The TIGRE prototype results for 511 and 900 keV gamma rays

A small prototype of the Tracking and Imaging Gamma-Ray Experiment (TIGRE) has been assembled and tested at 511 keV and 900 keV. TIGRE uses multi-layers of silicon strip detectors both as a gamma ray converter and to track Compton recoil electrons and positron-electron pairs. Our 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 so that an unique direction and energy can be found for each tracked event. In the 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% FWHM. An important requirement of TIGRE will be its ability to separate atmospheric induced upward gamma rays from downward gamma rays within the field of view. For tracked events this is done by defining a Direction of Motion (DOM) parameter far 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 60% and 65% for tracked events which constitute 5.5% and 11% of the data. Monte Carlo simulations show similar results and shows the percentage increasing to 98% at 6 MeV in which nearly all of the events are tracked