles feeding the background.

Photons are the results of nuclear capture of thermal neutrons. They are created via (n,γ) reactions. The

typical energy of these photons is 100 keV up to several MeV. Photons observed are usually produced in

the outermost centimeters of the materials (others are absorbed). These photons in turn, can produce

electrons and positrons, giving rise to most of the low‐energy electron background. Neutrons, and to a

smaller extent photons, are scattered many times before being captured giving rise to a relatively

uniform and isotropic “gas” of low‐energy background particles that is called “the uncorrelated

background’. The correlated background includes primary collision products, such as prompt muons

from heavy particle decays and hadronic debris of calorimeter showers

Consequently, we have to test the robustness of our detector with long exposure of the particles which

will be present in HL‐LHC muon spectrometer: neutrons and photons (of different energies)

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Three kinds of experiments are thus conducted:

‐ X ray radiation in order to simulate the total charge accumulated by the detector during long

time operation

‐ Neutron radiation to prove the invariability of the detector material properties and ageing due

to nuclear interactions

‐ Gamma radiation to visualize the robustness and ageing hardness

II‐3AgeingstudieswithX‐rays

These studies were done before I joined the group. I conducted myself some tests using the X‐rays and

will present them later on. However, the steps that have to be done to pursue a good test with X‐rays

will briefly be summarized.

The preliminary step is to characterize the X‐rays gun, knows its rate and the nature of the spectrum.

This task was successfully accomplished by Javier Galan, the post‐doc in the group. He showed that X‐

rays gun produces two main peak energies:

X‐ray gun peaks Energy Interaction rate

Low energy peak 3.51 keV 500 kHz/mm²

High energy peak 5.28 keV 1.5MHz/mm²

Table 3: Characteristics of the X‐ray gun

The X‐ray beam is mainly determined by 3 parameters: the cathode composition, the electron current

(mA) and the accelerating voltage (kV).

The tests were performed with two new prototypes detectors, called R17a and R17b. They are resistive o

but with a 2‐dimensinal readout (copper standard strips in X and Y directions, like in figure [26]). They

were manufactured at the CERN workshop by Rui De Oliveira and borrowed by the MAMMA

collaboration.

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Geometrical properties and materials

Strip pitch 250 µm

Strip width 150 µm

R‐strips thickness 35 µm

Insulator (coverlay) 60 µm

Y strips thickness (90° to R‐strips) 9 µm Cu

Insulator (FR4) 75 µm

X‐strips thickness (as R‐strips) 9 µm Cu

Resistance to GND 80‐140 MΩ for R17A 60‐100 MΩ for R17B

Resistance along strips 45‐50 MΩ/cm for R17A 35‐40 mΩ/cm for R17B

Table 4: Properties of R17a and R17b Micromegas detectors

Figure 24: scheme of 2D (X and Y readouts) Micromegas

This detector is able to detect a particle and read its associated signal in 2 dimensions, which leads to

much more precise results.

II‐2‐1X‐raystestsset‐up

We still use a very similar set‐up as for the characterization described before. The gas is still 90%Ar +

10%CO2. The flow is so that there is one renewal per hour (0.5 L.h‐1).

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Figure 25: X‐rays test set‐up

The control module of the X‐ray gun allows us to modify and monitor the electron current (mA) and the

accelerating voltage (kV). The X‐ray gun works only if the protection cage is safely closed, to avoid

irradiations.

Figure 26: Photo of detector R17a with its mask before irradiation

The detector is due to a mask only exposed over an area of 4cm2 in order to have a controlled exposed

region.

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II‐2‐1Testsandresults

I will first briefly present some tests performed before I joined the group. I later had to make others

experiments to explain the results previously found.

The detector R17a was exposed to the X‐rays gun. The total charge generated was of 765mC in 4cm²

during an exposure time of 11days and 21hours. It corresponds to the equivalent charge generated

during 5 years of HL‐LHC (with a factor 5 of security). All along the exposure, the mesh current due to the

ions collected on the mesh and directly related to the gain of the detector. If remains stable, it

means that the detector can resist and cope to the irradiation. On the contrary, if it goes down, it means

that the Micromegas endures damages and loses gain performances.

The first result was the following (figure [29]):

Figure 27: Evolution of the mesh current during the X‐rays exposure

Results are astonishing. One would expect a decrease or stagnation in current, but we got an increase.

The current rose by 30% which means that the detector gets a better behavior and a better charge

collection with respect to the time, which is impossible. Moreover, the gain of the detector was

measured before and after the exposure. One can also notice that there is no degradation but an

increase of the gain.

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Figure 28: Gain curves comparison before and after the irradiation

They thought a lot about this incoherence and the increasing gain effect remained unexplained. One

possible solution has been raised. There was no end cap (a connector) to ensure the connection of the

standard strips to the ground. It could also be attributed to a curing effect: charges could have been

trapped in the insulator or resistive layers and released during the operating period, improving the

electrical behavior of the detector, thus the gain and charge collection.

A new ageing session was conducted with three modifications: first, they added end‐caps to ground the

standard strips. Then, they connected the second detector R17b in parallel without being exposed. It

played a role of reference and was used to do gain control measurements to cross‐check the gain

changes coming from possible environmental or gas mixture effects. Finally, they changed the exposed

region of the detector.

Here are the second results:

Figure 29: Evolution of the mesh current during the second exposure

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The results are much more satisfying. One can see that the current is very stable. The variations are due

to climatic condition differences (pressure…) between days and nights. No degradation of the detector is

observed. This is very promising because it proves the ageing robustness of micromegas.

Another result proves the hardness of the detector: the gain comparison. Indeed, we compared the gain

of the detector before and after the exposure. Several gain measurements were performed on different

zones of the detector thanks to a mask (the process is more detailed in part II‐4‐4).

Figure 30: Gain comparison on irradiated detector R17a (left) and non‐irradiated R17b (right)

One can observe that there is no significant change of the gain on the exposed region, compared to

others. The trends are similar. The detector is consequently not much degraded and still works as before.

However, the increasing gain effect of the first ageing‐session has to be explained. I had to conduct some

new tests after the neutron exposure (see II‐4) in order to investigate the problem. The job was to do the

same tests again, with the same set‐up and of course the same detector R17a. The R17b was still

installed in parallel without being exposed.

Nevertheless, the X‐ray gun showed some instabilities. It stopped itself every two days because of a

water‐cooling problem. Moreover, I had to daily make a calibration with the Iron source, in order to see

how the gain of the R17a detector evolved to compare it with the R17b detector. It is believed that there

are some problems with the gun itself: the flux might not be constant. The exposure lasted 10 days but

none of the part of the results or graphs is conclusive. Date is thus not presented her. The next step will

be to recalibrate the gun and test it, in order to be sure that it is stable.

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II‐4AgeingstudieswithNeutronsThe purpose of this experiment is to measure the effect of a long neutron irradiation of the detector. It

will provide very useful information about the ageing of Micromegas.

II‐4‐1Set‐upandinstallation

The test took place at CEA‐Saclay. Indeed, the CEA has several nuclear reactors on the site of Saclay. The

one we used is called “ORPHEE”. It is a power plant delivering 15 MWh and is only dedicated to research.

All research teams at CEA can freely use this wonderful tool. The purpose of this reactor is to provide

neutrons beams.

Orphee was thus the perfect tool for the present study because one can irradiate our samples with a

high intensity thermal neutron beam. Several neutrons lines or guides are available. Here are the

characteristics of the guide used:

Neutron flux 8.108neutrons/cm²/s

Neutron energy 2.5 to 10 meV Table 5: Neutrons beam's specificities

Neutrons come directly from the heart of the reactor. They pass through an tank filled with helium and

are cooled down in it. They are “cold neutrons”, which means they are slower than thermal neutrons.

However, their velocity is about 2 times the speed of sound: 614m/s.

Figure 31: Detector's emplacement at Orphee reactor neutron guide

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The intensity of the beam is so that one hour of exposure is equivalent to 4.5 months in HL‐LHC.

Indeed, 10 years at HL‐LHC is around 5.108s (since HL‐LHC run 200days per year). The neutron flux at the

level of the potential place where Micromegas detectors would be installed is 15.104 neutrons cm‐2.s‐1. A

security factor of 3 was taken. So, the accumulated rate is of 8.1013 neutrons.cm‐2 .

At Orphee, the intensity of the beam is 8.108neutrons.cm‐2.s‐1. In 1 hour, we collect 3.1012neutrons.cm‐2,

which represents between 4 and 5 months of HL‐LHC!

The set up of the installation was not easy. We need to put the active area right in the beam. Technicians

built thus a lifting device to maintain the detector at the right place. An aluminum layer (few millimeters

thick) is placed behind the detector in order to reproduce the support and the shielding present in ATLAS

experiment. The beta particles produced from the decay of 28Al have energy up to 2.86 MeV.

Figure 32: Photo of the detector R17a in the neutron guide

The electronics set‐up is the same as for a characterization. The gas flow is 1L.h‐1 which corresponds to

one renewal per hour. We put a bubbler on the gas‐line to check if the gas is actually passing by. The gas

is finally rejecting in the ambient air near a hood.

It was decided to connect the second detector R17b in series out of the neutron guide to play a role of

reference.

Data are taken in the same way as the X‐rays exposure presented before. We brought a computer with a

MCA to be able to visualize the number of events with respect to the energy.

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II‐4‐2Calibrationandactivationofthedetectors

A calibration is needed to know the correlation between MCA channels and detected particles energies.

This calibration is done with the well‐known 55Fe source.

We first launched a short irradiation. It lasted only 10 minutes but it was enough to activate materials of

the detector. Components such as Copper or gold were activated by the neutrons. Particles emitted by

these new activated materials were detected by the MicroMegas and induced a huge background with

an order of magnitude around 2kHz.

The number of events was more important that the radioactive emission of the 55Fe source. Then, it was

impossible to calibrate the device.

Figure 33: Spectrum from the MCA after 5 minutes neutron exposure

After 10 hours, the noise was still above 500 Hz. The pre‐calibration was done before coming at Orphee.

The adopted strategy was the following: Irradiation of 1, 2, 5, 10 and 20 h long were successively

launched. In between, we had to wait for radioactive decays of materials in the Micromegas. During the

exposure time, some new materials can be activated. However, the activation rate one can measure

saturates and reaches a limit of 250 kHz which does not increase with exposures longer than 2 hours.

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Time Rate

11h08 206 kHz

14h04 35 kHz

16h03 26 kHz

18h16 21 kHz

8h52* 881 Hz

Figure 34: Spectrum from the MCA after 2 hours exposure

II‐4‐3Resultsandconclusions

During each irradiation, we monitored the current in the mesh. It is given by the power supply itself. The

current produced in the mesh is generated by both electrons and ions moving in the amplification gap. If

is stable, it means that the detector can resist and cope to the irradiation. On the contrary, if it

decreases, it means that the Micromegas endures damages.

Figure 35: Evolution of the mesh current during neutrons exposure

First, the total exposure time is 40 hours which is equivalent to 15 years of HL‐LHC.

The results for all irradiation periods show no degradation of the detector. The current remained stable

during all tests no matter how long they were. Results are thus very encouraging and make us believe

that we can be optimistic about the robustness of Micromegas.

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However, these results do not mean that all properties of detectors have managed to stay the same as

before. We needed to test and characterize the irradiated detector in our laboratory to check if it still

behaves as we expected and to prove the invariability of material properties.

Of course, the check‐up was planned before the irradiation tests and a pre‐characterization was made. It

means that we measured the gain of the detector in laboratory before. The purpose is then to compare

these results with new results that a post‐characterization will give. The investigation has however to be

done after a complete deactivation of the detector.

II‐4‐4Postcharacterization

I conducted a new characterization after the exposure on the detector R17a, the irradiated one. It is

mandatory to use exactly the same set‐up and the same electronics devices for the pre‐ and post‐

characterization.

For these tests we used a new mask with 9 holes. We are then able to compute the gain with respect to

the position on the detector. We moved the iron source over the mask and measured the gain.

Figure 36: photo of R17b, ready for a gain measurement

The pre‐characterization was performed on October 2011 and the post‐characterization of January 2012.

The detector was still activated a bit after the neutrons exposure, so the background was quite high but

low enough to see the iron peak.

As can be observed in the following figure, a comparison between gains before and after the irradiation

for each position was plotted. The gain is normalized with respect to the maximal one. We can see that

the two trends are very much the same and detector has the same behavior before and after the test.

Nevertheless, a degradation of the energy resolution is noticeable (order of magnitude: 25%). The

energy resolution is represented by the length of rods in blue or red. This effect is not explained yet but

the change is not dramatically big.

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Figure 37: Gains comparison before and after neutrons irradiation of detector R17a

The conclusion of this neutron radiation is then very positive since we did not notice any significant

degradation.

II‐5AgeingstudieswithGammaSources

Different ageing tests have been conducted here in Saclay. They all have shown a very acceptable

behavior of the detector. Its properties and its materials have the capacity to remain stable while being

hit by a high‐flux of particles. However, it is still needed to continue investigations with other prototypes

and other configurations. The research team in Saclay has also in mind to test the detector with Gamma

radiation.

There are some facilities in Saclay to expose Micromegas to very powerful gammas sources. For

example, a facility called COCASE owns a huge gammas source which emits with an enormous intensity

of 500 mGy.h‐1. This is a 60‐cobalt source which emits two gammas with 1.1 MeV and 1.7 MeV

We set‐up the experiment on March 22nd, i.e.one week before my internship ended up. So, I helped for

the installation and the beginning of the data taking but I was not involved in the analysis. At this

moment, results are not finalized but I can present them as preliminaries. The exposure lasted 18days in

order to acquire the equivalent of 10 years of HL‐LHC. The detector is fixed at 50 cm from the source in

order to control the flux of incoming particles. We proceed exactly the way as for neutron exposure. We

monitor the current in the mesh and analyze its evolution.

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Figure 38: Detector R17a, the Cobalt source and its shielding (in Orange)

The first results are quite promising. The current in the mesh is higher than for the X‐rays or neutrons

exposure (around 800nA) but the detector can easily handle such currents. Moreover, the current is

roughly stable. It is decreasing slowly but unfortunately it is too early to conclude or try to find an

explanation. A zoom of the curve shows small oscillations. They are probably a consequence of a change

of climatic conditions (pressure) during nights and days.

Figure 39: Evolution of the mesh current in COCASE for the first four days (bottom: zoom of the top)

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II‐6Conclusion

All the ageing tests conducted in Saclay were very promising and have proved that a Micromegas

detector with resistive technology can handle a very hostile environment. We have tested the

robustness with X‐rays, cold neutrons and gammas. Nevertheless, the ageing studies are just at the

beginning. Many other detectors and prototypes have to be tested. One also has to test the ageing

properties of large size detectors. Moreover, it is still needed to conduct new tests with new particles of

the background such as thermal or fast neutrons.

We have only tested two out of the three observables in ageing tests (the gain comparison and the mesh

current). One still has to inspect the deterioration of the detector’s components. Some damages may

have been created and caused by the irradiation. The plan is to come back soon to Orphée and the

neutron guide, put a “nude” Micromegas detector (without connectors, electrical devices or monitoring)

and strongly irradiate it. Afterwards, one will look at it very precisely with powerful microscope. This job

has also to be done with large detectors.

In conclusion, a lot of works is required in terms of ageing or R&D, but they are very stimulating because

if they are concluding Micromegas will fulfill the milestone and will be accepted in the New‐Small‐Whell

of ATLAS!

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IIIAlignmentandAssemblyProcessoftheMicromegasSmall‐wheel

III‐1Astrongneedofprecision

As explained in the first part, muons provide one of the best signatures of physics at LHC. That is

why an impressive system has been implemented on ATLAS: the muon spectrometer. It consists of

different kinds of detectors. The basic principle is to have 3 wheels of detectors (one in the barrel and

two as end‐caps). The muon, a charged particle, undergoes a deflection in the magnetic field produced

by the toroidal magnets. Muons then traverse three stations of chambers and we are able to reconstruct

their tracks and compute their momentum.

Figure 40: Current muon spectrometer's scheme. In blue, the three wheels of MDT chambers.

The whole device is designed to have an amazing resolution of∆ / 10% for pT’s up to 1 TeV. This

implies several consequences in term of precision, alignments, electronics, triggering system, etc.

In this chapter, the proposition made by the MAMMA collaboration to build a small‐wheel filled by

Micromegas detector is described. A part of my task during my internship was to propose of a way to

build and assemble detectors chambers of the Small‐wheel and find out how to keep precision and

alignment in‐between detectors. CEA‐Saclay is involved in all this questions on behalf of the MAMMA

collaboration.

The reconstruction of particle tracks requires the knowledge of the position of the detector chambers

with accuracy better than 30 µm (in addition to a high spatial resolution of the chambers). To reach such

a precision, one needs to precisely build each part of the detector, precisely know where the detecting

components are (strips of copper for example) inside the detector, and precisely know the position of

the detector with respect to all the others. This is achieved by a system of optical alignment monitoring

sensors.

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III‐2MechanicalmeasurementoftheNewSmallWheel

III‐2‐1Detectorcomponents

The mechanical precision of a few MicroMegas detectors was measuring using a 3‐dimensional

microscope.

The specifications claim that the precision of the position of strips in such a detector must be around

30µm. This means that the distance between the strips, the parallelism and the straightness have to be

kept within a few microns.

The PCB and copper strips, which is the base of a Micromegas detector, was measured. The total size of

the active zone can reach 10cm, so the number of strips can reach 200 or more. It is necessary to use a

3D‐microscope to measure these distances and detect faults along the axis perpendicular to the sample.

Figure 41: Inspection of a Micromegas with a 3D‐microscope

The 3D‐microscope used was a Mitutoyo model Quick Vision Ace. It works conjointly with a software

which is unfortunately not optimal for the needs here. It is almost impossible to write a code to

automate a measurement process. Consequently, 3 zones on the sample was chosen, and the results

extrapolated the results to the rest of the detector.

a) Widthandpitch:Specifications state that width is 380µm and pitch is 500µm. The results are shown below:

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Figure 42: Geometrical properties of the Micromegas detector

b) Straightness

The strip edge can be measured detected thanks to a dedicated function. The microscope can detect a

change in the intensity or luminosity on the sample and consider it as an edge. It can then select some

points on the edge, memorize values in a buffer, create a line and measure the straightness. However, it

is necessary to preselect a zone where the “edge detection” can work which complicates the process. I

took around 1000 points to create a complete line and the mean value for the straightness over the

detector was 16 µm.

c) ParallelismThere are several ways to check the parallelism of strips.

First, the function “parallelism” of the software gives back the diameter of a cylinder comprising the

selected line and having for axis another line selected as reference. The mean value found was 16 µm.

Nevertheless, one cannot rely on this method: it was seen to return a measured deviation from

parallelism of 17 µm between a line and itself!

Preferably it was performed “by hand”, measuring the angle between two lines belonging to two strips

with the microscope, and computing from that value the parallelism. On average, two strips form an

angle of 0.000036° between then, which corresponds to 4µm at the end of the strips

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d) Conclusion: The error on the strips dimensions are within 3μ .

The deviation from straightness is within 16μ .

The strips are parallel within 16μ according to the software and 4μ according to the

measurements.

Differences between requirements and existing detectors are, although the values are small, too large.

Indeed, the precision on the copper strips is raises a major issue.

e) MeasurementresolutionThe values given are only statistical data because it is way too complex to automate measurements.

Moreover, there are several sources of inaccuracy: the cursor (or pointer) is 2 µm big, the ambient

temperature is 25°C which can dilate the PCB, the “edge detection” function is not precise enough, and

the difficulty to create a perfect reference was associated to the Micromegas. The flatness of the sample

is not perfect and can cause some shifts…

For all these reasons, a more accurate and evolved microscope was searched for. A more advanced 3D

one is installed in the clean room. The microscope works under a laminar flow where the air is filtered,

comes from the top and flows down to not move dusts potentially present. Temperature is also

monitored. These conditions are more suitable for precise measurements. Though, Voyager5, the

software used to control the microscope, is an antiquity and its documentation disappeared long time

ago and had to be aborted after several attempts.

III‐2‐2Chamberprecision

As a reminder, a Micromegas detector is composed by a PCB (Print Circuit Board: a few mm thick) where

standard strips, coverlay and resistive strips are placed. Then, pillar, mesh, cathode and gas box are

mounted.

The Micromegas chamber that will be installed in the New‐Small‐Wheel is composed of two multilayers

separated and supported by a spacer structure. A multilayer is composed of four detectors: two times

two detectors back‐to‐back separated by an aluminum layer.

In order to distinguish, detect and reject out‐of‐time tracks, two micromegas are mounted back‐to‐back.

In such a way, out‐of‐time tracks will not be collinear in the two neighboring planes. The Micromegas

detectors are identical. The plan is to assemble two sets of back‐to‐back detectors together. In such a

way, a particle will pass through four active layers which will maximize the detection power and

minimize errors. This arrangement is called “a multilayer”.

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Figure 43: Scheme of a Micromegas multilayer

Finally, two multilayers are joined to form a Micromegas Chamber. They are separated by a spacer

structure connected to the Small‐Wheel (SW). The readout electronics is placed along the sides of the

chambers. The full layout will be described later on.

Figure 44: schematic view of a Micromegas chamber

III‐2‐3TheNew‐Small‐Wheel

The MicroMegas chambers have a trapezoidal shape in order to better fill the wheel. It is always a good

idea to copy the nature: the wheel will look like a flower and the petals will be the chambers. The layout

of the chambers follows what already exists, with large and small chambers and an overlap between

them. A preliminary integration study and a possible layout are shown here:

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Face

Back

Figure 45: Scheme of the New‐Small‐Wheel (from Saclay's engineering department)

These designs and drawings have been made by the Engineering Department in CEA‐Saclay. They are

studying the best structure to support all chambers and avoid dead‐zones.

First, let us investigate the segmentation. There are two sectors: the large one and the small one. One

sector (one petal) is composed of 4 trapezoidal chambers which have different sizes. The overall number

of chambers is 128. As mentioned before, each chamber has 8 detection layers.

Figure 46: Typical dimensions and lengths of micromegas chambers

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Because of the electronics, the chamber housing and the Small‐wheel structure itself, the New‐Small‐

Wheel will have many dead spaces between chambers; this problem needs special attention. The current

idea to manage dead zones is to build the chambers such that neighboring chambers overlap radially

with at least two active gaps. In such a way, in the worst case, a particle would hit a minimum of two

MicroMegas per multilayer. The overlapping is shown here:

Figure 47: Layout of the chambers to avoid dead‐zones and detect all particles

One can see that there is a small offset in the radial direction between two multilayers of a chamber so

that the active areas overlap. This solution solves efficiently the problem but has some consequences:

the price to pay for such an arrangement is a more complex mechanical structure and a more complex

installation procedure.

III‐3AlignmentandAssemblyprocess

All the experimental tests introduced in the first part of this thesis were performed on small prototypes

(typically 10 cm*10 cm). Nevertheless, the current chosen solution requires being able to produce large

MicroMegas detectors, at least 1 m*1 m. It thus raises a major issue: since one needs to produce

detectors with accuracy around 30 µm, it will be even more difficult to design large‐size detectors with

such constraints. Another difficulty will be to perfectly align all the detectors (or layers) together, in a

same multilayer and in the same chamber.

That is why a whole assembly and fabrication process study is mandatory. It should stipulate how step‐

by‐step to pay attention step by step to the alignment and precision of pieces.

III‐3‐1Preliminaries

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The precision and the alignment must be a priority at every level. First, by construction, copper strips

positions are defined with respect to each other since they are on the same PCB.

The strips of the two detectors which are back‐to‐back are on a single structure. The copper of the strips,

depending on its thickness, may create internal stresses and constrains in the PCB, which can curve

under the pressure. To annihilate it, the structure must undergo a specific cycle, called annealing cycle:

repeated extreme warming in an oven and fast cooling, on and on. This process is well‐controlled and

can be automated in a laboratory at CERN.

However, an alternative solution has been proposed [24]: one can use a stiff foam (ROHACELL type) and

glue two layers of Aluminum on each side (one can even glue a whole aluminum frame around the foam

to strengthen the structure). This structure should behave like a thick layer of aluminum, but with a

lower mass. Indeed, the foam ROHACELL does not stress the aluminum.

Then, a Micromegas detector can be constructed on each side of this structure: two PCBs are glued on

each side of the structure, Copper strips are added and the whole bulk technology [25] is implemented.

The aluminum layer has to be thick enough (3 mm) to not be affected by the BULK and constrains that it

could create on.

In this way, the precision of the whole device will be dictated by the mechanical properties of the

aluminum and not anymore by the composite material of the detector itself.

The purpose is of course to remove as much as possible internal mechanical stresses to avoid any

displacement and misalignment. We have also to control the temperature and the humidity over all the

procedure to prevent any dilation.

III‐3‐2MultilayerAssemblyandalignment

One can design very precise patterns on the PCB, and perfectly know its position with respect to the

strips. The pattern might be a target of 4mm but more likely a central point of 100µm with concentric

circles around, in order to find it more efficiently.

The aim is then to drill several holes in the PCB. Holes have to be round and oblong in order to let one

degree of freedom (dilation…). Other targets have to be drawn as references, to know positions of all

holes.

The drilling must be very precise and performed by an optical drill. The resolution of the camera can’t be

higher than 10 µm. A perfect drilling on such a long sample (around 1m) is impossible. A precision of 30

µm only can reasonably be reached. The drilling must take place in a reinforced PCB’s zone, certainly

with a layer of aluminum (thickness to be determined). One can then drill all the PCB of a multilayer, and

align them together.

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Figure 48: Schematic view of a PCB and copper strips

The overall idea is then to slide pins into each of the holes. The rods are fixed to a granite table. One can

thus put all the layers of a multilayer and align them perfectly. Limits are mechanical. The length of the

rod cannot be too long to avoid a bending. Depending on the critical length, one can align parts of

varying sizes.

Figure 49: Scheme of the potential assembly device

The assembly process of two full multilayers and a spacer in‐between is still under investigation. One

solution can be similar to what been used to mount MDT chambers in ATLAS [26].

This solution can be a base but some questions are pending: How long does it take ? How can we adapt the machines to MicroMegas? Etc.

III‐3‐3Qualityandcontrol

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a) ControlduringtheprocessFor the Micromegas technique, the position of each (full) layer depends only on 3 parameters: the two

displacements and the rotation of the plane. A solution to control the quality of the process can use a

camera‐control on granite.

One can indeed print a mask on the PCB itself. It may enable to control the alignment and positions via

an optical system (a camera, mounted on another part of the structure, can look at this mask and

determine the accuracy of the position).

The PCB can be built with a mechanical extension, where we can design a mask. Thus, one will be able to

perform an optical control via a camera mounted on a sliding base linked with the granite table via a

system of ruler and rail.

Figure 50: Schematic view of a possible device from optical control during the assembly

This is also preliminary and we have to think to more precise systems and how to build them.

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b) InternalControlIn the present Atlas small wheel currently exists an internal control system [27]. This is a complex system

made with optical cameras and devices. A very interesting challenge will be to adapt this system to the

new wheel, or rather how to adjust the new wheel and its configurations to change the current system

as little as possible.

This system has been designed and created by Jim Bensinger and Christoph Amelung from Brandeis

University in the US and will have to be upgraded by them. The NSW will be equipped with the new

optical alignment systems and sensors and be integrated with the remainder of the existing system.

First, the Atlas muon alignment system is designed as an absolute system: it means that they use optical

sensors measurements at time t to determine actual chamber positions at time t.

The optical system uses CCD cameras and masks. The basic principle is to see the mask with the camera

through a lens, as shown in figure [55].

Figure 51: Scheme of the optical control system with lenses, masks and cameras

As it is a very precise absolute system, the sensors positions have to be well‐known. Alignment sensors

are mounted on the structure via a “sensor mount” composed by three different kinds of holes to obtain

a well‐defined and reproducible positioning.

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Figure 52: Possibility to design slots for a camera and masks on the PCB

The system uses alignment bars. The New Small Wheel will need eight bars full of sensors. Sensors on

chambers link chambers to bars and sensors on bars link to other bars. The following figure 55 shows the

actual small wheel and the potential future positions of alignment bars.

Figure 53: The current Small‐wheel with eight alignment bars full of optical devices

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Finally, the new system will be very like the current one which is modeled in the following scheme.

Figure 54: The full optical control system

One important quality and control system is the “In‐plane” one. In the chamber itself, it can detect any

bending, movement or sag.

Figure 55: The In‐plane system mounted on a chamber

Finally, once the full wheel is assembled in Atlas, the straight tracks, which occur when magnets are off,

can calibrate the whole device.

Of course, all this work is going on. There are pending questions and all steps are not fully finalized. Even

though the overall idea is here, such a set up deserves much more preparation and detailed processes.

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III‐4MechanicalanalysisofMicromegaschamber

An alignment system and a precise construction are needed. We have also to know how the structure

will behave with respect to the time. All chambers are heavy and long (cf table [6]). They will undergo

huge strains and therefore move a bit. That is the reason why the engineering department made a

complete mechanical analysis of a micromegas chamber. I did not do this analysis myself but I had to

present the results to the MAMMA collaboration during the weekly meeting. I will thus summarize

important conclusions.

This study was conducted by P.Graffin and supervised by P.Ponsot with ANSYS software using a finite‐

element model. The purpose is to visualize the behavior of Micromegas chambers under different

conditions.

III‐4‐1ThemodelsThey built 4 different models. The first two ones are just composed of one multilayer. Model 3 and

Model 4 are two multilayers coming from model 1 and 2 linked through an aluminum frame.

Figure 56: Models designed for the study

Only model 4, which is the more realistic one, is focused on here. It is formed with two multilayers from

the model 2. As in the New Small Wheel, a multilayer is composed by two detectors, or more precisely

by two times two detectors back‐to‐back. In the following zoom, one can see that a detector is

composed by a base, the blue FR4 epoxy layer and a grey aluminum layer which is placed above and is

just an empty frame (gas is supposed to be inside).Let us consider that detectors are 2D‐ones (strips

along y and z axis)

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Figure 57: Zoom on a multilayer from model 2

Some simplifications had to be made to create the model. First, the glue is not modeled. Then, the

mechanical bounds are considered as perfect. Finally, there are some simplifications of computations

which give birth to an optimistic estimation, depending on the glue. All the characteristics of the model

for are shown here:

Table 6: Specifications of the model

III‐4‐2Firststudy:displacementsunderthermalconditions

In order to model the real conditions, the chamber will first undergo strong thermal conditions. One side

is at the initial temperature (22°C). Then, the temperature of the other side is increased by 10°C. Such a

strong gradient can be caused by all electronical devices that are around active zones of detectors.

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Figure 58: Thermal conditions applied on model 4

Consequently, it results in some displacements of the structure. Due to the temperature, the structure

can dilate or contract and the chamber curve. The results are presented in maps of the displacement

module. For the model 4, the maximal flexion of the chamber is 0.6mm. Moreover, the displacement is

strongly inhomogeneous, due to the trapezoidal shape. For such a structure and such conditions, this is

very a very reasonable behavior.

Figure 59: Displacement module (in millimeters)

Nevertheless, this bending produces a longitudinal distortion. It means that the layers slide with respect

to each other in the y‐direction. It creates an offset between their positions and a misalignment of the

transverse strips (along z‐axis). Indeed, strips that are designed on these layers are no longer the one

above the other because, since the layer dilates, the pitch between two strips gets bigger. This distortion

is embedded between 0.01 and 0.3mm.

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Figure 60: Longitudinal distortion (along the y‐axis) causing a misalignment of strips along z‐axis

The same distortion appears in other directions. The maximal displacement is summarized in the table:

Module Ortho x Transversal z Longitudinal y

0,59 0,58 ‐0,14 ± 0,28

Table 7: Maximal displacement upon axis in millimeters

This is a significant problem that has to be tackled. A solution may be to put pins around the structure

which will be able to recover this distortion. In that way, the structure will be strengthened and layers

will be fixed.

Results can also be seen in terms of mechanical stress. The following drawing shows the stresses on the

aluminum frame only. This is where the strongest constraints are applied.

Figure 61: Von Mises stresses over the frame (in MPa)

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The frame undergoes most of the mechanical stresses. Anyways, all stresses over the structure are

inferior to the tensile strength, which is the value where the structure breaks. The tensile strength is

around 30MPa for FR4 material.

Alu layers Alu frame FR4 layers

6,8 11,2 1,13

Table 8: Von Mises maximal stress over the different components (in MPa)

III‐4‐3Secondstudy:displacementunderitsownweight

This analysis is only conducted for the model 1: a unique multilayer. This study deserves attention

because its conditions can be found during the construction phase. The multilayer is just supported along

its two sides (in B and C red zones on figure) and undergoes its own weight.

Figure 62: Conditions for study 2

Then, we visualize the map of displacements of the structure. It is very stiff and rigid. The maximal

displacement reaches 0.5 millimeters, which is very few for a sample of 324kg.

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Figure 63: Displacement module of model 1 for study 2

III‐4‐4Conclusion

Results are very encouraging. Indeed, the gradient of temperature of the study 1 is highly pessimistic and

such thermal conditions will not be found in ATLAS. However, the reaction of the chamber is very

positive. According to its weight and its dimension, we can think that we will be able to find technical

solutions to recover displacements and distortions over the sample. Moreover, in the study 2, the

multilayer is horizontal. But chambers will be put vertically in the Small‐wheel and it seems obvious that

chambers will be less deformed in this case.

Thus, studies and conditions are very pessimistic but gave birth to quite good results, which allow us to

be more confident.

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IV3D‐Micromegasbehavior’ssimulationwithLorentz‐3D

All along this report, some experimental tests are introduced, aiming to better understand the overall

behavior of a micromegas detector and effects of the modifications of some characteristics such as

standard strips widths.

All the conclusion or clues of explanations we found experimentally have to be checked or proven by the

simulation. There exist several powerful and efficient softwares capable to model the electrical field in a

detector. In the present work, Lorentz‐3D was used. It is possibly not the most appropriate and powerful

tool, but it is really easy to use and fast to master..

IV‐1Thesoftware

LORENTZ [28] is a software developed by INTEGRATED Engineering Software. It is a computer program

for simulating the effects of electric and magnetic fields on the motion of particles. Lorentz is designed to

analyse specific problems inherent in charged particle trajectory applications and allows us to design and

inspect different architectures of detectors. Furthermore, It allows to simulate virtual prototypes on the

computer and thus provides far greater insight into design optimization and verification.

Figure 64: Logo of Lorentz

To perform an analysis with Lorentz, you have basically to follow three steps. First, you must create a

physical model; then use a field solver and obtain a solution; finally, analyze electrical fields or charged

particle motion in the fields.

There are two kinds of Lorentz sofware : the 2 and the 3 dimensions. For the first work, the 3D version

which was the only one available at CEA was used. The micromegas design turned out to be too detailed

and complex for this software.

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IV‐23D‐models

A whole micromegas detector (X and Y strips, coverlay, resistive strips, mesh and cathode) was modeled

in 3 dimensions.

Figure 65: Schematic views of Micromegas in 3D with LORENTZ

The computation time was way too long and results not reasonable, mainly because of the complexity of

such a design. It will be irrelevant to present them in this report.

Therefore, the 2dimensional version was purchased and conclusive results were obtained.

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IV‐32D‐models

To create a complex design is easier in 2D. All the characteristics of the model used in this work is

described below.

The geometry MicroMegas‐like is close to reality. There are only 3 standard strips and 3 resistive strips

above them. This is however enough to visualize the global behavior of the detector if one chooses the

good boundary conditions.

Figure 66: Geometrical properties of the 2D model

The materials of the components were selected so that the model has the same properties as

MicroMegas. The gas is substituted by air (default of the software).

Figure 67: Materials used for 2D model

It is imperative to carefully chose the appropriate boundary conditions. The voltages of drift and mesh

had to be computed in order to respect the conditions of operation. A tricky detail is to not forget to put

conditions on the box‐frame. There are two solutions: either one builds a sample and one apply

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symmetry and periodicity conditions on it, or one builds a part of the detector and closes it in a box with

appropriate boundary conditions.

Figure 68: Boundaries conditions and voltages of 2D model

IV‐4Resultsandcomparisons

Several different models were created in order to understand how the software works and to visualize

the behavior of different prototypes. The study on the 3 prototypes with different resistive strips width

received from CERN and which were characterized. It was seen that the gain of the detectors changes a

lot depending on the resistive strips width. Thus, the 3 different micromegas with 3 different widths

were modeled in order to explain the differences between gains and to see how the electrical field

evolves.

For each model, the streamlines and the equipotential lines are plotted.

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IV‐4‐1Streamlines

As you will see in the next figures, the streamlines follow a particular scheme: Due to the well‐chosen

ratio between the conversion and amplification gaps, a funnel effect can be observed. Electrons follow

these lines and are then collected efficiently on the strips.

a) Samewidthofresistiveandstandardstrips

Figure 69: Streamlines for Micromegas detector. Resistive width: 300µm; Standard strips width: 300µm. (Top: the whole, Bottom: zoom on the central strip, Left: Scale of E‐field)

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b) Smallerwidthofresistivestrips

Figure 70: Streamlines for Micromegas detector. Resistive width: 200µm; Standard strips width: 300µm.

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c) Smallestwidthofresistivestrips

Figure 71: Streamlines for Micromegas detector. Resistive width: 100µm; Standard strips width: 300µm.

d) ConclusionsOne can observe that the larger the resistive strips width is, the higher the maximum of the E‐field is.

Indeed, the field near the strips is the highest. The maximum reaches 9.3*106 for 300µm width, 8*106 for

200µm and only 7.2*106 V.m‐1 for 100µm. The E‐field is thus stronger for the first case. We can however

suspect a “peak effect” on the sharpe corners of strips.

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Moreover, large strips catch more field‐lines than small ones. More lines are of course led to the

insulator layers in case 3 than 1 and 2.

Consequently, the size and the shape of resistive strips have an effect on the streamlines and on the

behavior of the electrical fields. However, the relation with the previous results about the

characterizations of prototype 2 is not obvious at all. We can nevertheless assume that the differences of

E‐fields can be the cause of the differences of gains.

IV‐4‐2Equipotential

Figure 72: Equipotential lines near the strips (only from 0V to ‐500V) for the three configurations

It is hard to draw a conclusion from these graphs, but it seems that the lines are more condensed and

more numerous in the case of wide resistive strips, which is coherent with the first results.

Unfortunately, these simulations are not complete enough to find an appropriate and detailed

explanation to the effect seen in the characterizations. However, the models and the first analysis

presented here may be a good starting point for further studies.

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CONCLUSION

Micromegas detectors will be in ATLAS by a few years and the team will have the opportunity to be even

more involved in one the biggest and most exciting experiment ever imagined.

However, many studies are still needed and necessary to prove that Micromegas can operate efficiently

in ATLAS for 10 years. The muon decisional committee has created some milestones that the MAMMA

collaboration has to fulfill.

All the laboratories involved will have to work together to perform more advanced ageing tests to prove

the capability of hardness and robustness. Moreover, a industrialization process has to be found: one

needs to find out how to construct and assemble with precision a large amount of large chambers. By

the end of the year, a scale‐1 prototype is supposed to be assembled and tested. Finally, there is still a

strong need of advanced simulation to model the behavior of a MicroMegas and to understand all the

particularities.

It is a great feeling the HL‐LHC team to know that all the work that was done on the development of the

Micromegas technology was appreciated and rewarded:

All along this internship, I have had the possibility and the opportunity to deal with almost all the aspects

of an international physics project.

First, I was involved in R&D studies. I had to set up many experiments, to learn how to use specific

electrical devices, to analyze data and present them. No characterization like the ones we did, were

performed before and it was a great experience to bring a contribution to detectors science.

Then, I helped to conduct ageing tests in many different environments such as a nuclear reactor, rooms

with X‐rays gun or very powerful radioactive sources. We had to comply with many security and safety

rules. We also had to tackle several technical problems and try to find out how to solve them efficiently

and quickly in order to not waste our time.

Furthermore, I worked on a real engineer’s problem. To figure out how to build, assemble and align 128

detectors chambers on a 10meters‐diameter wheel is very challenging. This kind of tasks cannot be

fulfilled alone and you need to discuss and meet many people with many different specialties: designers,

IT‐ engineers, mechanics, physicists… and it is a good workout.

Finally, I had to complete the whole scientific process: Anticipate results, test them experimentally, and

prove them with simulations. My time was running out when I started this last but not least phase of

simulation but it helped me to understand the importance and the utility of such a work.

On a personal point of view, I have learned a lot with this trainee and not about physics only. For

example, I had to present some results to the MAMMA collaboration or to my research team. I also

made a review of all the works I have done to the whole detectors unit.

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I attended personally to some organization meeting of MAMMA and had the possibility to see how

twenty laboratories achieve to work together (or not!), and what kinds of troubles you have to face in

such a huge international collaboration.

Finally, I have spent six months with very talented, patient, passionate and open‐minded physicists

which it was very easy to discuss with. It was the perfect occasion to think about my future carrier and

evaluate the pros and cons of being a physicist or scientist in this time of crisis.

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References

[1] LHC web page http://lhc.web.cern.ch/lhc/

[2] CERN web page http://public.web.cern.ch/public/

[3] Higgs boson’s website http://press.web.cern.ch/press/pressreleases/releases2011/pr25.11e.html

[4] ATLAS, CMS, ALICE, http://lhc.web.cern.ch/lhc/LHC_Experiments.htm

[5] ATLAS web page http://atlas.web.cern.ch/Atlas/Collaboration/

[6] A.Engl et al., ATLAS monitored drift tube chambers for super‐LHC, Nucl.Instrum.Meth. in Physics

Research A623:91‐93 (2010)

[7] T.Argyropoulos et al., Cathode Strip Chambers in ATLAS: Installation, Commissioning and in

Situ Performance , Nuclear Science, IEEE Transactions on, 56 Issue: 3 (2009)

[8] Giordano Cattani (On behalf of the RPC group), The Resistive Plate Chambers of the ATLAS

experiment: performance studies, J. Phys.: Conf. Ser. 280 012001 (2011)

[9] Koichi Naga, Thin gap chambers in ATLAS, Nucl. Instrum. and Meth. A384 Issue 1 (1996)

[10] L. Shekhtman ,Micro‐pattern gaseous detectors, Nucl. Instrum. Meth A 494 (2002)

[11] I. Giomataris et al., MICROMEGAS: a high‐granularity position‐sensitive gaseous detector for high

particle‐flux environments, Nucl. Instrum. Meth. A 376 (1996)

[12] A. Oed, Position sensitive detector with microstrip anode for electron multiplication with gases,

Nucl.Instrum. Meth. A 263 (1988)

[13] G. Charpak, Proc. Int. Symp. Nuclear Electronics (Versailles 10‐13 Sept 1968).

[14] S. Ramo, Proc. IRE 27 (1939) 584

[15] The COMPASS Collaboration, COMPASS: A proposal for a COmmon Muon and Proton Apparatus for

Structure and Spectroscopy CERN/SPSLC 96‐14, SPSC/P 297(1996)

[16] D. Thers et al., Micromegas as a large microstrip detector for the COMPASS experiment, Nucl. Instr.

and Meth. A 469 (2001) 133

[17] J Beucher, Large bulk‐MICROMEGAS as amplification device for the T2K time projection chambers,

Nuclear Science Symposium Conference Record, (2008)

[18] A. Bay et. al. Study of sparking in MicrOMEGAs chambers, Nucl. Instrum. Meth. A488 (2002) 162

[19] H. Raether Electron avalanches and breakdown in gases, Butterworth, Washington U.S.A (1964)

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[20] F.Jeanneau et al., Micromegas study for the sLHC environment, JINST 5 P02003 (2010)

[21] M.S. Dixit and A. Rankin Simulating the charge dispersion phenomena in micro pattern gas detectors

with a resistive anode, Nucl. Instrum. Meth. A566 (2006) 281

[22] J.Burnens et al., A spark‐resistant bulk‐micromegas chamber for high‐rate applications, CERN–PH–

EP–2010–061 (2010)

[23] F.J. Iguaz et al., Characterization of microbulk detectors in argon‐ and neon‐based mixtures, JINST 7

P04007 (2012)

[24]I. Giomataris et al., Micromegas in a bulk, Nucl. Instr. and Meth. A 560 (2006) 405.

[25] Private communication with Rui De Oliveira , Philippe Schune, Florian Bauer, Sebastien Herlant and

Guillaume CAUVIN, CERN (December 2011)

[26] Atlas Collaboration, Letter of Intent for the Phase‐I Upgrade of the Atlas Experiment version 35,

CERN‐LHC 2011‐012 (2011) 26‐37

[27] C.Amelung et al., The Optical alignment system of the Atlas Muon Spectrometer Endcaps, ATL‐

MUON‐PUB‐2008‐003, Atlas Muon note, revised version (2008)

[28] Lorentz software web page: http://www.integratedsoft.com/products/lorentz

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APPENDIXTwo alternative proposals for the New Small Wheel Upgrade

Two proposals presented in this document. They are both using a single trigger system over the whole

New Small Wheel surface based on the TGC technology.

Such a trigger system can identify efficiently track segments coming from the IP and provide a muon

trigger with a sharp pt threshold. An independent detector(s) provides the tracking capability required.

This detector may also be used for a complimentary trigger system. In parallel sTGC may as well improve

the track measurement accuracy. In both cases MicroMegas technology is used as a part of the system

due to its intrinsic background rate tolerance. This technology seems very promising and relevant

applications within ATLAS should be promoted.

Advantages of this approach

Separate trigger – tracking chambers (at least initially). Inherent redundancy

The trigger system may achieve design performance form the first day of operation

Construction of sTGCs of the required size may start very soon

Simple sTGC trigger scheme

sTGCs have adequate time resolution to provide BC ID

sTGC can be used to complement the tracking chambers, providing with points with 100 μm precision

sTGC construction ensures the formation of new experts. They may eventually contribute to the maintenance of the full detector. Such experts may not be available otherwise

Existing expertise and infrastructure can be re‐used for the sTGC construction

Common sTGC and MicroMegas front‐end electronics development and production process

A single common ROD platform can and should be developed for all detectors (provided identical readout links are used)

Issues to be resolved

High precision tracking resolution still to be demonstrated for a large surface sTGC and MicroMegas detector

The MicroMegas alignment (both internal and global) has to be studied

sTGC alignment (both internal and global) has to be studied

Offline reconstruction of MicroMegas tracks has to be developed

Limited number of Institutes with required expertise for sTGC

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Homogeneous Solution (sTGC + MicroMegas)

A detector designed for full redundancy in both trigger and tracking functions. Take advantage of the

synergies between the two detectors for a completely integrated New Small Wheel System.

Advantages

Full redundancy. Each detector can fulfill both trigger and tracking requirements. May be considered as a second stage upgrade of the system

Adequate operation in high background environments. May easily cope with unexpected background conditions at the highest luminosity foreseen

Optimal space‐point resolution and track separation

Simpler service distribution for MicroMegas (fiber bundles directly from detector to ROD)

Solution with the lowest cost of the two (development, alignment, industrialization etc costs are independent of number of detectors)

Potential to engage community on a large scale (due to number/size of detectors). Concentrate efforts, resources to only two technologies

Develop innovative technology(ies) for big size detector to be used eventually elsewhere in ATLAS as well (for example further muon spectrometer upgrades)

Disadvantages

Extensive R/D required to prove feasibility of large size MicroMegas chambers

No experience of large scale production with all quality control constrains implemented

MicroTPC operation performance has to be demonstrated. Therefore Space point accuracy for inclined tracks and final detector granularity needs still to be studied and defined

Between the two solutions proposed, this has the highest number of channels and power consumption

Different pattern recognition and tracking algorithm as a function of track incident angle (centroid vs microTPC).

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Split Solution (sTGC + sMDT/MicroMegas)

A large part of the detector is based on established technologies. The transition radius between the two

tracking technologies may be defined once the exact performance of the detectors is known. A large

safety factor will have to be taken into account due to background uncertainties. sMDT to be used

where the rate is appropriate, maintaining both efficiency and resolution required. Inner ring to be

equipped with MicroMegas.

Advantages

sMDT has well established performance and is very robust for tracking except maybe at regions of the highest background rates/occupancy

sMDT production can start immediately, no R&D is required. Fail‐safe solution for tracking

Existing expertise and infrastructure can be re‐used for the sMDT construction

Precision of the sMDT mechanical assembly and alignment system is demonstrated

CSC size MicroMegas production technology already exists at CERN

The MicroMegas is still a technology under development and if we find unforeseen problems we can switch to the construction of a full MDT+ TGC NSW.

The accuracy of the read out plane (strips on PCB) is probably easier to be kept on smaller planes.

sMDT construction ensures the formation of new experts. They may eventually contribute to the maintenance of the full detector. Such experts may not be available otherwise.

sMDT may allow tube based or tagged trigger. It can be a testing ground for the necessary trigger upgrade for phase 2 where the same technique can be used on the Big Wheel tubes

Disadvantages

Higher cost. ½ number of channels but high development costs and high fixed part of production/installation cost (three technologies)

3 systems which require different HW/SW development – trigger/readout/monitoring/analysis/controls/services

sMDT front‐end electronics require a different ASIC

Complexity in the integration of the different systems and communities

Front‐end on‐chamber electronics raise questions on space available on the NSW rim and maintenance due to limits of accessibility

Documents

A study of MicroMegas detectors with resistive anodes for muon