Calibration of the ComPair Balloon Instrument

TL;DR

ComPair calibrates a prototype AMEGO instrument to quantify MeV gamma-ray performance, focusing on angular resolution, energy resolution, and effective area to benchmark simulations for Compton and pair events. The paper details the four-subsystem design (DSSD Tracker, CZT Calorimeter, CsI Calorimeter, ACD), the dedicated calibration campaign with radioactive sources, and the data-processing pipelines that reconstruct events. Results at 662 keV show energy resolution around 4% and ARM values that validate ComPair’s imaging capabilities, with simulations generally in good agreement though DEE refinements are ongoing. The work provides critical baseline metrics and informs improvements for ComPair-2 and AMEGO, highlighting the need for larger active area and optimized thresholds to reach mission-level performance.

Abstract

ComPair, the prototype of the All-sky Medium Energy Gamma-ray Observatory (AMEGO) mission concept, is a combined Compton imager and pair production telescope. It consists of four subsystems: a double-sided silicon strip detector (DSSD) Tracker, a virtual Frisch-grid cadmium zinc telluride (CZT) Low Energy Calorimeter, a cesium iodide (CsI) High Energy Calorimeter, and a plastic scintillator Anti-Coincidence Detector (ACD) to reject the charged particle background. These subsystems work together to reconstruct events, by tracking the locations and energies of gamma-ray scatters and pair production events. To quantify ComPair's scientific capabilities prior to a balloon launch in 2023, calibrations were performed to benchmark the instrument's performance in terms of angular resolution, energy resolution, and effective area. In this paper we provide an overview of the ComPair instrument and detail the calibration campaign. Finally, we compare our results to the expected performance based on simulations.

Figures (26)

• Figure 1: A CAD model of ComPair with the active area boxed in red. Each subsystem's detectors and electronics are integrated into aluminum enclosures stacked on top of each other, including the DSSD Tracker (magenta), the CZT Calorimeter (teal), and the CsI Calorimeter (orange). Surrounding the instrument stack is the ACD (green) that rejects charged particles.
• Figure 2: ComPair's design enables reconstruction of both Compton and pair events. Below $\sim$10 MeV, photons are more likely to Compton scatter in the Tracker (magenta), before the scattered photon gets absorbed in the CZT Calorimeter (teal). Above $\sim$10 MeV, photons will likely convert into an electron-positron pair in the Tracker, and the pair products are tracked through the CZT Calorimeter before being absorbed in the CsI Calorimeter (orange) Valverde_2023.
• Figure 3: Eight out of 10 layers of ComPair's silicon Tracker stacked on top of the CZT Calorimeter. The DSSD (boxed in red) takes up a quadrant of each layer, with the surrounding readout electronics taking up the other 3 quadrants.
• Figure 4: The inside of ComPair's CZT Calorimeter pressure vessel enclosure. The active detector area, composed of 9 crates, is boxed in red. The HV supply and FPGA are located on the left side of the enclosure.
• Figure 5: An exploded view of a CZT crate, showing the 4$\times$4 CZT bars AMEGO_Kierans.