  LXeGRIT is a balloon-borne Compton telescope for MeV gamma-ray astrophysics,
based on a liquid xenon time projection chamber with charge and light readout.
The energy and direction of an incident gamma-ray is reconstructed from the
three-dimensional locations and energy deposits of individual interactions
taking place in the homogeneous detector volume. While the charge signals
provide energy information and X-Y positions, the fast xenon scintillation
light signal is used to trigger the detector. The drift time measurement,
referred to the time of the trigger signal, gives the Z-position with the known
drift velocity. The light is detected by four UV-sensitive photomultiplier
tubes (PMTs). The logical OR of the PMT signals triggers the data acquisition
system with an efficiency which depends on the event energy and location, as
well as on the discriminator thresholds used on the individual PMTs. Results
from experiments with a tagged 22Na source give the spatial distribution of the
light trigger efficiency for 511 keV gamma-rays. When averaged over the whole
sensitive volume amd all PMTs, the trigger efficiency is 47% or 40% for two
discriminator windows used during the 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% for interactions taking place in a small volume
right above the PMT.
