Date of Award

Winter 2025

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Mark L McConnell

Second Advisor

Amy Keesee

Third Advisor

Fabian Kislat

Abstract

This dissertation presents the end-to-end development, characterization, and in-flight validation of the 3-dimensional Gamma-Ray Polarimeter Experiment (GRAPE), a wide-field-of-view (FoV) Compton polarimeter designed for high-energy astrophysics. The primary scientific motivation is to address unresolved questions in the physics of Gamma-Ray Burst (GRB) jets, for which gamma-ray polarimetry is a critical diagnostic. GRAPE is designed to measure the degree and angle of $\gamma$-ray polarization within a targeted energy band of \SIrange{50}{500}{keV} to cover the peak energy output (E$_{pk}$) of observed GRB spectra. This work establishes the instrument's scientific readiness through two principal contributions. First, the instrument's fundamental low-energy response was characterized through a dedicated monoenergetic photon beam campaign. These measurements provide an experimental validation of the Compton scatter kinematics below 100 keV for this instrument, establishing a low-energy threshold crucial for observing the peak energy output of GRBs. Second, a multi-component calibration framework was developed to correct for the complex thermal gain variations encountered during a high-altitude balloon flight. This calibration involves two main steps: a ground-based thermal model that provides initial adjustments for temperature-induced variations, and an in-flight spectral detrending algorithm that refines corrections by capturing residual systematic effects using real-time data. The successful application of this framework improved the energy resolution of a measurement of the atmospheric 511 keV positron-electron annihilation line, achieving a resolution of approximately 26\% (from $\sim$58\% without correction) at 511 keV. This provides an end-to-end validation of the instrument's spectral range and the analysis pipeline. Together, these contributions demonstrate that GRAPE is a validated, calibrated, and scientifically capable instrument, poised to make significant contributions to the field of GRB science on future long-duration balloon missions.

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