INTEGRAL/SPI $\gamma$-ray line spectroscopy

TL;DR

This paper analyzes 13.5 years of INTEGRAL/SPI gamma-ray spectra to understand detector degradation, background variations, and spectral response changes over time, aiding in precise background modeling for space-based gamma-ray observations.

Contribution

It provides a comprehensive analysis of the spectral response degradation and background component variations of INTEGRAL/SPI detectors over 13.5 years, including effects of annealing and solar activity.

Findings

Spectral resolution degrades up to 15% within half a year.

Annealing operations recover spectral resolution losses.

Background intensity varies anti-correlated with solar activity.

Abstract

INTEGRAL/SPI is a space-based coded mask telescope featuring a 19-element Germanium detector array for high-resolution gamma-ray spectroscopy, encapsulated in a scintillation detector assembly that provide a veto for background from charged particles. In space, cosmic rays irradiate spacecraft and instrument, which results in a large instrumental background from activation of those materials, and leads to deterioration of the charge collection properties of the Ge detectors. We use 13.5 years of INTEGRAL/SPI spectra for each detector and for each pointing of the satellite. Spectral fits provide us with details about separated background components. From the strongest background lines, we first determine how the spectral response changes with time. We then determine how the instrumental background components change in intensities and other characteristics, most-importantly their relative distribution among detectors. Spectral resolution of Ge detectors in space degrades with time, up to 15% within half a year, consistently for all detectors, and across the SPI energy range. Semi-annual annealing operations recover these losses, yet there is a small long-term degradation. The intensity of instrumental background varies anti-correlated to solar activity, in general. There are significant differences among different lines and with respect to continuum. Background lines are found to have a characteristic, well-defined and long-term consistent intensity ratio among detectors. We use this to categorise lines in groups of similar behaviour. The dataset of spectral-response and background parameters as fitted across the INTEGRAL mission allows studies of SPI spectral response and background behaviour in a broad perspective, and efficiently supports precision modelling of instrumental background.