On the background rate in the LXeGRIT Instrument during the 2000 Balloon Flight

LXeGRIT is the first prototype of a novel Compton telescope for MeV γ-ray astrophysics based on a Liquid Xenon Time Projection Chamber (LXeTPC), sensitive in the energy band of 0.15-10 MeV. In this homogeneous, 3D position sensitive detector, γ-rays with at least two interactions in the sensitive volume of 2800 cm3, are imaged as in a standard Compton telescope. γ-rays with a single interaction cannot be imaged and constitute a background which can be easily identified and rejected. Charged particles and localized β-particles background is also easily suppressed based on the TPC localization capability with millimeter resolution. A measurement of the total γ-ray background rate in near space conditions and the background rejection power of the LXeTPC was a primary goal of the LXeGRIT balloon flight program. We present here a preliminary analysis addressing this question, based on balloon flight data acquired during the Oct 4-5, 2000 LXeGRIT balloon flight from Ft. Sumner, NM. In this long duration balloon experiment, the LXeGRIT TPC was not surrounded by any γ-ray or charged particle shield. Single site events and charged particles were mostly rejcted on-line at the first and second trigger level. The remaining count rate of single-site γ-ray events, at an average atmospheric depth of 3.2 g cm-2, is consistent with that expected from atmospheric and diffuse γ-ray background, taking into account the instrument mass model and response.