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 InGrid “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 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 3

Electrons in gases : drift, ionization and avalanche

E Mean free path l =n s (0.4 m 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) At higher fields

ionization

takes place (gain 10 V in 2 m m =50kV/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/ m s (or 50 m m/ns) Higher with CF 4 mixtures Lower with CO 2 mixtures Beijing, January 9, 2008 P. Colas - Micromegas TPC 6

Attachment

electron capture by the molecules N e = N e 0 exp(-az) a can be from m m -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

σ l



C DL .

z σ t



C DT .

z

limits z resolution (typically 200-500 m / √cm) Limits r f resolution at high z (“diffusion limit”) s

T

(

B

)  s

T

(

B

 0 ) 1 + (  )² B field greatly reduces the diffusion  =eB/m e ,  = time between collisions (assumed isotropic)  = from ~1 to 15-20 (note  ~ V drift B/E)

Drift

Langevin equation v(E,B) -> ExB effect Beijing, January 9, 2008 P. Colas - Micromegas TPC 8

Electrons in gases : drift, ionization and avalanche

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

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 9

TPC: Time Projection Chamber

electrons diffuse and 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 x 10 y

Micromegas: How does it work?

Y. Giomataris, Ph. Rebourgeard, JP Robert and G. Charpak, NIM A 376 (1996) 29 S1 Micromesh Gaseous Chamber: a micromesh supported by 50-100 m m insulating pillars, and held at V anode – 400 V 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

) Funnel field lines: electron

transparency

very close to 1 for thin meshes Small gap:

fast

collection of ions S2 S2/S1 = E drift /E amplif ~ 200/60000= 1/300 Beijing, January 9, 2008 P. Colas - Micromegas TPC 11

Beijing, January 9, 2008 P. Colas - Micromegas TPC 12

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

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

Gain uniformity in Micromegas

The nicest property of Micromegas • Gain (=e a d ) • 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.) 16 Beijing, January 9, 2008 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 17

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

K α escape line K β escape line 13.6 % FWHM Gap : 50 μm; Trou, pas : 32 μm, Ø : 14 μm K β removed by using a Cr foil

11.7 % FWHM

Beijing, January 9, 2008 P. Colas - Micromegas TPC 18

2007 MM1_001 prototype

Gain uniformity measurements

Y- vs-X 55 Fe 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 19

Gain uniformity

Inactive pads (V mesh connection) MM1_001 prototype 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

MM0_007: gain uniformity

487 / 1726 tested pads V mesh =

-

350V 7.4 % rms @ 5.9 keV Average resolution = 21% FWHM @ 5.9 keV

Beijing, January 9, 2008 P. Colas - Micromegas TPC 21

Bopp micromesh

AFTER

MM1_002 : gain uniformity and energy resolution

21% FWHM @ 5.9 keV

Measured non-uniformities (%)

ORTEC amplifier : 12 pads / measurement

-

5.6

-

4.7

-

3.9

-

4.4

-

4.4

-

5.8

-

1.0

+

1.6

+

0.6

-

2.8

+

1.4

1.0

+ +

1.4

+

1.4

+

0.0

+

2.8

+

0.8

2.2

+

4.1

+

3.0

+

4.4

+

5.2

+

3.8

+

1.9

RMS = 3.3%

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Transparency

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

Micromesh Gantois Bopp pitch (

m

m)

f

(

m

m) 57 19 63 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 23

Natural suppression of ion backflow

S1

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 = 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.

Beijing, January 9, 2008 P. Colas - Micromegas TPC S2 24

Feedback : theory and simulation

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

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

Theoretical ion feedback Results 500 lpi (sigma/l=0.25) 1000 lpi (sigma/l=0.5) 1500 lpi (sigma/l=0.75)

ion

_

feedback

 2 .

5

field

_

ratio

Beijing, January 9, 2008

ion

_

feedback

 1 .

03

field

_

ratio

P. Colas - Micromegas TPC

ion

_

feedback

 1

field

_

ratio

27

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

Ion backflow measurements X-ray gun V drift I 1 (drift) Primaries+backflow V mesh I 2 (mesh) 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 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 (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 30

Gain and spark rates

E. Mazzucato et al., T2K

95

m

m 128

m

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.

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Discharge probability in a hadron beam

2.5 mm conversion gap 100 µ amplif. gap ~20 ~14 ~10 Ne-C 2 H 6 -CF 4 gain ~ 10 4 P = 10 -6 Note that discharges are not destructive, and can be mitigated by resistive coating Beijing, January 9, 2008

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

Future, pion beam: -remove CF4 -lower the gain -increase the gap to compensate P. Colas - Micromegas TPC 32

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 Electroformed etched Laser etching, Plasma etching…

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) Wowen 200 m m Beijing, January 9, 2008 P. Colas - Micromegas TPC Deposited by vaporization 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 T2K TPC under construction Beijing, January 9, 2008 P. Colas - Micromegas TPC 34

The Bulk technology

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The T2K TPC has been tested successfully at CERN

(9/2007) 36x34 cm 2 1728 pads Pad pitch 6.9x9 mm 2 Beijing, January 9, 2008 P. Colas - Micromegas TPC 36

T2K TPC

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

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 (s avalanche ~12µm) Beijing, January 9, 2008 P. Colas - Micromegas TPC 38

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 39

25 µm mylar with Cermet (1 M W /□) glued onto the pads with 50 µm thick dry adhesive Cermet selection and gluing technique are essential

Al-Si Cermet on mylar Drift Gap MESH Amplification Gap 50

m

m pillars

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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 W /□ and C~1pF per pad area matches µs signal duration.     

t

 1

RC

   2 

r

 2 + 1

r

  

r

    (

r

,

t

) 

RC

2

t e

-

r

2

RC

4

t



(r)

Beijing, January 9, 2008

mm

P. Colas - Micromegas TPC

Q

 (

r,t)

integral over pads

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.

Beijing, January 9, 2008 Mesh voltage (V) 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 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 43 Beijing, January 9, 2008 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 44

Beijing, January 9, 2008 P. Colas - Micromegas TPC 45

4 GeV/c



+

s

x

 s 0 2 +

beam, B=1T (KEK)

C d

2 

z N eff

Effect of diffusion: should become negligible at high magnetic field for a high  gas Beijing, January 9, 2008 P. Colas - Micromegas TPC 46

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 47 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 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

Ar+5% isobutane B=5 T Analysis: Curved track fit P>2 GeV f < 0.05

Beijing, January 9, 2008 P. Colas - Micromegas TPC 51

Resolution = 50 µ independent of the drift distance

‘T2K gas’ Beijing, January 9, 2008 P. Colas - Micromegas TPC 52

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

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

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

Cathode

Principle of the digital TPC

Micromegas Ionizing particle Gas volume Every single ionization electron is + ultimate resolution amplification system (MPGD) + TimePix chip Beijing, January 9, 2008 P. Colas - Micromegas TPC 56

TimePix/Micromegas CERN/Nikhef-Saclay Fenêtre pour sources X Capot 6 cm Fenêtre pour source b Cage de champ Mesh Micromegas Beijing, January 9, 2008 P. Colas - Micromegas TPC Puce Medipix2/TimePix 57

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 Beijing, January 9, 2008 P. Colas - Micromegas TPC 58

SiProt: protection against sparks

NIKHEF Timepix chip + SiProt 20 μm + Micromegas Introduce 228 Th in the gas to provoke sparks 228 Th  220 Rn 6.8 MeV 6.3 MeV 2.5×10 5 e 2.7×10 5 e Ar/Iso (80:20) Mode TOT z = 10 mm V mesh = -420 V Beijing, January 9, 2008 P. Colas - Micromegas TPC 59

SPARKS, but the chip’s still alive

NIKHEF Timepix chip + SiProt 20 μm + Micromegas 228 Th  220 Rn Ar/Iso (80:20) Mode TOT z = 10 mm V mesh = -420 V Beijing, January 9, 2008 P. Colas - Micromegas TPC 60

Beijing, January 9, 2008 P. Colas - Micromegas TPC 61