Abstract
Gaseous Electron Multipliers (GEM) are well known for stable operation at high particle fluxes. We present a study of the intrinsic limits of GEMdetectors when exposed to very high particle fluxes of the order of MHz/mm2. We give an interpretation to the variations of the effective gain, which, as a function of the particle flux, first increases and then decreases. We also discuss the reduction of the ion back-flow with increasing flux. We present measurements and simulations of a triple GEM detector, describing its behaviour in terms of accumulation of positive ions that results in changes of the transfer fields and the amplification fields. The behaviour is expected to be common to all multi-stage amplification devices where the efficiency of transferring the electrons from one stage to the next one is not 100%.
Highlights
We present a study of the intrinsic limits of Gaseous Electron Multipliers (GEM) detectors when exposed to very high particle fluxes of the order of MHz/mm2
We present measurements and simulations of a triple GEM detector, describing its behaviour in terms of accumulation of positive ions that results in changes of the transfer fields and the amplification fields
Gaseous Electron Multipliers (GEM) [1] are known to maintain stable gains when exposed to 6 keV X-ray fluxes of at least 106 Hz/mm2 for a single stage device operated at a gain of about 102 [2], and at least 105 Hz/mm2 for double stage devices operated at a gain of 2×104 [3]
Summary
Gaseous Electron Multipliers (GEM) [1] are known to maintain stable gains when exposed to 6 keV X-ray fluxes of at least 106 Hz/mm for a single stage device operated at a gain of about 102 [2], and at least 105 Hz/mm for double stage devices operated at a gain of 2×104 [3]. For triple stage devices it was observed that at high fluxes of soft X-rays the effective gain as a function of the flux first increases and decreases [4]. This effect was assumed to be due to space-charge effects resulting in field distortions in the detector. Considerable efforts have been made to reduce the back-flow of positive ions into the conversion volume of multi GEM detectors to reduce space-charge induced field distortions [5][6]. Further studies [7] show a decrease of the ion back-flow of a triple GEM detector due to ion space-charge when exposed to high X-ray fluxes.
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