| 1. |
- Aoyagi, M., et al.
(författare)
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Systematic effects on a Compton polarimeter at the focus of an X-ray mirror
- 2024
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 158
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Tidskriftsartikel (refereegranskat)abstract
- XL-Calibur is a balloon-borne Compton polarimeter for X-rays in the ∼15–80 keV range. Using an X-ray mirror with a 12 m focal length for collecting photons onto a beryllium scattering rod surrounded by CZT detectors, a minimum-detectable polarization as low as ∼3% is expected during a 24-hour on-target observation of a 1 Crab source at 45° elevation. Systematic effects alter the reconstructed polarization as the mirror focal spot moves across the beryllium scatterer, due to pointing offsets, mechanical misalignment or deformation of the carbon-fiber truss supporting the mirror and the polarimeter. Unaddressed, this can give rise to a spurious polarization signal for an unpolarized flux, or a change in reconstructed polarization fraction and angle for a polarized flux. Using bench-marked Monte-Carlo simulations and an accurate mirror point-spread function characterized at synchrotron beam-lines, systematic effects are quantified, and mitigation strategies discussed. By recalculating the scattering site for a shifted beam, systematic errors can be reduced from several tens of percent to the few-percent level for any shift within the scattering element. The treatment of these systematic effects will be important for any polarimetric instrument where a focused X-ray beam is impinging on a scattering element surrounded by counting detectors.
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| 2. |
- Iyer, Nirmal, et al.
(författare)
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The design and performance of the XL-Calibur anticoincidence shield
- 2023
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 1048
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Tidskriftsartikel (refereegranskat)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.
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