  A Hadron Blind Detector (HBD) is being developed for the PHENIX experiment at
RHIC. It consists of a Cherenkov radiator operated with pure CF4 directly
coupled in a windowless configuration to a triple-GEM detector element with a
CsI photocathode and pad readout. The HBD operates in the bandwidth 6-11.5
eV(110-200 nm). We studied the detector response to minimum ionizing particles
and to electrons. We present measurements of the CsI quantum efficiency, which
are in very good agreement with previously published results over the bandwidth
6-8.3 eV and extend them up to 10.3 eV. Discharge probability andaging studies
of the GEMs and the CsI photocathode in pure CF4 are presented.
