1. HIGH RATE BEHAVIOUR AND DISCHARGE LIMITS IN 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 10 th - CERN

2. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN2 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

3. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN3 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

4. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN4 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 = I a /(R*n p *e) recorded for different operating voltages (anodic I a 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

5. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN5 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

6. Spontaneous breakdown in absence of radiation Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN6 The performance of the whole detector is determined by the intrinsic defects of the worst group

7. Rate-induced breakdown Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN7 Paulo Fonte “The physics of streamer and discharges”; 2 nd RD51 Collaboration meeting Paris 13-15 October 2008

8. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN8 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

9. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN9 Detectors experimental results (1) Standard MSGCMicromegas

10. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN10 Detectors experimental results (2) Standard GEMConical GEM

11. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN11 Detectors experimental results (3) Microcat/WELLMicrodot

12. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN12 Detectors experimental results (4) Standard MSGC + Standard GEM Double GEM

13. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN13 Summary Detector Gain without α’s irradiation (Max Voltage) Maximum Gain before disch* in presence of α’s (Dischage limit**) Stand. MSGC5000 (590)2000 (550) Micromegas4*10 4 (470)3000 (385) Stand. GEM5000 (540)1500 (485) Conical GEM N  W: 2500 (600) W  N: 3000 (660) N  W: 1500 (570) W  N: 2000 (640) Microcat/Well6000 (540)1500 (490) Microdot10 4 (580) St MSGC+St GEM (ΔV GEM = 400 V) 2*10 5 (V c =625) 10 4 (V c =450) Double GEM (ΔV GEM2 = 400 V) 10 4 (ΔV GEM1 = 460) * (**) Gain (Voltage) just below the first non zero discharge probability

14. Detector experimental results: GEM (1) Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN14 Gain and discharge probability on irradiation with alpha particles for the single, double and triple GEM

15. Detector experimental results: Sectored 10x10 cm 2 GEM Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN15 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 for a sectored GEM. S. Bachman et al, Discharge studies and prevention in the gas electron multiplier (GEM), Nucl. Instrum. Methods A479(2002)294

16. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN16 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 10 7 electrons (close Raether limit)

17. Gabriele Croci - RD51 WG2 Meeting - December the 10th 2008 - CERN17 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