Micromegas TPC CCAST P. Colas, Saclay Lectures at the TPC school, Tsinghua University, Beijing, January 7-11, 2008

P. Colas - Micromegas TPC OUTLINE PART I – operation and properties TPC, 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 Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC OUTLINE PART II – Micromegas experiments 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 Beijing, January 9, 2008 P. Colas - Micromegas TPC

Electrons in gases : drift, ionization and avalanche 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 ionization takes place (gain 10 V in 2mm =50kV/cm) E Mean free path l=ns (0.4 mm at 1eV) magboltz Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. 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, 2008 P. Colas - Micromegas TPC

Electrons in gases : drift, ionization and avalanche 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/ms (or 50 mm/ns) Higher with CF4 mixtures Lower with CO2 mixtures Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Attachment electron capture by the molecules Ne = Ne0 exp(-az) a can be from mm-1 to (many m) -1 Attachment coefficient = 1 / attenuation length 2-body : e- + A -> A- ; 3-body : e- + A -> A*-, A *- B -> AB-, a a [A][B] 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 Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Diffusion limits z resolution (typically 200-500 m/√cm) Limits rf resolution at high z (“diffusion limit”) B field greatly reduces the diffusion w=eB/me, t = time between collisions (assumed isotropic) wt = from ~1 to 15-20 (note wt ~Vdrift B/E) Drift Langevin equation v(E,B) -> ExB effect Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

TPC: Time Projection Chamber electrons diffuse and drift due to the E-field Ionizing Particle electrons are separated from ions E B A magnetic field reduces electron diffusion y x Micromegas TPC : the amplification is made by a Micromegas Localization in time and x-y Beijing, January 9, 2008 P. Colas - Micromegas TPC

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 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 ions S2/S1 = Edrift/Eamplif ~ 200/60000= 1/300 Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

P. Colas - Micromegas TPC A GARFIELD simulation of a Micromegas avalanche (Lanzhou university) Small size => Fast signals => Short recovery time => High rate capabilities micromesh signal strip signals Electron and ion signals seen by a fast (current) amplifier In a TPC, the signals are usually integrated and shaped Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

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

Gain uniformity in Micromegas The nicest property of Micromegas Gain (=e ad) Townsend a increases with field Field 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, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Gain stability Very good gain stability (G. Puill et al.) Optimization in progress for CAST <2% rms over 6 months Beijing, January 9, 2008 P. Colas - Micromegas TPC

This leads to excellent energy resolution 11.7 % @ 5.9 keV in P10 Max Chefdeville et al (NIKHEF/Saclay) + Twente Univ. 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 q Kα escape line Kβ escape line 13.6 % FWHM Gap : 50 μm; Trou, pas : 32 μm, Ø : 14 μm Kβ removed by using a Cr foil 11.7 % FWHM Beijing, January 9, 2008 P. Colas - Micromegas TPC

Gain uniformity measurements 2007 MM1_001 prototype Gain uniformity measurements Y- vs-X 55Fe source illumination 404 / 1726 tested pads Gain ~ 1000 7% rms @ 5.9 keV AFTER based FEE Average resolution = 19% FWHM @ 5.9 keV Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Gain uniformity MM1_001 prototype Inactive pads (Vmesh connection) 55Fe source near module edge 55Fe source near module centre Gain uniformity within a few % Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC MM0_007: gain uniformity 487 / 1726 tested pads Vmesh = 350V 7.4 % rms @ 5.9 keV Average resolution = 21% FWHM @ 5.9 keV Beijing, January 9, 2008 P. Colas - Micromegas TPC

MM1_002 : gain uniformity and energy resolution Measured non-uniformities (%) Bopp micromesh ORTEC amplifier : 12 pads / measurement 5.6 1.4 1.4 4.1 4.7 1.0 3.0 3.9 1.6 0.0 4.4 4.4 0.6 2.8 5.2 2.8 0.8 3.8 5.8 1.0 2.2 1.9 AFTER 21% FWHM @ 5.9 keV RMS = 3.3% Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Transparency Collection efficiency reaches a plateau (100%?) at high enough field ratio Micromesh Gantois Bopp pitch (m) 57 63  19 18 Operation point of MicroMegas detectors in T2K is in the region where high micromesh transparencies are obtained Beijing, January 9, 2008 P. Colas - Micromegas TPC

THE SECOND NICEST PROPERTY OF MICROMEGAS Natural suppression of ion backflow THE SECOND NICEST PROPERTY OF MICROMEGAS S1 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. S2 Beijing, January 9, 2008 P. Colas - Micromegas TPC

Feedback : theory and simulation Hypothesis on the avalanche Periodical structure Gaussian diffusion Avalanche Resolution 2s l Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

P. Colas - Micromegas TPC Theoretical ion feedback Results 1500 lpi (sigma/l=0.75) 1000 lpi (sigma/l=0.5) 500 lpi (sigma/l=0.25) Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

P. Colas - Micromegas TPC Ion backflow measurements X-ray gun Vdrift I1 (drift) Primaries+backflow I2 (mesh) Vmesh I1+I2 ~ G x primaries The absence of effect of the magnetic field on the ion backflow suppression has been tested up to 2T One gets the primary ionisation from the drift current at low Vmesh One eliminates G and the backflow from the 2 equations P. Colas, I. Giomataris and V. Lepeltier, NIM A 535 (2004) 226 Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC 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 s/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) Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Gain and spark rates E. Mazzucato et al., T2K 95m 128m Threshold = 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. Beijing, January 9, 2008 P. Colas - Micromegas TPC

Discharge probability in a hadron beam 2.5 mm conversion gap 100 µ amplif. gap <Z> ~20 Number of discharges per hadron <Z> ~14 <Z> ~10 Ne-C2H6-CF4 gain ~ 104 P = 10-6 Future, pion beam: -remove CF4 -lower the gain -increase the gap to compensate Note that discharges are not destructive, and can be mitigated by resistive coating D.Thers et al. NIM A 469 (2001 )133 Beijing, January 9, 2008 P. Colas - Micromegas TPC

MESHES 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. Chemically etched Deposited by vaporization Electroformed Wowen Laser etching, Plasma etching… PILLARS 200 mm 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, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC The Bulk technology Fruit of a CERN-Saclay collaboration (2004) Mesh fixed by the pillars themselves : No frame needed : fully efficient surface Very robust : closed for > 20 µ dust Possibility to fragment the mesh (e.g. in bands) … and to repair it Used by the T2K TPC under construction Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

P. Colas - Micromegas TPC The T2K TPC has been tested successfully at CERN (9/2007) 36x34 cm2 1728 pads Pad pitch 6.9x9 mm2 Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

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 (s avalanche ~12µm) Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. 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, C M.S.Dixit and A. Rankin NIM A566 (2006) 281 SIMULATION MEASUREMENT Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC 25 µm mylar with Cermet (1 MW/□) glued onto the pads with 50 µm thick dry adhesive Cermet selection and gluing technique are essential 50 m pillars Drift Gap MESH Amplification Gap Al-Si Cermet on mylar Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC 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 MW/□ and C~1pF per pad area matches µs signal duration. (r) Q (r,t) integral over pads mm ns Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC 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. Mesh voltage (V) Beijing, January 9, 2008 P. Colas - Micromegas TPC

Reminder of past results 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 Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC The Carleton chamber Carleton-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, 2008 P. Colas - Micromegas TPC

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

P. 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, 2008 P. Colas - Micromegas TPC

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 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=55°C in the magnet: no damage Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

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

P. Colas - Micromegas TPC Residuals in z slices Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Resolution = 50 µ independent of the drift distance Analysis: Curved track fit P>2 GeV f < 0.05 Ar+5% isobutane B=5 T Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Resolution = 50 µ independent of the drift distance ‘T2K gas’ Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Average residual vs x position Before bias correction After bias correction ±20 m Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC B=0.5 T Resolution at 0 distance ~50 µ even at low gain Gain = 2300 Gain = 4700 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 Beijing, January 9, 2008 P. Colas - Micromegas TPC

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

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

Principle of the digital TPC Cathode Micromegas Every single ionization electron is detected with an accuracy matching the avalanche size -> maximal information, ultimate resolution ~50 µm 80 kV/cm + - Ionizing particle Gas volume amplification system (MPGD) TimePix chip Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC TimePix/Micromegas CERN/Nikhef-Saclay Fenêtre pour sources X Capot 6 cm Fenêtre pour source b Cage de champ Mesh Micromegas Puce Medipix2/TimePix Beijing, January 9, 2008 P. Colas - Micromegas TPC

P. Colas - Micromegas TPC Timepix chip 65000 pixels (500 transistors each) + SiProt 20 μm + Micromegas 55Fe Ar/Iso (95:5) Mode Time z = 25 mm Vmesh = -340 V Beijing, January 9, 2008 P. Colas - Micromegas TPC

SiProt: protection against sparks Timepix chip + SiProt 20 μm + Micromegas Introduce 228Th in the gas to provoke sparks 228Th220Rn Ar/Iso (80:20) Mode TOT z = 10 mm Vmesh = -420 V NIKHEF 2.5×105 e- 6.3 MeV 6.8 MeV 2.7×105 e- Beijing, January 9, 2008 P. Colas - Micromegas TPC

SPARKS, but the chip’s still alive NIKHEF Timepix chip + SiProt 20 μm + Micromegas 228Th220Rn Ar/Iso (80:20) Mode TOT z = 10 mm Vmesh = -420 V Beijing, January 9, 2008 P. Colas - Micromegas TPC