Collection of photoelectrons from a CsI photocathode in triple GEM detectors

A study has been made of the parameters affecting the extraction and collection of photoelectrons from the surface of a CsI photocathode in a triple GEM detector. The purpose of this study was to optimize the photoelectron collection efficiency and GEM operating conditions for the PHENIX Hadron Blind Detector (HBD) at RHIC. The parameters investigated include the electric field at the surface of the photocathode, the voltage across the GEM, the electric field below the GEM, the medium into which the photoelectrons are extracted (gas or vacuum), and the wavelength dependence of the extraction efficiency. A small, calibrated light source, or “scintillation cube”, was used to measure the photoelectron yield from the CsI photocathode using ²⁴¹Am particles to produce scintillation light in CF4 and an ⁵⁵Fe source to measure the GEM gas gain. The absolute scintillation light yield (photons/MeV) in CF4 gas was also determined using a CsI photocathode GEM detector. Results are presented on the study of the parameters affecting the photoelectron collection efficiency, the construction and calibration of the scintillation cube, and the measurement of the absolute scintillation light yield in CF4.