Performance of the light trigger system in the liquid xenon gamma-ray imaging telescope LXeGRIT

LXeGRIT is a balloon-borne Compton telescope for MeV gamma -ray astrophysic s, based on a liquid xenon time projection chamber with charge and light re adout. The energy and direction of an incident gamma -ray are reconstructed from the three-dimensional locations and energy deposits of individual int eractions taking place in the homogeneous detector volume. While the charge signals provide energy information and X-Y-positions, the fast xenon scint illation light signal is used to trigger the detector. The drift time measu rement, referred to the time of the trigger signal, gives the Z-position wi th the known drift velocity. The light is detected by four ultraviolet-sens itive photomultiplier tubes (PMTs). The logical OR of the PMT signals trigg ers the data-acquisition system with an efficiency that depends on the even t energy and location, as well as on the discriminator thresholds used on t he individual PMTS. Results from experiments with a tagged Na-22 source giv e the spatial distribution of the light trigger efficiency for 511-keV gamm a -rays. When averaged over the whole sensitive volume and all PMTS, the tr igger efficiency is 47% or 40% for two discriminator windows used during th e LXeGRIT balloon flight of 1999. These values are strongly affected by the different sensitivity of each PMT. The corresponding average efficiency at 511 keV for the best of the four PMTS is in fact 63% and approaches 100% f or interactions taking place in a small volume right above the PMT.