  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.
