A new light readout system for the LXeGRIT time projection chamber

LXeGRIT is a liquid xenon time projection chamber (LXeTPC) used as balloon-borne Compton telescope for imaging cosmic sources in the MeV energy band. The three-dimensional position sensitive charge readout is triggered by the xenon scintillation light. In the original chamber design, the light is detected by four UV sensitive, 2ʹʹ PMTʹs (EMI 9813), coupled to the liquid xenon vessel by quartz windows. In order to improve the trigger efficiency and uniformity, a new light readout system has been studied. It consists of 12 UV sensitive, compact 2ʹʹ PMTʹs (Hamamatsu R6041Q), mounted in Teflon frames which cover the four sides of the LXeTPC active volume. These all-metal PMTʹs were especially developed to work at liquid xenon temperature and up to 3.5 atm overpressure. Light simulations promise an increase in light collection efficiency by more than a factor of ten for the new readout. We present simulations of the light collection efficiency and initial results from the successful operation of the new PMT fully immersed in liquid xenon.