The effect of the first dynode's geometry on the detection efficiency of a 119EM electron multiplier used as a highly charged ion detector

The ion counting as well as the current gain of an electron multiplier type 119EM with BeCu dynodes having a Venetian blind structure was measured in terms of the dependence on the position where ions impact the surface of the first dynode. The dependence of the 119EM's detection efficiency on the impact position exhibited large variations across the Venetian blind of the first dynode. The highest detection efficiency was localized at the first dynode's surface near its output area into the second dynode region. The lowest one was measured at the input area of the first dynode region that is far from the second dynode. The measurements also show that the 119EM is not very reliable for ion counting. The analog particle gain derived from the mean current gain measured across a single slat increases with increasing charge state as well as with ion energy for Coq+(10 less than or equal to q less than or equal to 26) and Taq+(14 less than or equal to q less than or equal to 34) ions with kinetic energy per charge from 33 keV/q to 163 keV/q. The gains were derived from comparison with the Faraday cup measurements. (C) 1999 Published by Elsevier Science B.V. All rights reserved.