Ion Back-Flow Suppression in GEM-MIGAS

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 μm , 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 μm. The studies were carried out with a GEM-MIGAS coupled to a semitransparent CsI-photocathode operated in Ar/5%CH₄ gas mixture at atmospheric pressure. The increase of the induction gap thickness from 50 μm to 300 μm leads to an increase of the maximum achievable charge gain by a factor of 100, from ~ 2 × 10³ to ~ 2 × 10⁵. 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 ~ 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 ~ 1% and ~ 4%, respectively, was obtained for the corresponding charge gains of ~ 5 × 10⁴ and ~ 2 × 10⁵, respectively.