Transcript HIGH RATE BEHAVIOR AND DISCHARGE LIMITS IN MICRO

HIGH RATE BEHAVIOUR AND
DISCHARGE LIMITS
IN MICRO-PATTERN
DETECTORS
Nuclear Instruments and Methods in Physics Research A 424 (1999) 321- 342
A. Bressan, M. Hoch, P. Pagano, L. Ropelewski and F. Sauli
(CERN, Geneva, Switzerland)
S. Biagi
(Univ. Liverpool)
A. Buzulutskov
(Budker Institute for Nuclear Physics, Novosibirsk, Russia)
M. Gruwé
(DESY-Univ. Hamburg, Germany)
G. De Lentdecker
(ULB Bruxelles, Belgium)
D. Moermann
(Univ. Karlsruhe, Germany)
A. Sharma
(GSI Darmsdtadt, Germany)
Presented by Gabriele Croci (CERN-GDD Group)
RD51 Working Group 2 Meeting – December the 10th - CERN
GOAL
• Measure the maximum gain of gaseous
proportional micropattern detectors when
irradiated with high-rate soft X-Rays and
heavely ionizing alpha particles
List of MPGD Tested:
• Micro-strips
• Micromegas
• Micro-dot
• Gas electron multiplier (GEM)
• Micro-CAT or Well
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Discharges in MPGD
High electric field present in a large
fraction or all gaps between anode and
cathode. The field is not uniform and it
is higher at the metal/dielectric
boundaries
y
GEM
y = 25 µm
x
High irradiation rate and/or exposure to
heavily ionizing tracks can induce
transitions from proportional avalanche to
streamer probably followed by a discharge
(harmful and fatal for the electronics)
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Experimental Setup and
Procedures
•
All measurements on variuos kinds of detector
performed in identical conditions (as far as possible)
•
The most appopriate gas used for each detector
1.
Absolute gain calibration: different gain G = Ia/(R*np*e)
recorded for different operating voltages (anodic Ia
current measurement)
2.
Full volume detector irradiation: For each setting of the
X-rays flux, the voltage is increased until reaching
instabilities or discharges
3.
Exposure to heavily ionizing particles
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Ways of discharges development in
MPGDs
• Spontaneous breakdown in absence of
radiation: geometry and position-linked
(essential role of quality and local defects)
• Rate-induced breakdown: reduction of the
maximum operating voltage
• Heavily ionizing tracks exposure:
considerable decrease of the maximum
safe operating voltage
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Spontaneous breakdown in
absence of radiation
The performance of the whole detector is determined by the intrinsic defects of the worst group
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Rate-induced breakdown
Paulo Fonte “The physics of streamer and discharges”; 2 nd RD51 Collaboration meeting Paris 13-15 October 2008
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Exposure to heavily ionizing particles
The gas flow is open to a
bypass containing a thorium
oxide compound. The mixture
is enriched with radon whose
main decay mode produces 6.4
MeV α particles
Measurements of discharge
rate.
A discharge is defined as an
event causing an overload of
the current-limited power
supplies set at a threshold of
about ten times the average
normal current
Discharge probability: fraction of signals with exceedingly large amplitude normalized to alpha flux
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Detectors experimental results (1)
Standard MSGC
Micromegas
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Detectors experimental results (2)
Standard GEM
Conical GEM
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Detectors experimental results (3)
Microcat/WELL
Microdot
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Detectors experimental results (4)
Standard MSGC + Standard GEM
Double GEM
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Summary
Detector
Gain without α’s
irradiation (Max
Voltage)
Maximum Gain before
disch* in presence of
α’s (Dischage limit**)
Stand. MSGC 5000 (590)
2000 (550)
Micromegas
4*104 (470)
3000 (385)
Stand. GEM
5000 (540)
1500 (485)
Conical GEM
NW: 2500 (600)
WN: 3000 (660)
NW: 1500 (570)
WN: 2000 (640)
Microcat/Well
Microdot
6000 (540)
104 (580)
1500 (490)
104 (580)
St MSGC+St GEM
(ΔVGEM = 400 V)
2*105 (Vc=625)
104 (Vc=450)
Double GEM
104 (ΔVGEM1= 460)
104 (ΔVGEM1= 460)
(ΔVGEM2= 400 V)
* (**) Gain (Voltage) just below the first non zero discharge probability
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Detector experimental results:
GEM (1)
Gain and discharge probability on irradiation with alpha particles for the single, double and triple GEM
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Detector experimental results: Sectored 10x10 cm2
GEM
Resistor partition
network used to
power a sectored
GEM
Discharge propagation probability as a
function of induction field for
a sectored GEM.
Discharge signals on anodes for increasing GEM
capacitance, obtained by grouping one to four sectors.0
S. Bachman et al, Discharge studies and prevention in the gas
electron multiplier (GEM), Nucl. Instrum. Methods A479(2002)294
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Conclusions (1)
• The difference in max gain reached in a low
irradiation environment shown by different single
stage devices tends to vanish in presence of
heavily ionizing particles.
• In this conditions all single stage devices but
microdot shown a non-negligeble probability of
transition from avalanche to streamer at gain
between 1000 and 3000
• This transition begin to occur when the average
avalanche size exceeds 2-3 107 electrons (close
Raether limit)
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Conclusions (2)
• Sharing the amplification results in a shift
upwards by at least an order of magnitude
of the maximum gain
This may be explained by:
– Field strength dependence of Raether limit
(higher for lower electric field)
– Reduction of charge density induced by
additional spread due to diffusion in double
devices
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