The design and performance of the XL-Calibur anticoincidence shield

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