The TIGRE desktop prototype results for 511 and 900 keV gamma rays

A small desktop prototype of the Tracking and Imaging Gamma-Ray Experiment (TIGRE) has been assembled and tested at 511 keV and 900 keV. TIGRE was designed to observe cosmic gamma ray sources at energies of 0.3 to 100 MeV. Its major feature is its use of multi-layer silicon strip detectors to track Compton recoil electrons and positron-electron pairs. Our small prototype consists of 7 double sided silicon strip detectors 3.2 cm×3.2 cm×300 micron with 1 mm pitch in both the x and y directions. The direction and energy of the Compton scattered gamma ray is measured with small CsI(Tl) photodiode detectors. Knowing the energy and momentum of the scattered electron and scattered photon allows us to determine the incident direction uniquely. In the small prototype 36 CsI(Tl) crystals of 1 cm×1 cm×1.7 cm were used. Non-tracked events, those interacting in only a single silicon plane, can only be determined to within the Compton scatter ring. The silicon strips were calibrated using the 60 keV photons from Am²⁴¹ and the Landau peak obtained from a Sr⁹⁰ beta source. The energy resolution of the silicon was measured to be 8 keV (1σ) at 60 keV and 7.8% FWHM for CsI at 900 keV. Total energy resolutions at 511 and 900 keV were measured to be 11% and 8.9% FWHM respectively. An important requirement of TIGRE will be its ability to separate the upward moving gamma rays produced by cosmic ray interactions in the atmosphere from the downward moving gamma rays. For tracked events this is done by defining a direction of motion (DOM) parameter for the electron by its energy deposition and multiple scattering in the silicon layers. Measurements at 511 and 900 keV show that the DOM parameter is correctly predicted at 70% and 75% for tracked events which constitute 9% and 20% of the data. Monte Carlo simulations show similar results and show the percentage increasing to 98% at 6 MeV in which nearly all of the events are tracked