A VUV gas transmission monitor and recirculating gas system for the PHENIX hadron blind detector

The PHENIX Hadron Blind Detector (HBD) is a high performance Cherenkov counter that uses pure CF₄ to detect electrons in relativistic heavy ion collisions at RHIC. It requires extremely high purity gas in order to achieve a high photoelectron yield, preserve the quality of its cesium iodide photocathodes, and maintain stable gas gain in its GEM detectors. In particular, water and oxygen at even the few ppm level can cause significant optical absorption in the wavelength range from 120-180 nm, which is the region of sensitivity of the Csl photocathodes. In order to ensure good optical transparency and low water and oxygen content of the operating gas, a transmission monitor has been constructed to measure the transmittance of the input and output gases of the HBD. The monitor is based on a McPherson 234/302 VUV spectrometer, along with a custom, computer controlled movable mirror system to measure the transmittance of the gas flowing to and from the detector. The monitor is used in conjunction with a recirculating gas system that is designed to supply high purity gas with minimal consumption. Both the transmission monitor and the recirculating gas system are described, along with their performance when used with the Hadron Blind Detector during the 2006-07 heavy ion run at RHIC.