Micromegas TPC P Colas Saclay Lectures at the

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

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 In. Grid “bulk” technology Resistive anode Micromegas Digital TPC Beijing, January 9, 2008 P. Colas - Micromegas TPC 2

OUTLINE PART II – Micromegas experiments The COMPASS experiment The CAST experiment The KABES beam spectrometer The T 2 K 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 3

Electrons in gases : drift, ionization and avalanche Typical (thermic) energy of an electron in a gas: 0. 04 e. V E Low enough electric field (<1 k. V/cm) : collisions with gas atoms limit the electron velocity to vdrift = f(E) (effective friction force) Mean free path l=ns (0. 4 mm at 1 e. V) At higher fields ionization takes place (gain 10 V in 2 mm ionization =50 k. V/cm) magboltz Beijing, January 9, 2008 P. Colas - Micromegas TPC 4

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 5

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 CF 4 mixtures Lower with CO 2 mixtures Beijing, January 9, 2008 P. Colas - Micromegas TPC 6

Attachment Ne = Ne 0 exp(-az) electron capture by the molecules 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 : O 2, CF 4 Exemple of 3 -body attachment : O 2, O 2+CO 2 Very small (10 ppm) contamination of O 2, H 2 O, or some solvants, can ruin the operation of a TPC Beijing, January 9, 2008 P. Colas - Micromegas TPC 7

Diffusion limits z resolution (typically 200 -500 m/√cm) Limits rf resolution at high z (“diffusion limit”) B field greatly reduces the diffusion w=e. B/me, t = time between collisions (assumed isotropic) wt = from ~1 to 15 -20 (note wt ~Vdrift B/E) Drift Beijing, January 9, 2008 Langevin equation v(E, B) -> Ex. B effect P. Colas - Micromegas TPC 8

Electrons in gases : drift, ionization and avalanche E At high enough fields (5 – 10 k. V/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 Me. V to Ge. Vs), 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 9

t TPC: Time Projection Chamber electrons diffuse and Ionizing Particle drift due to the E-field electrons are separated from ions B E A magnetic field reduces electron diffusion Micromegas TPC : the amplification is made by a Micromegas Beijing, January 9, 2008 Localization in time and x-y P. Colas - Micromegas TPC y x 10

Micromegas: How does it work? Y. Giomataris, Ph. Rebourgeard, JP Robert and G. Charpak, S 1 NIM A 376 (1996) 29 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) stage Funnel field lines: electron transparency very close to 1 for thin meshes Small gap: fast collection of ions fast Beijing, January 9, 2008 S 2/S 1 = Edrift/Eamplif ~ 200/60000= 1/300 P. Colas - Micromegas TPC 11

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Small size => Fast signals => Short recovery time => High rate capabilities A GARFIELD simulation of a Micromegas avalanche (Lanzhou university) 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 13

Gain of Ar mixtures measured with Micromegas (D. Attié, PC, M. Was) Beijing, January 9, 2008 P. Colas - Micromegas TPC 14

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) : e. A -> e. A* A*B ->AB+e Beijing, January 9, 2008 P. Colas - Micromegas TPC 15

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 16

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 17

Max Chefdeville et al (NIKHEF/Saclay) + Twente Univ. • This leads to excellent energy resolution 11. 7 % @ 5. 9 ke. V in P 10 That is 5% in r. m. s. obtained by grids postprocessed on silicon substrate. Similar results obtained with Microbulk Micromegas Kα escape line Kβ escape line 13. 6 % FWHM Gap : 50 μm; Trou, pas : 32 μm, Ø : 14 μm – with F = 0. 14 & Ne = 229 one can estimate the gain fluctuation parameter q Beijing, January 9, 2008 Kβ removed by using a Cr foil P. Colas - Micromegas TPC 11. 7 % FWHM 18

Gain uniformity measurements 2007 MM 1_001 prototype Y- vs-X 55 Fe source illumination 404 / 1726 tested pads Gain ~ 1000 7% rms @ 5. 9 ke. V AFTER based FEE Average resolution = 19% FWHM @ 5. 9 ke. V Beijing, January 9, 2008 P. Colas - Micromegas TPC 19

Gain uniformity MM 1_001 prototype Inactive pads (Vmesh connection) 55 Fe source near module edge 55 Fe source near module centre Gain uniformity within a few % Beijing, January 9, 2008 P. Colas - Micromegas TPC 20

MM 0_007: gain uniformity 487 / 1726 tested pads Vmesh = 350 V 7. 4 % rms @ 5. 9 ke. V Average resolution = 21% FWHM @ 5. 9 ke. V Beijing, January 9, 2008 P. Colas - Micromegas TPC 21

MM 1_002 : gain uniformity and energy resolution Measured non-uniformities (%) Bopp micromesh AFTER ORTEC amplifier : 12 pads / measurement 21% FWHM @ 5. 9 ke. V 5. 6 1. 4 4. 1 4. 7 1. 0 1. 4 3. 0 3. 9 1. 6 0. 0 4. 4 0. 6 2. 8 5. 2 4. 4 2. 8 0. 8 3. 8 5. 8 1. 0 2. 2 1. 9 RMS = 3. 3% Beijing, January 9, 2008 P. Colas - Micromegas TPC 22

Transparency Collection efficiency reaches a plateau (100%? ) at high enough field ratio Micromesh Gantois Bopp pitch ( m) 57 63 ( m) 19 18 Operation point of Micro. Megas detectors in T 2 K is in the region where high micromesh transparencies are obtained Beijing, January 9, 2008 P. Colas - Micromegas TPC 23

Natural suppression of ion backflow S 1 THE SECOND NICEST PROPERTY OF MICROMEGAS 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 S 2/S 1 = 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. S 2 This has been experimentally thoroughly verified. Beijing, January 9, 2008 P. Colas - Micromegas TPC 24

Feedback : theory and simulation Hypothesis on the avalanche Periodical structure Gaussian diffusion Avalanche Resolution 2 s l Beijing, January 9, 2008 P. Colas - Micromegas TPC 25

ion backflow calculation Sum of gaussian diffusions 2 D Beijing, January 9, 2008 3 D P. Colas - Micromegas TPC 26

Theoretical ion feedback Results 500 lpi (sigma/l=0. 25) Beijing, January 9, 2008 1000 lpi (sigma/l=0. 5) P. Colas - Micromegas TPC 1500 lpi (sigma/l=0. 75) 27

Ion backflow (theory) Beijing, January 9, 2008 P. Colas - Micromegas TPC 28

Ion backflow measurements X-ray gun Vdrift I 1 (drift) Primaries+backflow Vmesh I 2 (mesh) I 1+I 2 ~ 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 2 T P. Colas, I. Giomataris and V. Lepeltier, NIM A 535 (2004) 226 Beijing, January 9, 2008 P. Colas - Micromegas TPC 29

Ion backflow measurements A new technique to make perfect meshes with various pitches and gaps has been set up (In. Grid at Twente) and allowed 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 30

Gain and spark rates E. Mazzucato et al. , T 2 K 95 m 128 m Threshold = 100 n. A The T 2 K/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 31

Discharge probability in a hadron beam 2. 5 mm conversion gap 100 µ amplif. gap Number of discharges per hadron Ne-C 2 H 6 -CF 4 gain ~ 104 P = 10 -6 <Z> ~20 <Z> ~14 <Z> ~10 Future, pion beam: -remove CF 4 -lower the gain Note that discharges are not destructive, and can be mitigated by resistive coating Beijing, January 9, 2008 -increase the gap to compensate D. Thers et al. NIM A 469 (2001 )133 P. Colas - Micromegas TPC 32

MESHES Many different technologies have been developped for making meshes (Back-buymers, CERN, 3 M-Purdue, Gantois, Twente…) Exist in many metals: nickel, copper, stainless steel, Al, … also gold, titanium, nanocristalline copper are possible. Chemically etched Laser etching, Plasma etching… Electroformed Wowen Deposited by vaporization 200 mm PILLARS 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 33

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 T 2 K TPC under construction Beijing, January 9, 2008 P. Colas - Micromegas TPC 34

The Bulk technology Beijing, January 9, 2008 P. Colas - Micromegas TPC 35

The T 2 K TPC has been tested successfully at CERN (9/2007) 36 x 34 cm 2 1728 pads Pad pitch 6. 9 x 9 mm 2 Beijing, January 9, 2008 P. Colas - Micromegas TPC 36

T 2 K TPC (beam test events) Beijing, January 9, 2008 P. Colas - Micromegas TPC 37

Resistive anode Micromegas • With 2 mm x 6 mm pads, an ILC-TPC has 1. 2 106 channels, with consequences on cost, cooling, material budget… • 2 mm still too wide to give the target resolution (100 -130 µm) Not enough charge sharing, even for 1 mm wide pads in the case of Micromégas (s avalanche ~12µm) Beijing, January 9, 2008 P. Colas - Micromegas TPC 38

Solution (M. S. Dixit et. al. , NIM A 518 (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 A 566 (2006) 281 SIMULATION MEASUREMENT Beijing, January 9, 2008 P. Colas - Micromegas TPC 39

25 µm mylar with Cermet (1 MW/□) glued onto the pads with 50 µm thick dry adhesive Cermet selection and gluing technique are essential Drift Gap Al-Si Cermet on mylar MESH Amplification Gap 50 m pillars Beijing, January 9, 2008 P. Colas - Micromegas TPC 40

A point charge being deposited at t=0, r=0, the charge density at (r, t) is a solution of the 2 D telegraph equation. Only one parameter, RC (time per unit surface), links spread in space with time. R~1 MW/□ and C~1 p. F per pad area matches µs signal duration. (r) Q (r, t) integral over pads mm Beijing, January 9, 2008 P. Colas - Micromegas TPC ns 41

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 42

Reminder of past results • Demonstration with GEM + C-loaded kapton in a X-ray collimated source (M. S. Dixit et. al. , Nucl. Instrum. Methods A 518 (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 + Al. Si cermet (A. Bellerive et al. , in Proc. of LCWS 2005, Stanford) • Beam test and cosmic-ray test in B=1 T at KEK, October 2005 Beijing, January 9, 2008 P. Colas - Micromegas TPC 43

The Carleton chamber Carleton-Saclay Micromegas endplate with resistive anode. 128 pads (126 2 mmx 6 mm 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 44

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4 Ge. V/c + beam, B=1 T (KEK) Effect of diffusion: should become negligible at high magnetic field for a high t gas Beijing, January 9, 2008 P. Colas - Micromegas TPC 46

The 5 T 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% CF 4+2% isobutane (so-called T 2 K 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 47

The gain is independent of the magnetic field until 5 T within 0. 5% Beijing, January 9, 2008 P. Colas - Micromegas TPC 48

Pad Response Function Beijing, January 9, 2008 P. Colas - Micromegas TPC 49

Residuals in z slices Beijing, January 9, 2008 P. Colas - Micromegas TPC 50

• Resolution = 50 µ independent of the drift distance Analysis: Ar+5% isobutane Curved track fit B=5 T P>2 Ge. V f < 0. 05 Beijing, January 9, 2008 P. Colas - Micromegas TPC 51

Resolution = 50 µ independent of the drift distance ‘T 2 K gas’ Beijing, January 9, 2008 P. Colas - Micromegas TPC 52

Average residual vs x position Before bias correction After bias correction ± 20 m Beijing, January 9, 2008 P. Colas - Micromegas TPC 53

• 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=2 m Beijing, January 9, 2008 P. Colas - Micromegas TPC 54

Further developments • Make bulk with resistive foil for application to T 2 K, 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 55

Principle of the digital TPC Micromegas Cathode Ionizing particle Gas volume amplification system (MPGD) + Every single ionization electron is ~50 µm detected with an accuracy matching the 80 k. V/cm avalanche size -> maximal information, ultimate resolution - + - Time. Pix chip Beijing, January 9, 2008 P. Colas - Micromegas TPC 56

Time. Pix/Micromegas CERN/Nikhef-Saclay Fenêtre pour sources X Capot 6 cm Fenêtre pour source b Cage de champ Mesh Micromegas Puce Medipix 2/Time. Pix Beijing, January 9, 2008 P. Colas - Micromegas TPC 57

Timepix chip 65000 pixels (500 transistors each) + Si. Prot 20 μm + Micromegas 55 Fe Ar/Iso (95: 5) Mode Time z = 25 mm Vmesh = -340 V Beijing, January 9, 2008 P. Colas - Micromegas TPC 58

Si. Prot: protection against sparks NIKHEF Timepix chip + Si. Prot 20 μm + Micromegas Introduce 228 Th in the gas to provoke sparks 228 Th 220 Rn 6. 3 Me. V 6. 8 Me. V 2. 5× 105 e 2. 7× 105 e- Ar/Iso (80: 20) Mode TOT z = 10 mm Vmesh = -420 V Beijing, January 9, 2008 P. Colas - Micromegas TPC 59

SPARKS, but the chip’s still alive NIKHEF Timepix chip + Si. Prot 20 μm + Micromegas 228 Th 220 Rn Ar/Iso (80: 20) Mode TOT z = 10 mm Vmesh = -420 V Beijing, January 9, 2008 P. Colas - Micromegas TPC 60

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