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HIGH RATE BEHAVIOUR AND DISCHARGE LIMITSIN MICRO-PATTERN
DETECTORS
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)
Nuclear Instruments and Methods in Physics Research A 424 (1999) 321- 342
Presented by Gabriele Croci (CERN-GDD Group)
RD51 Working Group 2 Meeting – December the 10th - CERN
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 2
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
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 3
Discharges in MPGD
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)
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
GEM
y
x
y = 25 µm
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 4
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
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 5
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
Spontaneous breakdown in absence of radiation
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 6
The performance of the whole detector is determined by the intrinsic defects of the worst group
Rate-induced breakdown
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 7
Paulo Fonte “The physics of streamer and discharges”; 2nd RD51 Collaboration meeting Paris 13-15 October 2008
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 8
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
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 9
Detectors experimental results (1)Standard MSGC Micromegas
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 10
Detectors experimental results (2)
Standard GEM Conical GEM
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 11
Detectors experimental results (3)
Microcat/WELL Microdot
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 12
Detectors experimental results (4)Standard MSGC + Standard GEM Double GEM
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 13
SummaryDetector 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 NW: 2500 (600)
WN: 3000 (660)
NW: 1500 (570)
WN: 2000 (640)
Microcat/Well 6000 (540) 1500 (490)
Microdot 104 (580) 104 (580)St MSGC+St GEM (ΔVGEM = 400 V)
2*105 (Vc=625) 104 (Vc=450)
Double GEM(ΔVGEM2= 400 V)
104 (ΔVGEM1= 460) 104 (ΔVGEM1= 460)
* (**) Gain (Voltage) just below the first non zero discharge probability
Detector experimental results: GEM (1)
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 14
Gain and discharge probability on irradiation with alpha particles for the single, double and triple GEM
Detector experimental results: Sectored 10x10 cm2 GEM
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 15
Resistor partition network used to power a sectored GEM
Discharge signals on anodes for increasing GEM capacitance, obtained by grouping one to four sectors.0
Discharge propagation probability as a function of induction field fora sectored GEM.
S. Bachman et al, Discharge studies and prevention in the gas electron multiplier (GEM), Nucl. Instrum. Methods A479(2002)294
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 16
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)
Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN 17
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