Micromegas TPC

Micromegas TPC

P. Colas, SaclayLectures at the TPC school, Tsinghua University, Beijing,

January 7-11, 2008

Beijing, January 9, 2008 P. Colas - Micromegas TPC 2

OUTLINEOUTLINETPC, drift and amplification

Micromegas principle of operation

Micromegas properties

Gain stability and uniformity, optimal gap

Energy resolution

Electron collection efficiency and transparency

Ion feedback suppression

Micromegas manufacturing

meshes and pillars

InGrid

“bulk” technology

Resistive anode Micromegas

Digital TPC

PART I – operation and properties


OUTLINEOUTLINE

The COMPASS experiment

The CAST experiment

The KABES beam spectrometer

The T2K ND-280 TPC

The Large Prototype for the ILC

Micromegas neutron detectors

TPCs for Dark Matter search and neutrino studies

PART II – Micromegas experiments


Electrons in gases : drift, ionization and avalancheElectrons in gases : drift, ionization and avalanche

E

Mean free path =nm at 1eV)

Typical (thermic) energy of an electron in a gas: 0.04 eV

Low enough electric field (<1kV/cm) : collisions with gas atoms limit the electron velocity to vdrift = f(E)

(effective friction force)

At higher fields ionizationionization takes place (gain 10 V in 2m =50kV/cm)

magboltz


Cross-sections of most common quenchers follow the same kind of shape, but not all (noticeably, not He); Dip due to Ramsauer effect (interf. when e-wavelength~mol.size)

Note : attachment



Thanks to the Ramsauer effect, there is a maximum drift velocity at low drift field : important for a TPC, to have a homogeneous time to z relation

Typical drift velocities : 5 cm/s

(or 50 m/ns)

Higher with CF4 mixtures

Lower with CO2 mixtures


AttachmentAttachment

Ne = Ne0 exp(-az) a can be from m-1 to (many m) -1

Attachment coefficient = 1 / attenuation length

2-body : e- ; 3-body : ea

Exemple of 2-body attachment : O2, CF4

Exemple of 3-body attachment : O2, O2+CO2

Very small (10 ppm) contamination

of O2, H2O, or some solvants, can

ruin the operation of a TPC

electron capture by the molecules


DriftDrift

DiffusionDiffusion

z.Cσ DTt

)²(1

)0()(

BB T

T

z.Cσ DLl limits z resolution (typically 200-500 /√cm)

Limits r resolution at high z (“diffusion limit”)

B field greatly reduces the diffusion

=eB/me, = time between collisions (assumed isotropic)

= from ~1 to 15-20 (note Vdrift B/E)

Langevin equation v(E,B) -> ExB effect



E At high enough fields (5 – 10 kV/cm) electrons acquire enough energy to bounce other electrons out of the atoms, and these electrons also can bounce others, and so on… This is an avalancheavalanche

In a TPC, electrons are extracted from the gas by the high energy particles (100 MeV to GeVs), these electrons drift in an electric field, and arrive in a region of high field where they produce an avalanche.

Wires, Micromegas and GEMs provide these high field regions.


TPC: Time Projection ChamberTPC: Time Projection Chamber

E

Ionizing Particle

electrons are separated from ions

electrons diffuse and drift due to the E-field

Localization in time and x-y

B

t

x

y

A magnetic field reduces electron diffusion

Micromegas TPC : the amplification is made by a Micromegas


Micromegas: How does it work?Micromegas: How does it work?

Y. Giomataris, Ph. Rebourgeard, JP Robert and G. Charpak,

NIM A 376 (1996) 29

S1

S2

Micromesh Gaseous Chamber: a micromesh supported by 50-100 m insulating pillars, and held at Vanode – 400 V Multiplication (up to 105 or more) takes place between the anode and the mesh and the charge is collected on the anode (one stageone stage)

Funnel field lines: electron transparencytransparency very close to 1 for thin meshes

Small gap: fastfast collection of ions S2/S1 = Edrift/Eamplif ~ 200/60000= 1/300



Small size =>

Fast signals =>

Short recovery time =>

High rate capabilities

micromesh signal

strip signals

A GARFIELD simulation of a

Micromegas avalanche

(Lanzhou university)

Electron and ion signals seen by a fast (current) amplifier

In a TPC, the signals are usually integrated and shaped


GainGain of Ar mixtures measured with Micromegas (D.Attié, PC, M.Was)


Gain

Compared with the “simple” picture, there are complications :

-due to photon emission (which can re-ionize if the gas is transparent in the UV domain and make photo-electric effect on the mesh). This increases the gain, but causes instabilities. This is avoided by adding a (quencher) gas, usually a polyatomic gas with many degrees of freedom (vibration, rotation) to absorb UVs

-due to molecular effects : molecules of one type can be excited in collisions and the excitation energy can be transferred to a molecule of another type, with sufficiently low ionization potential, which releases it in ionization (Penning effect) :

ee*

e


Gain uniformity in MicromegasThe nicest property of Micromegas

• Gain (=e d) • Townsend

increases with field• Field decreases with

gap at given V• => there is a

maximum gain for a given gap (about 50 for Ar mixt. and 100 for He mixt.)


Gain stabilityVery good gain stability (G. Puill et al.)

Optimization in progress for CAST

<2% rms over 6 months


• This leads to excellent energy resolution

11.7 % @ 5.9 keV in P10

That is 5% in r.m.s.

obtained by grids post-processed on silicon substrate. Similar results obtained with Microbulk Micromegas

–with F = 0.14 & Ne = 229 one can estimate the gain fluctuation parameter

Kα escape line

Kβ escape line 13.6 % FWHM

Kβ removed by using a Cr foil

11.7 %

FWHM

Max Chefdeville et al (NIKHEF/Saclay) + Twente Univ.

Gap : 50 μm; Trou, pas : 32 μm, Ø : 14 μm


Gain uniformity

measurements Y- vs-X 55Fe source illumination

404 / 1726 tested pads

Gain ~ 1000 7% rms@ 5.9 keV

Average resolution = 19% FWHM

AFTER based FEE

2007 MM1_001 prototype

@ 5.9 keV


Gain uniformityMM1_001 prototype

Inactive pads (Vmesh connection)

55Fe source near module edge

55Fe source near module centre

Gain uniformity within a few %


MM0_007: gain uniformity

Vmesh = 350V 7.4 % rms @ 5.9 keV

487 / 1726 tested pads

Average resolution = 21% FWHM

@ 5.9 keV


MM1_002 : gain uniformity and energy resolution

Bopp micromesh

21% FWHM @ 5.9 keV

5.6 1.4 1.4 4.1

4.7 1.0 1.4 3.0

3.9 1.6 0.0 4.4

4.4 0.6 2.8 5.2

4.4 2.8 0.8 3.8

5.8 1.0 2.2 1.9

Measured non-uniformities (%)

RMS = 3.3%

ORTEC amplifier : 12 pads / measurement

AFTER


Transparency

Gantois Bopp

pitch

(m) 57 63

(m) 19 18

Micromesh

Operation point of MicroMegas detectors in T2K is in the region where high micromesh transparencies are obtained

Collection efficiency reaches a plateau (100%?) at high enough field ratio


S1

S2

Natural suppression of ion backflowNatural suppression of ion backflowNatural suppression of ion backflowNatural suppression of ion backflow

Electrons are swallowed in the funnel, then make their avalanche, which is spread by diffusion.

The positive ions, created near the anode, will flow back with negligible diffusion (due to their high mass). If the pitch is comparable to the avalanche size, only the fraction S2/S1 = EDRIFT/EAMPLIFICATION will make it to the drift space. Others will be neutralized on the mesh : optimally, the backflow fraction is as low as the field ratio.

This has been experimentally thoroughly verified.

THE SECOND NICEST PROPERTY OF MICROMEGAS


Hypothesis on the avalanche

Gaussian diffusionPeriodical structure

l2

Avalanche Resolution

Feedback : theory and simulationFeedback : theory and simulationFeedback : theory and simulationFeedback : theory and simulation


ion backflow calculation

Sum of gaussian diffusions

2D 3D


Results1500 lpi (sigma/l=0.75)1000 lpi (sigma/l=0.5)500 lpi (sigma/l=0.25)

5.2_

_ ratiofield

feedbackion 03.1_

_ ratiofield

feedbackion 1_

_ ratiofield

feedbackion

Theoretical ion feedback


Ion backflow (theory)


Ion backflow measurements

Vmesh

Vdrift

I2 (mesh)

I1 (drift)

X-ray gun

Primaries+backflow

I1+I2 ~ G x primaries

One gets the primary ionisation from the drift current at low Vmesh

One eliminates G and the backflow from the 2 equations

The absence of effect of the magnetic field on the ion backflow suppression has been tested up to 2T

P. Colas, I. Giomataris and V. Lepeltier, NIM A 535 (2004) 226


Ion backflow measurements

A new technique to make perfect meshes with various pitches and gaps has been set up (InGrid at Twente) and allowed the theory to be thoroughly tested (M. Chefdeville et al., Saclay and Nikhef)

rms avalanche sizes are 9.5, 11.6 and 13.4 micron resp. for 45, 58 and 70 micron gaps.

The predicted asymptotic minimum reached about /pitch ~0.5 is observed. Red:data

Blue:calculation

In conclusion, the backflow can be kept at O(1 permil) : does not add to primary ionisation (on average)


Gain and spark rates

95m

128mThreshold = 100nA

The T2K/TPC will be operated at moderate gas gains of about 1000 where spark rates / module are sufficiently low (< 0.1/hour). TPC dead time < 1% achievable.

E. Mazzucato et al., T2K


Nu

mb

er

of

dis

ch

arg

es

pe

r h

ad

ron

Discharge probability in a hadron beam

D.Thers et al. NIM A 469 (2001 )133

<Z> ~20

<Z> ~10

Ne-C2H6-CF4

gain ~ 104

P = 10-6

<Z> ~14

Note that discharges are not destructive, and can be mitigated by resistive coating

2.5 mm conversion gap 100 µ amplif. gap

Future, pion beam:

-remove CF4

-lower the gain

-increase the gap to compensate


200 m

MESHESMESHES

ElectroformedChemically

etched Wowen

PILLARSPILLARS

Deposited by vaporization

Laser etching, Plasma etching…

Many different technologies have been developped for making meshes (Back-buymers, CERN, 3M-Purdue, Gantois, Twente…)

Exist in many metals: nickel, copper, stainless steel, Al,… also gold, titanium, nanocristalline copper are possible.

Can be on the mesh (chemical etching) or on the anode (PCB technique with a photoimageable coverlay). Diameter 40 to 400 microns.

Also fishing lines were used (Saclay, Lanzhou)


The Bulk technologyFruit of a CERN-Saclay collaboration (2004)Mesh fixed by the pillars themselves :

No frame needed : fully efficient surfaceVery robust : closed for > 20 µ dustPossibility to fragment the mesh (e.g. in bands)… and to repair it

Used by the T2K TPC under construction


The Bulk technology


The T2K TPC has been tested successfully at CERN

(9/2007)

36x34 cm2

1728 pads

Pad pitch 6.9x9 mm2


T2K TPC (beam test events)


Resistive anode Micromegas• With 2mm x 6mm pads, an ILC-TPC has 1.2 106

channels, with consequences on cost, cooling, material budget…

• 2mm still too wide to give the target resolution (100-130 µm)

Not enough charge sharing, even for 1mm wide pads in the case of Micromégas

avalanche ~12µm)


Solution ((M.S.Dixit et.al., NIM M.S.Dixit et.al., NIM A518A518 (2004) (2004)

721.721.)) Share the charge Share the charge between several between several neighbouring pads after neighbouring pads after amplification, using a amplification, using a resistive coating on an resistive coating on an insulator. insulator. The charge is spread in The charge is spread in this continuous network of this continuous network of R, CR, C

SIMULATION

MEASUREMENT

M.S.Dixit and A. Rankin NIM A566 (2006) 281M.S.Dixit and A. Rankin NIM A566 (2006) 281


25 µm mylar with Cermet (1 M25 µm mylar with Cermet (1 M//□□) ) glued onto the pads with 50 µm thick glued onto the pads with 50 µm thick dry adhesivedry adhesive

50 m pillars

Drift Gap

MESHAmplification Gap

Al-Si Cermet on mylar

Cermet selection and gluing technique are essential


(r,t) integral over pads

(r) Q

mm ns

A point charge being deposited at t=0, r=0, the charge density at (r,t) is a solution of the 2D telegraph equation.

Only one parameter, RC (time per unit surface), links spread in space with time. R~1 M/□ and C~1pF per pad area matches µs signal duration.

t

1

RC

2r2

1

r

r

(r, t) RC

2t

r 2RC

4 te


Mesh voltage (V)

Another good property of the resistive foil: it prevents charge build-up, thus prevents sparks.

Gains 2 orders of magnitude higher than with standard anodes can be reached.


• Demonstration with GEM + C-loaded kapton in a X-ray collimated source (M.S.Dixit et.al., Nucl. Instrum. Methods A518 (2004) 721)

• Demonstration with Micromegas + C-loaded kapton in a X-ray collimated source (unpublished)

• Cosmic-ray test with GEM + C-loaded kapton (K. Boudjemline et.al.,

to appear in NIM)• Cosmic-ray test with Micromegas + AlSi cermet (A. Bellerive et al.,

in Proc. of LCWS 2005, Stanford)• Beam test and cosmic-ray test in B=1T at KEK, October 2005

Reminder of past results


The Carleton chamberCarleton-Saclay Micromegas endplate with resistive anode.

128 pads (126 2mmx6mm in 7 rows plus 2 large trigger pads)

Drift length: 15.7 cm

ALEPH preamps + 200 MHz digitizers



4 GeV/c + beam, B=1T (KEK)

eff

dx N

zC

22

0

Effect of diffusion: should become negligible at high magnetic field for a high gas


The 5T cosmic-ray test at DESY

4 weeks of data taking (thanks to DESY and T. Behnke et al.)

Used 2 gas mixtures:

Ar+5% isobutane (easy gas, for reference)

Ar+3% CF4+2% isobutane (so-called T2K gas, good trade-off for safety, velocity, large

Most data taken at 5 T (highest field) and 0.5 T (low enough field to check the effect of diffusion)Note: same foil used since more than a year. Still works

perfectly.

Was ~2 weeks at T=55°C in the magnet: no damage


The gain is independent of the magnetic field until 5T within 0.5%


Pad Response Function


ResidualsResiduals

in z slicesin z slices


• Resolution = 50 µ independent of the drift distance

Ar+5% isobutane

B=5 T

Analysis:

Curved track fit

P>2 GeV

< 0.05


Resolution = 50 µ independent of the drift distance

‘T2K gas’


±20

Average residual vs x position

Before bias correction

After bias correction


• B=0.5 T• Resolution at 0 distance ~50 µ even at low gain

Gain = 4700 Gain = 2300

Neff=25.2±2.1

Neff=28.8±2.2

At 4 T with this gas, the point resol° is better than 80 µm at z=2m


Further developments

• Make bulk with resistive foil for application to T2K, LC Large prototype, etc…

• For this, several techniques are available: resistive coatings glued on PCB, serigraphied resistive pastes, photovoltaïc techniques


Principle of the digital TPC

+

-

+

-

TimePix chip

Ionizing particle

Gas

volume

amplification system (MPGD)

Cathode

~50 µm

80 kV/cm

Micromegas

Every single ionization electron is detected with an accuracy matching the avalanche size -> maximal information, ultimate resolution


TimePix/Micromegas

Cage de champ

Capot

MeshMicromeg

as

Puce Medipix2/TimePix

Fenêtre pour sources X

Fenêtre poursource

CERN/Nikhef-Saclay

6 cm


Timepix chip65000 pixels(500 transistors

each)+ SiProt 20 μm+ Micromegas

55Fe

Ar/Iso (95:5)

Mode Time

z = 25 mm

Vmesh = -340 V


SiProt: protection against sparksTimepix chip

+ SiProt 20 μm+ Micromegas

Introduce 228Th in the gas to provoke sparks

228Th220Rn

Ar/Iso (80:20)

Mode TOT

z = 10 mm

Vmesh = -420 V

2.5×105 e-

2.7×105 e-

6.3 MeV

6.8 MeV

NIKHEF


SPARKS, but the chip’s still alive

Timepix chip+ SiProt 20 μm+ Micromegas

228Th220Rn

Ar/Iso (80:20)

Mode TOT

z = 10 mm

Vmesh = -420 V

NIKHEF


Documents

Micromegas TPC