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Ion Back-Flow Suppression in GEM-MIGAS

2010; Institute of Electrical and Electronics Engineers; Linguagem: Inglês

10.1109/tns.2010.2084591

1558-1578

Ana Sofia Conceicao, J.A. Mir, J.M. Maia, J.M.F. dos Santos,

Photocathodes and Microchannel Plates

The Gas Electron Multiplier with a MIcromegas Gap Amplifying Structure (GEM-MIGAS) is obtained by the coupling of a GEM to a short induction gap, typically 50 $\mu{\rm m}$ , where additional charge multiplication occurs. In this work, the GEM-MIGAS gain and ion back-flow are investigated, for induction regions in the range of 50–300 $\mu{\rm m}$ . The studies were carried out with a GEM-MIGAS coupled to a semitransparent CsI-photocathode operated in ${\rm Ar}/5\%{\rm CH}_{4}$ gas mixture at atmospheric pressure. The increase of the induction gap thickness from 50 $\mu{\rm m}$ to 300 $\mu{\rm m}$ leads to an increase of the maximum achievable charge gain by a factor of 100, from $\sim 2\times 10^{3}$ to $\sim 2\times 10^{5}$ . Moreover, the high field ratio between amplification and conversion regions, which prevents ions to drift towards the conversion region, allows a strong reduction, by a factor $\sim$ 20, of the ion back-flow ratio to the drift region, when compared with the operation in GEM-mode (i.e. at low induction field). For typical drift fields of 0.1 and 0.5 kV/cm, an ion back-flow fraction $\sim$ 1% and $\sim$ 4%, respectively, was obtained for the corresponding charge gains of $\sim 5\times 10^{4}$ and $\sim 2\times 10^{5}$ , respectively.