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Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
1
Micromegas TPC R&D Micromegas TPC R&D (and wire chamber)(and wire chamber) Micromegas TPC R&D Micromegas TPC R&D (and wire chamber)(and wire chamber)
• First measurements with a 2T First measurements with a 2T
magnetic fieldmagnetic field
ion feed-backion feed-back
• New studies (New studies (mainlymainly simulations) on: simulations) on: – some gas properties some gas properties
– ion feedbackion feedback
• Cosmic set-up under constructionCosmic set-up under construction
• How to improve the r-φ resolution?How to improve the r-φ resolution?
• ConclusionConclusion
F. Bieser1, R. Cizeron2, P. Colas3, C. Coquelet3,E. Delagnes3, B. Genolini4, A. Giganon3,Y. Giomataris3, G. Guilhem2, S. Herlant3, J. Jeanjean2, V. Lepeltier2, J. Martin3, A. Olivier3, J. Peyré4, J. Pouthas4, Ph. Rebourgeard3, M. Ronan1 (and many others, not mentioned)
1) LBL, 2) LAL Orsay, 3) DAPNIA Saclay, 4) IPN Orsay
ECFA-DESY workshopworkshop
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
2
First measurements in a magnetic fieldFirst measurements in a magnetic fieldFirst measurements in a magnetic fieldFirst measurements in a magnetic field By P. Colas1, Y. Giomataris1, J. Jeanjean2, V. Lepeltier2, J. Martin1, A. Olivier1
1) DAPNIA Saclay 2) LAL Orsay
We took data end of June with a small-gap wire-chamber TPC and a micromegas TPC (both 1cm drift) in a 1-2 Tesla magnetic field
The primary ionization was provided by
•an 55Fe X-ray source (25 MBq, gain typically 100,000) for the wire
chamber
•a 90Sr -ray source (1 GBq, gain a few 100) for the micromegas
The currents from the supplies were monitored, allowing ion feedback
measurements
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
3
55Fe
0 +2kV 0 - 300 V
Cathodegridwires
90Sr
0 -340 V - 640 V
Cathodemeshanode
2mm 2mm 1cm 50 m 1cm
Small-gap wire TPC Micromegas TPC
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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ION FEEDBACK MEASUREMENTS
Vmesh
Vdrift
I2 (mesh)
I1 (drift)
X-ray gun or -source
primary ions + feedback
I1+I2 ~ G x primary current
Obtain primary from G=1 ( at small Vmesh)
Eliminate G between the 2 equations to obtain the feedback fraction
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
5
2T SaclayNMR magnet
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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B//E
The total current (primary ionisation x gain) is reasonably constant with B in the wire chamber case.
The current increase seen with micromegas is very likely due to in increase in primary ionisation
( electrons are spiraling
better electron collection)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
7
The ion feedback does not depend significantly on the magnetic field
It is in quite good agreement with prediction for Ar-CH4 (90:10) and for a 500 lpi grid.
ED/EA
~4xED/EA
90Sr -source
Ar-CH4 10%
expected value
measurement
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
8
The ion feedback does not depend on B for the wire chamber (however the MC predicts a 50% increase from 0 to 2 Tesla)
An effective field ratio can be defined, and the ion feedback is equal to the field ratio .
The feedback is a bit worse for Ar-CH4, as expected from a lower diffusion
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
9
new gas studies (simulations)
amplification properties
the quencher has:
•a very small influence on the optimal gap, which is determined mainly by the dominant atomic mass (argon)
•a large influence on the gain
max. gain at ~25 μm
with Argon
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
10
Stationnarity of the gainStationnarity of the gainStationnarity of the gainStationnarity of the gain
• VeryVery The gain is stationnary (maximal) as a function of the gap around a few 10m.
decrease of the gas atomic mass the optimal gap and the maximum gain
good dE/dx potential
for a micromegas TPC
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
11
Gas studies Constraints on the gas mixture:
Drift properties: to obtain a high drift velocity plateau at low E-field, an Ar-dominated carrier is required (good also for dE/dx)
Hydrogen should be avoided because of neutron background: no CH4
Use of CF4 as a quencher improves T
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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Gas studies
Ar-CF4 (1 to 4%) mixtures ~vxB/E
1% ~24
2% ~ 19
4% ~14
3% ~16
Drift properties:
A plateau drift velocity of ~8 cm/s with E<200 V/cm can be obtained with Ar-CF4 mixtures (confirmed by measurements)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
13
Gas studies
attachment is negligible below 500 V/cm (supported by our measurements) and above 15 kV/cm (dominated by Townsend)
with Micromegas, the transition between drift and multiplication spaces is very short (a few m) so the loss of electrons is expected to be negligible (a few %)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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Ar CF4 (98:2) has many nice properties:
drift velocity plateau at 180 V/cm
with 7 cm/s
good gain
electron attachment is negligible
below ~500V/cm
(need absorption less than 1/(10m)
to be checked with the prototype
was suspected of aging.
--> aging test (presented by Paul C. at St- Malo and Jeju):
NO aging
100
1000
104
105
360 380 400 420 440 460
Gain Ar+2% CF4Edrift=100 V/cm
Ga
in
HV2 (V)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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Ion feed-back modelisation: Ion feed-back modelisation: funnel effectfunnel effect
Ion feed-back modelisation: Ion feed-back modelisation: funnel effectfunnel effect
• VeryVeryS1/S2 ~ Eamplif / Edrift (Gauss theorem)
due to diffusion of the electrons in the avalanche, the ions are unlikely to follow back the field lines to the drift space.
the dominant parameter
is the ratio diff/pitch
typically for 100m and 40kV/cm at 1 atm.:
~12-15 m
ion feed-back and
related space charge
effects are suppressed
S1
S2
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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Hypotheses on the avalanche
Gaussian diffusionPeriodical structure
l
2
Avalanche Dispersion
Ion feedback theoryIon feedback theoryIon feedback theoryIon feedback theory
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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ion feedback
0,2 0,3 0,4 0,5 0,6 0,7 0,8
sigma/l
ion
fe
ed
ba
ck
/ f
ield
ra
tio
Field ratio
Feedback 2D
Feedback 3D
conclusion: the ion feedback is close to its optimum (equal to the field ratio) if /l > 0.5
1000 l.p.i. meshes (for usual gases and 50-100 μm gap)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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ion feedback measurements
as expected scales as 1 / field ratio reasonably well (/l ~0.7)
Ar + 10% isobutane
1500 lpi mesh
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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Building a TPC for a cosmic test in a magnetic Building a TPC for a cosmic test in a magnetic field field
Building a TPC for a cosmic test in a magnetic Building a TPC for a cosmic test in a magnetic field field
• 2 tesla magnet brought to operation end of March 20022 tesla magnet brought to operation end of March 2002
• STAR front-end electronics. STAR front-end electronics. Full wave sampling on 1028 Full wave sampling on 1028
channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits
ADC, 512 time buckets deep, low noiseADC, 512 time buckets deep, low noise
• removable detector endplate removable detector endplate (plan to test micromegas, (plan to test micromegas,
wires, +options for e-cloud spreading)wires, +options for e-cloud spreading)
• Supplies recuperated from LEPSupplies recuperated from LEP
• Copper grid (Nickel was shown to deform strongly in a Copper grid (Nickel was shown to deform strongly in a
magnetic field)magnetic field)
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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TPC for the cosmic test
Field cage
Detector
Front end electronics
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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2x10 mm2 pads
1024 pads
1x10 mm2 pads
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
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STAR READOUT ELECTRONICS
TEST BENCH
Front end cards
Pulse generator
Mother board
Optical link
VME processor
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
23
HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?
1. problem:
the intrinsic optimal r-φ resolution is roughly
σrφ = σtr.diff./Ne , with σtr.diff.(цm) =500xLdrift/(1+22)
2. in our case: Ne ~ 60 for a 6mm-long pad
and ~ 17 with Ar-2%CF4 at 1 atm. , 180V/cm and 4T
for Ldrift ~250cm: tr.diff. =500 m and r = 62 m
“ 100cm: tr.diff. =300 m and r = 40 m
“ 25cm: tr.diff. =150 m and r = 20 m
micr. ~ 20 m
3. (rectangular) optimal pad width l < 3-4 tr.diff in order to spread over 2-3
pads
l ~ 500m ... and 6 millions channels !!!
if l =2mm need to diffuse charge over ~500-600 m
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
24
HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?HOW TO IMPROVE THE r-φ RESOLUTION?
1. Chevron pads
2. Diffusion of electrons AFTER multiplication
3. Resistive sheet
4. Pad segmentation
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
25
DIFFUSION OF ELECTRONS AFTER MULTIPLICATION
------------------------
------------------------
micromegas meshmesh
drift
E~40 kV/cm multiplication
E~ 160V/cm
E~ 8-10 kV/cm diffusion ~1cm ~500m
pad plane
But: 1/ bad electron transparency from multiplication to diffusion region (20%?)
2/ attachment problem with CF4
will be tested soon with Subatech lab (Nantes) at CERN or PSI
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
26
for example: diffuse over 1cm at ~8-10kV/cm ~ 500m
but: attachment+Townsend many + and - ions producing a very long
( >100s !!!) and huge signal Nions~ ~ 2xe2xe1010 ~2000020000
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
27
USE OF A RESISTIVE SHEET
------------------------
micromegas mesh
resistive sheet~1M/
drift
E~40 kV/cm multiplication
E~ 160V/cm
50 m mylar pad plane
idea: the resistive sheet spreads the signal over a larger surface cf Kisten Sachs’s talk at Jeju and Madhu Dixit’s at Prague
will be tested soon at Saclay with a plotted micro-mesh
simple calculations made by Eric Delagnes at Saclay
Prague , November 15-18th, 2002
Vincent Lepeltier Micromegas TPC R&D
28
CONCLUSIONCONCLUSIONCONCLUSIONCONCLUSION• A strong collaboration within the PRC group is building a cosmic
test for a micromegas (and an asymmetric wire chamber) TPCs in a 2T magnetic field.
• In parallel, tests of various aspects of the micromegas behavior are conducted. They allow to assess the potentialities of this technology. Operation of a micromegas device in a magnetic field has been successfully tried for the first time.
• Ion feedback, aging, gain, drift velocities have been studied for
several gases. Ar+2% CF4 seems to be a very promising
mixture for the LC TPC in all these respects.
• Within a few months, 2 new high energy exp’ts using micromegas have successfully started (COMPASS and NA48-KABES at CERN)