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Compton Recoil Electron Tracking With the TIGRE Gamma-Ray Balloon Experiment

Abstract

Conventional Compton telescopes use a single converter and a calorimeter that results in azimuthal uncertainty in the imaging analysis. The University of California, Riverside's (UCR) Tracking and Imaging Gamma Ray Experiment (TIGRE) was designed to minimize the directional uncertainty of the incident photons by using the silicon converter simultaneously as a photon scatterer and an electron tracker to determine the recoil electron path. TIGRE's initial flight was conducted in June, 2007 from Ft. Sumner in New Mexico. This thesis discusses the instrument's design, calibration, flight, and data analysis for Compton events. The time interval of the flight data selected for this analysis was 18,436 seconds. The expected efficiencies of the TIGRE instrument in a balloon-borne stratospheric environment, obtained from MEGAlib, a version of the GEANT 4 high energy simulator, ranged from 0.09% to 0.87% in the (0.3 - 50) MeV energy range. However, the New Mexico flight yielded only 6.5% of the predicted number of Compton candidates which was attributed primarily to the lower number of functional detectors available during the flight. A repeat simulation with realistic telescope conditions provided the expected number of Compton events that matched the flight results within errors.

To take advantage of the tracking feature, one of the necessary procedures is to determine the Direction of Motion (DOM) of the electron track. The Pearson Correlation was used to determine the DOM. This method was successfully tested and verified by using a 90Sr calibration data-set. Using this method, the flight data had 698 downward electron tracks and 862 upward electron tracks, but this result was not successful in reconstructing the Compton events. The Compton angle, determined with the electron tracks, and the Compton angles, determined by the classic Compton formula, did not correlate. The discrepancy came from the low quality NaI calorimeters, its timing window with the silicon and the anti-coincident detectors, and an accidental inclusion of a module clear code within the flight software. The analysis work undertaken for this thesis allowed us to correct and update the telescope circuitry and software for the Australia balloon flight that was carried out in the spring of 2010 (the Australian flight results are not a part of this thesis).

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