ar X iv : a st ro - p h / 02 11 58 4 v 2 4 D ec 2 00 2 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 cm 3 , 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 (27 hr) balloon experiment, the LXeGRIT TPC was not surrounded by any γ-ray or charged particle shield. Single site events and charged particles were mostly rejected 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. 1. LXEGRIT 2000 BALLOON FLIGHT The Liquid Xenon Gamma Ray Imaging Telescope (LXeGRIT) uses a Liquid Xenon Time Projection Chamber (LXeTPC) to image MeV γ-rays from the energy deposits and 3D spatial coordinates, measured on an event-by-event basis. The instrument and results from laboratory and previous balloon flight experiments in 1997 and 1999 are discussed in various references. 1–5 Here we focus on an analysis of data acquired during the last LXeGRIT balloon flight, when the LXeTPC was used without any γ-ray or charged particle shield. The flight took place on Oct 4–5, 2000, from Ft. Sumner, NM and lasted 27 hours, including ascent. Fig. 1 shows the altitude of the balloon payload and the corresponding atmospheric depth, which varies between 5.7 and 3.2 g cm −2. The zenith distance of known celestial γ-ray sources during flight is shown in Fig. 2. The Crab nebula, among them the brightest in γ-rays, was in the LXeGRIT field of view-which we define within an angular distance of …