HIGH RATE BEHAVIOUR AND DISCHARGE LIMITS IN MICRO-PATTERN DETECTORS

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


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


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


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


The performance of the whole detector is determined by the intrinsic defects of the worst group

Rate-induced breakdown


Paulo Fonte “The physics of streamer and discharges”; 2nd RD51 Collaboration meeting Paris 13-15 October 2008


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


Detectors experimental results (1)Standard MSGC Micromegas


Detectors experimental results (2)

Standard GEM Conical GEM


Detectors experimental results (3)

Microcat/WELL Microdot


Detectors experimental results (4)Standard MSGC + Standard GEM Double GEM


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)


Gain and discharge probability on irradiation with alpha particles for the single, double and triple GEM

Detector experimental results: Sectored 10x10 cm2 GEM


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


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)


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

HIGH RATE BEHAVIOUR AND DISCHARGE LIMITS IN MICRO-PATTERN DETECTORS

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