2. Micromegas TPC P. Colas, CEA/Irfu Saclay MPGD Lectures, SINP, Kolkata October 20-22, 2014

3. Kolkata, October 20, 2014P. Colas - Micromegas2 OUTLINE 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 PART I – operation and properties

4. Kolkata, October 20, 2014P. Colas - Micromegas3 OUTLINE The CAST experiment The COMPASS 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 Practical operation and use of Micromegas PART II – Applications

5. Kolkata, October 20, 2014P. Colas - Micromegas4 Electrons in gases : drift, ionization and avalanche E Mean free path =n   m 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 v drift = f(E) (effective friction force) ionization At higher fields ionization takes place (gain 10 V in 2  m =50kV/cm) magboltz

6. Kolkata, October 20, 2014P. Colas - Micromegas5 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

7. Kolkata, October 20, 2014P. Colas - Micromegas6 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/  s (or 50  m/ns) Higher with CF 4 mixtures Lower with CO 2 mixtures

8. Kolkata, October 20, 2014P. Colas - Micromegas7 Attachment N e = N e 0 exp(-az) a can be from  m -1 to (many m) -1 Attachment coefficient = 1 / attenuation length 2-body : e -   ; 3-body : e        a  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 electron capture by the molecules

9. Kolkata, October 20, 2014P. Colas - Micromegas8 Drift Diffusion limits z resolution (typically 200-500  /√cm) Limits r  resolution at high z (“diffusion limit”) B field greatly reduces the diffusion  =eB/m e,  = time between collisions (assumed isotropic)  = from ~1 to 15-20 (note  V drift B/E) Langevin equation v(E,B) -> ExB effect

10. Kolkata, October 20, 2014P. Colas - Micromegas9 Electrons in gases : drift, ionization and avalanche E 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.

11. Kolkata, October 20, 2014P. Colas - Micromegas10 TPC: 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

12. Kolkata, October 20, 2014P. Colas - Micromegas11 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 V anode – 400 V one stage Multiplication (up to 10 5 or more) takes place between the anode and the mesh and the charge is collected on the anode (one stage) transparency Funnel field lines: electron transparency very close to 1 for thin meshes fast Small gap: fast collection of ions S2/S1 = E drift /E amplif ~ 200/60000= 1/300

13. Kolkata, October 20, 2014P. Colas - Micromegas12

14. Kolkata, October 20, 2014P. Colas - Micromegas13 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

15. Kolkata, October 20, 2014P. Colas - Micromegas14 Gain Gain of Ar mixtures measured with Micromegas (D.Attié, PC, M.Was)

16. Kolkata, October 20, 2014P. Colas - Micromegas15 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  e  *     e

17. Kolkata, October 20, 2014P. Colas - Micromegas16 Gain uniformity in Micromegas The 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.)

18. Kolkata, October 20, 2014P. Colas - Micromegas17 Gain stability Very good gain stability (G. Puill et al.) Optimization in progress for CAST <2% rms over 6 months

19. Kolkata, October 20, 2014P. Colas - Micromegas18 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 & N e = 229 one can estimate the gain fluctuation parameter  K α escape line K β escape line 13.6 % FWHM K β 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

20. Kolkata, October 20, 2014P. Colas - Micromegas19 Gain uniformity measurements Y- vs-X 55 Fe 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

21. Kolkata, October 20, 2014P. Colas - Micromegas20 Gain uniformity MM1_001 prototype Inactive pads (V mesh connection) 55 Fe source near module edge 55 Fe source near module centre Gain uniformity within a few %

22. Kolkata, October 20, 2014P. Colas - Micromegas21 MM0_007: gain uniformity V mesh =  350V 7.4 % rms @ 5.9 keV 487 / 1726 tested pads Average resolution = 21% FWHM @ 5.9 keV

23. Kolkata, October 20, 2014 P. Colas - Micromegas22 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

24. Kolkata, October 20, 2014 P. Colas - Micromegas23 Transparency Gantois Bopp pitch (  m) 5763  (  m) 1918 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

25. Kolkata, October 20, 2014P. Colas - Micromegas24 S1 S2 Natural 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 S 2 /S 1 = E DRIFT /E AMPLIFICATION 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

26. Kolkata, October 20, 2014P. Colas - Micromegas25 Hypothesis on the avalanche Gaussian diffusionPeriodical structure l 22 AvalancheResolution Feedback : theory and simulation

27. Kolkata, October 20, 2014P. Colas - Micromegas26 ion backflow calculation Sum of gaussian diffusions 2D3D

28. Kolkata, October 20, 2014P. Colas - Micromegas27 Results 1500 lpi (sigma/l=0.75)1000 lpi (sigma/l=0.5)500 lpi (sigma/l=0.25) Theoretical ion feedback

29. Kolkata, October 20, 2014P. Colas - Micromegas28 Ion backflow (theory)

30. Kolkata, October 20, 2014P. Colas - Micromegas29 Ion backflow measurements V mesh V drift I 2 (mesh) I 1 (drift) X-ray gun Primaries+backflow I 1 +I 2 ~ G x primaries One gets the primary ionisation from the drift current at low V mesh 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

31. Kolkata, October 20, 2014P. Colas - Micromegas30 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)

32. Kolkata, October 20, 2014P. Colas - Micromegas31 Gain and spark rates 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. E. Mazzucato et al., T2K

33. Kolkata, October 20, 2014P. Colas - Micromegas32 Number of discharges per hadron Discharge probability in a hadron beam D.Thers et al. NIM A 469 (2001 )133 ~20 ~10 Ne-C 2 H 6 -CF 4 gain ~ 10 4 P = 10 -6 ~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

34. Kolkata, October 20, 2014P. Colas - Micromegas33 200  m MESHES Electroformed Chemically etched Wowen PILLARS 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)

35. Kolkata, October 20, 2014P. Colas - Micromegas34 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

36. Kolkata, October 20, 2014P. Colas - Micromegas35 The Bulk technology

37. Kolkata, October 20, 2014P. Colas - Micromegas36 The T2K TPC has been tested successfully at CERN (9/2007) 36x34 cm 2 1728 pads Pad pitch 6.9x9 mm 2

38. Kolkata, October 20, 2014P. Colas - Micromegas37 T2K TPC (beam test events)

39. Kolkata, October 20, 2014P. Colas - Micromegas38 Resistive anode Micromegas With 2mm x 6mm pads, an ILC-TPC has 1.2 10 6 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)

40. Kolkata, October 20, 2014P. Colas - Micromegas39 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 SIMULATION MEASUREMENT M.S.Dixit and A. Rankin NIM A566 (2006) 281

41. Kolkata, October 20, 2014P. Colas - Micromegas40 25 µm mylar with Cermet (1 M  /□) glued onto the pads with 50 µm thick dry adhesive 50  m pillars Drift Gap MESH Amplification Gap Al-Si Cermet on mylar Cermet selection and gluing technique are essential

42. Kolkata, October 20, 2014P. Colas - Micromegas41  (r,t) integral over pads  (r) Q mmns 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.

43. Kolkata, October 20, 2014P. Colas - Micromegas42 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.

44. Kolkata, October 20, 2014P. Colas - Micromegas43 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

45. Kolkata, October 20, 2014P. Colas - Micromegas44 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

46. Kolkata, October 20, 2014P. Colas - Micromegas45

47. Kolkata, October 20, 2014P. Colas - Micromegas46 4 GeV/c  + beam, B=1T (KEK) Effect of diffusion: should become negligible at high magnetic field for a high  gas

48. Kolkata, October 20, 2014P. Colas - Micromegas47 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

49. Kolkata, October 20, 2014P. Colas - Micromegas48 The gain is independent of the magnetic field until 5T within 0.5%

50. Kolkata, October 20, 2014 P. Colas - Micromegas49 Pad Response Function

51. Kolkata, October 20, 2014 P. Colas - Micromegas50 Residuals in z slices

52. Kolkata, October 20, 2014P. Colas - Micromegas51 Resolution = 50 µ independent of the drift distance Ar+5% isobutane B=5 T Analysis: Curved track fit P>2 GeV  < 0.05

53. Kolkata, October 20, 2014P. Colas - Micromegas52 Resolution = 50 µ independent of the drift distance ‘T2K gas’

54. Kolkata, October 20, 2014P. Colas - Micromegas53 ±20  Average residual vs x position Before bias correction After bias correction

55. Kolkata, October 20, 2014P. Colas - Micromegas54 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

56. Kolkata, October 20, 2014P. Colas - Micromegas55 Further developments Make bulk with resistive foil for application to T2K, LC Large prototype, NSW, etc… For this, several techniques are available: resistive coatings glued on PCB, serigraphied resistive pastes, photovoltaïc techniques

57. Kolkata, October 20, 2014P. Colas - Micromegas56 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

58. Kolkata, October 20, 2014P. Colas - Micromegas57 TimePix/Micromegas Cage de champ Capot Mesh Micromegas Puce Medipix2/TimePix Fenêtre pour sources X Fenêtre pour source  CERN/Nikhef-Saclay 6 cm

59. Kolkata, October 20, 2014P. Colas - Micromegas58 Timepix chip 65000 pixels (500 transistors each) + SiProt 20 μm + Micromegas 55 Fe Ar/Iso (95:5) Mode Time z = 25 mm V mesh = -340 V

60. Kolkata, October 20, 2014P. Colas - Micromegas59 SiProt: protection against sparks Timepix chip + SiProt 20 μm + Micromegas Introduce 228 Th in the gas to provoke sparks 228 Th  220 Rn Ar/Iso (80:20) Mode TOT z = 10 mm V mesh = -420 V 2.5×10 5 e- 2.7×10 5 e- 6.3 MeV 6.8 MeV NIKHEF

61. Kolkata, October 20, 2014P. Colas - Micromegas60 SPARKS, but the chip’s still alive Timepix chip + SiProt 20 μm + Micromegas 228 Th  220 Rn Ar/Iso (80:20) Mode TOT z = 10 mm V mesh = -420 V NIKHEF

62. Kolkata, October 20, 2014P. Colas - Micromegas61 A few ‘historical’ Micromegas First Chinese Micromegas, with fishing lines (Zhang XiaoDong, Lanzhou) Japanese copy of the Saclay box (T. Matsuda, K. Fujii, KEK) One of the Tunis boxes

63. First Bulk in Aachen Box in Kolkata (copy from Saclay’s)

64. Kolkata, October 20, 2014P. Colas - Micromegas63 Practical use of Micromegas

65. Kolkata, October 20, 2014P. Colas - Micromegas64 ‘ Choose your material For first tests of a detector, a power supply with current limitation is preferred. Set the current limitation at 500 nA for instance. The CAEN N471A is ideal for testing, though not very precise. They have 2 chanels, you can use one for the mesh and one for the drift cathode. Check your gasbox for gas-tightness : must bubble down to 1 l/h. Before connecting the electronics, ‘cook’ your detector (see next slide). Preamp: use a protected fast preamp (for instance ORTEC 142 series) and an amplifier-shaper (0.5 or 1 microsecond peaking time), for instance ORTEC 472 or 672. Hunt noise (microphonic noise, radiated noise, noise from the grounds)

66. Kolkata, October 20, 2014P. Colas - Micromegas65 ‘Burning’ or ‘cooking’ your detector To make the detector stable for further operation, it must be ‘cooked’ : raise the voltage slowly to 550-600 V (50 micron gap) or 800-900 V (128 micron gap), step by step, to the level where it starts sparking. This has to be done in air It consists of burning small dusts (mostly fibres). A relatively high (ionic) current (200-250 nA) can remain. It will decrease after circulation of the gas and go down to 0(1nA). A detector which stands its voltage in air will always work in gas.