The design and performance of the XL-Calibur anticoincidence shield
Nirmal Iyer, M. Kiss, M. Pearce, T. Stana, Hisamitsu Awaki, R. Bose, A. Dasgupta, G. De Geronimo, Ephraim Gau, Tomohiro Hakamata, M. Ishida, Kota Ishiwata, W. Kamogawa, F. Kislat, T. Kitaguchi, H. Krawczynski, Lindsey Lisalda, Yoshitomo Maeda, H. Matsumoto, A. Miyamoto, T. Miyazawa, T. Mizuno, B. F. Rauch, Nicole Rodriguez Cavero, N. Sakamoto, J. Sato, S. Spooner, H. Takahashi, Mai Takeo, T. Tamagawa, Y. Uchida, Andrew West, K. Wimalasena, M. Yoshimoto
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.