A hadron blind detector for the PHENIX experiment at RHIC

A hadron blind detector (HBD) is being developed for an upgrade of the PHENIX experiment at RHIC. The HBD is a windowless Cherenkov detector, operated with pure CF₄ in a special proximity focus configuration. The detector consists of a 50 cm long radiator, directly coupled to a triple GEM detector which has a CsI photocathode evaporated on the top surface of the uppermost GEM foil, and a pad readout at the bottom of the GEM stack. Detailed studies of the detector performance, including hadron rejection, figure of merit, N₀, number of photoelectrons and efficiency are presented. These studies include measurements performed with a UV lamp, an ⁵⁵Fe X-ray source and an ²⁴¹Am alpha source. Results will also be given on aging studies of the GEM foils and the CsI photocathode in pure CF₄.