The design and performance of the XL-Calibur anticoincidence shield

TL;DR

This paper describes the design, construction, and in-flight performance of the XL-Calibur anticoincidence shield, demonstrating its effectiveness in reducing background noise for X-ray polarimetry during a balloon flight.

Contribution

It provides the first detailed account of the XL-Calibur shield's design, testing, and operational performance in a real flight environment.

Findings

In-flight background rate of ~0.5 Hz in 20-40 keV range

Veto threshold successfully set below 100 keV

Performance aligns with design expectations for scientific goals

Abstract

The XL-Calibur balloon-borne hard X-ray polarimetry mission comprises a Compton-scattering polarimeter placed at the focal point of an X-ray mirror. The polarimeter is housed within a BGO anticoincidence shield, which is needed to mitigate the considerable background radiation present at the observation altitude of ~40 km. This paper details the design, construction and testing of the anticoincidence shield, as well as the performance measured during the week-long maiden flight from Esrange Space Centre to the Canadian Northwest Territories in July 2022. The in-flight performance of the shield followed design expectations, with a veto threshold <100 keV and a measured background rate of ~0.5 Hz (20-40 keV). This is compatible with the scientific goals of the mission, where %-level minimum detectable polarisation is sought for a Hz-level source rate.