A hadron blind detector for the PHENIX experiment at RHIC

A hadron blind detector (HBD) is to be installed in the PHENIX experiment for Run7, starting in fall 2006. The HBD is a threshold Cerenkov detector designed to measure electrons in a field free region surrounding the collision vertex. The HBD's primary purpose is to tag background electrons originating from photon conversion and Dalitz decays. These background electrons can subsequently be excluded from a dilepton analysis, thereby reducing the combinatorial background by up to a factor of 100. The detector is realized by a proximity-focus windowless Cerenkov detector operating with pure CF4 and read out by a reflective photocathode consisting of a stack of gas electron multipliers (GEM), the first of which is coated with 300 nm of CsI. The avalanche charge from the GEM stack is collected on a PCB with pads having similar size to the Cerenkov blob. Dalitz and photon conversions are tagged by having twice the amplitude of a single blob. An excellent Quantum Efficiency of the photocathode is thus crucial to the success of the device. Outstanding results have been accomplished using a vacuum evaporation facility on loan from INFN Rome. We will give an overview of the experimental methods to identify blobs and techniques developed for photocathode production. (orig.)