UC Berkeley

Aluminum-26 (Al-26) is a radioactive isotope produced in massive star processes. High-resolution spectroscopy of its 1.809 MeV gamma-ray (γ-ray) decay signature constrains the dynamics of its emission as it is ejected from its progenitor sites and incorporated into the interstellar medium of the Milky Way Galaxy. Imaging reveals dominant emission in the Inner Galaxy and emission in localized regions of massive star activity. Taken together, spectroscopy and imaging of Al-26 shed light on the chemical evolution of the Galaxy over millions of years.

The Compton Spectrometer and Imager (COSI) is a compact Compton telescope designed to measure astrophysical γ-rays of energy 0.2–5 MeV. Its high-purity germanium detectors track incident photons as they Compton scatter throughout the detector volume. In 2016, COSI flew on a NASA ultra-long duration balloon for 46 days. This dissertation details the first analysis of Al-26 in the flight data and reports a measurement of 3.7σ significance above background and an Inner Galaxy flux of (8.6 ± 2.5) × 10−4 ph cm−2 s−1. All scientific achievements from the flight are predicated on calibrations performed before launch. These calibration procedures were repeated in advance of an intended 2020 balloon flight. Analyzing calibration data validates instrument performance and informs studies of detector effects, including charge sharing and charge trapping, that complicate the measurement process.

The next generation of COSI as a NASA Small Explorer satellite is slated for launch in 2027. It is anticipated to strengthen the spectroscopic measurement of Al-26 in the balloon flight and yield the most detailed images of Al-26 to date. Extensive testing of the imaging algorithm in COSI’s new analysis toolkit is presented as a first step towards producing these images. The desired culmination of these efforts is an enhanced understanding of Galactic Al-26 by way of thorough calibrations, novel insight into undesirable detector effects, and advanced analyses of high-resolution data from the COSI balloon and satellite instruments.