Transcript Micromegas TPC

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
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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
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Electrons in gases : drift, ionization and avalanche
Typical (thermic) energy of an electron in a gas: 0.04 eV
E
Mean free path l=ns
(0.4 mm at 1eV)
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)
magboltz
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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
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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
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Attachment
Ne = Ne0 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 : 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
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Diffusion
σl  CDL. z
limits z resolution (typically 200-500 m/√cm)
σt  CDT. z
Limits rf resolution at high z (“diffusion limit”)
sT
(B)

sT ( B  0 )
1 + ( )²
B field greatly reduces the diffusion
=eB/me,  = time between collisions
(assumed isotropic)
 = from ~1 to 15-20 (note  ~Vdrift B/E)
Drift
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Langevin equation v(E,B) -> ExB effect
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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.
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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
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y
x
Localization in time and x-y
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Micromegas: How does it work?
Y. Giomataris, Ph. Rebourgeard,
JP Robert and G. Charpak,
S1
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)
Funnel field lines: electron transparency
very close to 1 for thin meshes
Small gap: fast collection of ions
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S2
S2/S1 = Edrift/Eamplif ~ 200/60000= 1/300
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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
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Gain
Gain of Ar mixtures measured with Micromegas (D.Attié, PC, M.Was)
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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
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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.)
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Gain stability
Very good gain stability (G. Puill et al.)
Optimization in progress for CAST
<2% rms over 6 months
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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 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
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Kβ removed by using
a Cr foil
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11.7 %
FWHM
18
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
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Gain uniformity
MM1_001 prototype
Inactive pads (Vmesh connection)
55Fe
source
near
module edge
55Fe
source
near
module centre
Gain uniformity within a few %
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MM0_007: gain uniformity
487 / 1726 tested pads
Vmesh = -350V
7.4 % rms
@ 5.9 keV
Average resolution = 21% FWHM
@ 5.9 keV
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MM1_002 : gain uniformity
and energy resolution
Measured non-uniformities (%)
Bopp micromesh
AFTER
ORTEC amplifier : 12 pads / measurement
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
RMS = 3.3%
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Transparency
Collection efficiency reaches a plateau (100%?) at high enough field ratio
Micromesh
Gantois
Bopp
pitch
(mm)
57
63
f
(mm)
19
18
Operation point of MicroMegas detectors
in T2K is in the region where high
micromesh transparencies are obtained
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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
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.
S2
This has been experimentally
thoroughly verified.
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Feedback : theory and simulation
Hypothesis on the avalanche
Periodical structure
Gaussian diffusion
Avalanche
Resolution
2s
l
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ion backflow calculation
Sum of gaussian diffusions
2D
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3D
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Theoretical ion feedback
Results
500 lpi (sigma/l=0.25)
ion _ feedback
2.5
field _ratio
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1000 lpi (sigma/l=0.5)
ion _ feedback
1.03
field _ratio
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1500 lpi (sigma/l=0.75)
ion _ feedback
1
field _ratio
27
Ion backflow (theory)
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Ion backflow measurements
X-ray gun
Vdrift
I1 (drift)
Primaries+backflow
Vmesh
I2 (mesh)
I1+I2 ~ G x primaries
One gets the primary ionisation from the drift
current at low Vmesh
The absence of effect of the
magnetic field on the ion
backflow suppression has
been tested up to 2T
One eliminates G and the backflow from the 2
equations
P. Colas, I. Giomataris and V. Lepeltier, NIM A 535 (2004) 226
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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)
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Gain and spark rates
E. Mazzucato et al., T2K
95mm
128mm
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
Number of discharges per hadron
Ne-C2H6-CF4
gain ~ 104
P = 10-6
<Z> ~20
<Z> ~14
<Z> ~10
Future, pion
beam:
-remove CF4
-lower the gain
Note that discharges
are not destructive,
and can be mitigated
by resistive coating
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-increase the gap
to compensate
D.Thers et al. NIM A 469 (2001 )133
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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
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)
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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
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The Bulk technology
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The T2K TPC
has been
tested
successfully
at CERN
(9/2007)
36x34 cm2
1728 pads
Pad pitch
6.9x9 mm2
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T2K TPC (beam test events)
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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)
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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
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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 mm 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 MW/□ and C~1pF per pad area matches µs
signal duration.
 1 2  1  

 2+

t RC  r
r r 
 (r,t) 
RC
2t e
-r 2 RC
4t
(r)
Q
(r,t) integral
over pads

mm
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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)
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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
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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
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4 GeV/c + beam, B=1T (KEK)
Cd2  z
s x  s0 +
N eff
2
Effect of diffusion:
should become negligible
at high magnetic field for
a high  gas
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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
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The gain is independent of the
magnetic field until 5T within 0.5%
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Pad Response Function
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Residuals
in z slices
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• Resolution = 50 µ independent of the drift distance
Analysis:
Ar+5% isobutane
Curved track fit
B=5 T
P>2 GeV
f < 0.05
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Resolution = 50 µ independent of the drift distance
‘T2K gas’
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Average residual vs x position
Before bias correction
After bias correction
±20 m
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• 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
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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
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Principle of the digital TPC
Micromegas
Cathode
Ionizing particle
Gas
volume
amplification system (MPGD)
+
Every single ionization electron is
detected with an accuracy matching~50
theµm
80 kV/cm
avalanche size -> maximal information,
ultimate resolution
-
+
-
TimePix chip
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TimePix/Micromegas
CERN/Nikhef-Saclay
Fenêtre pour sources X
Capot
6 cm
Fenêtre pour
source b
Cage de champ
Mesh
Micromegas
Puce Medipix2/TimePix
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Timepix chip
65000 pixels
(500 transistors
each)
+ SiProt 20 μm
+ Micromegas
55Fe
Ar/Iso (95:5)
Mode Time
z = 25 mm
Vmesh = -340 V
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SiProt: protection against sparks
NIKHEF
Timepix chip
+ SiProt 20 μm
+ Micromegas
Introduce 228Th
in the gas to
provoke sparks
228Th220Rn
6.3 MeV
6.8 MeV
2.5×105 e2.7×105 e-
Ar/Iso (80:20)
Mode TOT
z = 10 mm
Vmesh = -420 V
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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
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