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
$\mu{\rm m}$
, 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.