Micromegas TPC

Micromegas TPCP. Colas, SaclayLectures at the TPC school, Tsinghua University, Beijing, January 7-11, 2008

Beijing, January 9, 2008P. Colas - Micromegas TPC*OUTLINETPC, drift and amplificationMicromegas principle of operationMicromegas propertiesGain stability and uniformity, optimal gapEnergy resolutionElectron collection efficiency and transparencyIon feedback suppressionMicromegas manufacturingmeshes and pillarsInGridbulk technologyResistive anode MicromegasDigital TPCPART I operation and properties

P. Colas - Micromegas TPC

Beijing, January 9, 2008P. Colas - Micromegas TPC*OUTLINEThe COMPASS experimentThe CAST experimentThe KABES beam spectrometerThe T2K ND-280 TPCThe Large Prototype for the ILCMicromegas neutron detectorsTPCs for Dark Matter search and neutrino studiesPART II Micromegas experiments


Beijing, January 9, 2008P. Colas - Micromegas TPC*Electrons in gases : drift, ionization and avalancheEMean free path l=ns (0.4 mm at 1eV)Typical (thermic) energy of an electron in a gas: 0.04 eVLow enough electric field (

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


Beijing, January 9, 2008P. Colas - Micromegas TPC*Electrons in gases : drift, ionization and avalancheThanks 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 relationTypical drift velocities : 5 cm/ms(or 50 mm/ns)Higher with CF4 mixturesLower with CO2 mixtures


Beijing, January 9, 2008P. Colas - Micromegas TPC*AttachmentNe = Ne0 exp(-az) a can be from mm-1 to (many m) -1Attachment coefficient = 1 / attenuation length2-body : e- + A -> A- ; 3-body : e- + A -> A*-, A *- B -> AB-, a a [A][B]Exemple of 2-body attachment : O2, CF4Exemple of 3-body attachment : O2, O2+CO2

Very small (10 ppm) contaminationof O2, H2O, or some solvants, canruin the operation of a TPC electron capture by the molecules


Beijing, January 9, 2008P. Colas - Micromegas TPC*DriftDiffusionlimits z resolution (typically 200-500 m/cm)

Limits rf resolution at high z (diffusion limit)B field greatly reduces the diffusionw=eB/me, t = time between collisions (assumed isotropic)wt = from ~1 to 15-20 (note wt ~Vdrift B/E)Langevin equation v(E,B) -> ExB effect


Beijing, January 9, 2008P. Colas - Micromegas TPC*Electrons in gases : drift, ionization and avalancheEAt 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 avalancheIn 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.


Beijing, January 9, 2008P. Colas - Micromegas TPC*TPC: Time Projection ChamberEIonizing Particleelectrons are separated from ionselectrons diffuse and drift due to the E-field Localization in time and x-yBtxyA magnetic field reduces electron diffusionMicromegas TPC : the amplification is made by a Micromegas


Beijing, January 9, 2008P. Colas - Micromegas TPC*Micromegas: How does it work?Y. Giomataris, Ph. Rebourgeard, JP Robert and G. Charpak, NIM A 376 (1996) 29Micromesh Gaseous Chamber: a micromesh supported by 50-100 mm 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 stage)

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

Small gap: fast collection of ionsS2/S1 = Edrift/Eamplif ~ 200/60000= 1/300


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


Beijing, January 9, 2008P. Colas - Micromegas TPC*Small size =>Fast signals =>Short recovery time =>High rate capabilitiesmicromesh signalstrip signalsA GARFIELD simulation of a Micromegas avalanche(Lanzhou university)Electron and ion signals seen by a fast (current) amplifierIn a TPC, the signals are usually integrated and shaped


Beijing, January 9, 2008P. Colas - Micromegas TPC*GainGain of Ar mixtures measured with Micromegas (D.Atti, PC, M.Was)


Beijing, January 9, 2008P. Colas - Micromegas TPC*GainCompared 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 UVsdue 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) :eA -> eA*A*B ->AB+e


Beijing, January 9, 2008P. Colas - Micromegas TPC*Gain uniformity in MicromegasThe nicest property of MicromegasGain (=e ad) Townsend a increases with fieldField decreases with gap at given V=> there is a maximum gain for a given gap (about 50 m for Ar mixt. and 100 m for He mixt.)


Beijing, January 9, 2008P. Colas - Micromegas TPC*Gain stabilityVery good gain stability (G. Puill et al.)Optimization in progress for CAST

Beijing, January 9, 2008P. Colas - Micromegas TPC*This leads to excellent energy resolution11.7 % @ 5.9 keV in P10That 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 q

K escape line K escape line13.6 % FWHMK removed by using a Cr foil11.7 % FWHMMax Chefdeville et al (NIKHEF/Saclay) + Twente Univ.Gap : 50 m; Trou, pas : 32 m, : 14 m


Beijing, January 9, 2008P. Colas - Micromegas TPC*Gain uniformity measurements Y- vs-X 55Fe source illumination404 / 1726 tested padsGain ~ 1000 7% [email protected] 5.9 keV

Average resolution = 19% FWHMAFTER based FEE2007 MM1_001 prototype @ 5.9 keV


Beijing, January 9, 2008P. Colas - Micromegas TPC*Gain uniformityMM1_001 prototypeInactive pads (Vmesh connection) 55Fe source near module edge 55Fe source near module centreGain uniformity within a few %


Beijing, January 9, 2008P. Colas - Micromegas TPC*MM0_007: gain uniformityVmesh = 350V 7.4 % rms @ 5.9 keV487 / 1726 tested pads Average resolution = 21% [email protected] 5.9 keV


Beijing, January 9, 2008P. Colas - Micromegas TPC*MM1_002 : gain uniformity and energy resolutionBopp micromesh21% FWHM @ 5.9 keVMeasured non-uniformities (%)RMS = 3.3%ORTEC amplifier : 12 pads / measurementAFTER

5.61.41.44.14.71.01.43.03.91.60.04.44.40.62.85.24.42.80.83.85.81.02.21.9


Beijing, January 9, 2008P. Colas - Micromegas TPC*TransparencyMicromeshOperation point of MicroMegas detectors in T2K is in the region where high micromesh transparencies are obtainedCollection efficiency reaches a plateau (100%?) at high enough field ratio

Gantois Bopppitch (m) 5763 (m) 1918


Beijing, January 9, 2008P. Colas - Micromegas TPC*S1S2Natural suppression of ion backflowElectrons 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


Beijing, January 9, 2008P. Colas - Micromegas TPC*Hypothesis on the avalancheGaussian diffusionPeriodical structurel2sAvalancheResolutionFeedback : theory and simulation


Beijing, January 9, 2008P. Colas - Micromegas TPC*ion backflow calculationSum of gaussian diffusions 2D 3D


Beijing, January 9, 2008P. Colas - Micromegas TPC*ResultsTheoretical ion feedback


Beijing, January 9, 2008P. Colas - Micromegas TPC*Ion backflow (theory)


Graph1

0.10.160.25

0.10.1340.179

0.10.1090.118

0.10.1010.103

0.10.10.1

Field ratio

Feedback 2D

Feedback 3D

/p

IBF (%)

Feuil1

Passage 2D ->3D

Remonte d'ions thorique

500 lpi (sigma/l=0.25)1000 lpi (sigma/l=0.5)1500 lpi (sigma/l=0.75)

fieldratioion 2Dion 3Dfieldratioion 2Dion 3Dfieldratioion 2Dion 3D

0.0010.001590.002590.0010.001010.001030.0010.0010.001

0.0020.00320.00510.0020.002030.002060.0020.0020.002

0.0040.00640.01020.0040.004060.004110.0040.0040.004

0.0080.0130.02040.0080.008110.008230.0080.0080.008

0.010.0160.0250.010.01010.01030.010.010.01

0.020.0320.0510.020.02030.02060.020.020.02

0.040.0640.1020.040.04060.04110.040.040.04

0.080.130.2040.080.08110.08230.080.080.08

0.10.160.250.10.1010.1030.10.10.1

sigma/l=0.4sigma/l=0.3

fieldratioion 2Dion 3Dfieldratioion 2Dion 3D

0.0010.001090.001180.0010.001340.00179

0.0020.002170.002350.0020.002680.00359

0.0040.004340.004710.0040.005360.00718

0.0080.00870.00940.0080.01070.0144

0.010.01090.01180.010.01340.0179

0.020.02170.02350.020.02680.0359

0.040.04340.04710.040.05360.0718

0.080.0870.0940.080.1070.144

0.10.1090.1180.10.1340.179

Feuil2

sigma/lfieldratiofeedback 2Dfeedback 3D

0.250.10.160.25

0.30.10.1340.179

0.40.10.1090.118

0.50.10.1010.103

0.750.10.10.1

Feuil3

Beijing, January 9, 2008P. Colas - Micromegas TPC*Ion backflow measurementsVmeshVdriftI2 (mesh)I1 (drift)X-ray gunPrimaries+backflowI1+I2 ~ G x primariesOne 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 2TP. Colas, I. Giomataris and V. Lepeltier, NIM A 535 (2004)226


Beijing, January 9, 2008P. Colas - Micromegas TPC*Ion backflow measurementsA 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 s/pitch ~0.5 is observed.Red:dataBlue:calculationIn conclusion, the backflow can be kept at O(1 permil) : does not add to primary ionisation (on average)


Beijing, January 9, 2008P. Colas - Micromegas TPC*Gain and spark rates95m128mThreshold = 100nAThe 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


Beijing, January 9, 2008P. Colas - Micromegas TPC*Number of discharges per hadronDischarge probability in a hadron beamD.Thers et al. NIM A 469 (2001 )133 ~20 ~10Ne-C2H6-CF4gain ~ 104P = 10-6 ~14Note that discharges are not destructive, and can be mitigated by resistive coating2.5 mm conversion gap 100 amplif. gapFuture, pion beam:-remove CF4-lower the gain-increase the gap to compensate


Beijing, January 9, 2008P. Colas - Micromegas TPC*200 mmMESHESElectroformedChemically etchedWowenPILLARSDeposited by vaporizationLaser etching, Plasma etchingMany 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)


Beijing, January 9, 2008P. Colas - Micromegas TPC*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 dustPossibility to fragment the mesh (e.g. in bands) and to repair it

Used by the T2K TPC under construction


Beijing, January 9, 2008P. Colas - Micromegas TPC*The Bulk technology


Beijing, January 9, 2008P. Colas - Micromegas TPC*The T2K TPC has been tested successfully at CERN(9/2007)

36x34 cm21728 padsPad pitch 6.9x9 mm2


Beijing, January 9, 2008P. Colas - Micromegas TPC*T2K TPC (beam test events)


Beijing, January 9, 2008P. Colas - Micromegas TPC*Resistive anode MicromegasWith 2mm x 6mm pads, an ILC-TPC has 1.2 106 channels, with consequences on cost, cooling, material budget2mm still too wide to give the target resolution (100-130 m)Not enough charge sharing, even for 1mm wide pads in the case of Micromgas(s avalanche ~12m)


Beijing, January 9, 2008P. Colas - Micromegas TPC*Solution (M.S.Dixit et.al., NIM A518 (2004) 721.) Share the charge between several neighbouring pads after amplification, using a resistive coating on an insulator. The charge is spread in this continuous network of R, CSIMULATIONMEASUREMENTM.S.Dixit and A. Rankin NIM A566 (2006) 281


Beijing, January 9, 2008P. Colas - Micromegas TPC*25 m mylar with Cermet (1 MW/) glued onto the pads with 50 m thick dry adhesiveCermet selection and gluing technique are essential


Beijing, January 9, 2008P. Colas - Micromegas TPC*(r,t) integral over pads(r)QmmnsA 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 MW/ and C~1pF per pad area matches s signal duration.


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


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


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


Beijing, January 9, 2008P. Colas - Micromegas TPC*4 GeV/c + beam, B=1T (KEK)Effect of diffusion: should become negligible at high magnetic field for a high t gas


Beijing, January 9, 2008P. Colas - Micromegas TPC*The 5T cosmic-ray test at DESY4 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 wt )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=55C in the magnet: no damage


Beijing, January 9, 2008P. Colas - Micromegas TPC*The gain is independent of the magnetic field until 5T within 0.5%


Beijing, January 9, 2008P. Colas - Micromegas TPC*Pad Response Function


Beijing, January 9, 2008P. Colas - Micromegas TPC*Residualsin z slices


Beijing, January 9, 2008P. Colas - Micromegas TPC*Resolution = 50 independent of the drift distanceAr+5% isobutaneB=5 TAnalysis:Curved track fitP>2 GeVf < 0.05


Beijing, January 9, 2008P. Colas - Micromegas TPC*Resolution = 50 independent of the drift distanceT2K gas


Beijing, January 9, 2008P. Colas - Micromegas TPC*20 mAverage residual vs x positionBefore bias correctionAfter bias correction


Beijing, January 9, 2008P. Colas - Micromegas TPC*B=0.5 TResolution at 0 distance ~50 even at low gainGain = 4700Gain = 2300Neff=25.22.1Neff=28.82.2At 4 T with this gas, the point resol is better than 80 m at z=2m


Beijing, January 9, 2008P. Colas - Micromegas TPC*Further developmentsMake bulk with resistive foil for application to T2K, LC Large prototype, etcFor this, several techniques are available: resistive coatings glued on PCB, serigraphied resistive pastes, photovoltac techniques


Beijing, January 9, 2008P. Colas - Micromegas TPC*Principle of the digital TPCTimePix chipIonizing particleGasvolumeamplification system (MPGD)CathodeMicromegas

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


Beijing, January 9, 2008P. Colas - Micromegas TPC*TimePix/MicromegasCage de champCapotMeshMicromegasPuce Medipix2/TimePixFentre pour sources XFentre poursource bCERN/Nikhef-Saclay6 cm


Beijing, January 9, 2008P. Colas - Micromegas TPC*Timepix chip65000 pixels(500 transistors each)+ SiProt 20 m+ Micromegas

55Fe

Ar/Iso (95:5)

Mode Time

z = 25 mm

Vmesh = -340 V


Beijing, January 9, 2008P. Colas - Micromegas TPC*SiProt: protection against sparksTimepix chip+ SiProt 20 m+ MicromegasIntroduce 228Th in the gas to provoke sparks

228Th220Rn

Ar/Iso (80:20)

Mode TOT

z = 10 mm

Vmesh = -420 V

2.5105 e-2.7105 e-6.3 MeV6.8 MeVNIKHEF


Beijing, January 9, 2008P. Colas - Micromegas TPC*SPARKS, but the chips still aliveTimepix chip+ SiProt 20 m+ Micromegas

228Th220Rn

Ar/Iso (80:20)

Mode TOT

z = 10 mm

Vmesh = -420 V

NIKHEF


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Text of Micromegas TPC

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Micromegas TPC

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