Characterisation of a prototype gamma-ray spectrometer for the COMCUBE-S CubeSat swarm

Sammanfattning

COMCUBE-S (Compton Telescope CubeSat Swarm) is a proposed mission aimed at understanding the radiation mechanisms of ultra-relativistic jets from Gamma-Ray Bursts (GRBs). It consists of a swarm of ∼27 16U CubeSats in low-Earth orbit designed to perform time-resolved spectroscopy and polarimetry, providing continuous full-sky coverage. The mission has completed the feasibility study phase (Phase A) with the European Space Agency. Each satellite carries a BGO Spectrometer Unit (BSU), consisting of a cuboid BGO crystal read out by SiPM arrays at both ends. The BSU extends the spectral coverage up to 10 MeV, beyond the primary Compton telescope. A breadboard implementation of the BSU has been assembled at KTH to characterise its performance and establish a baseline ahead of the next development phase. Energy calibration was established using sealed radioactive sources and validated against ambient background lines, yielding reconstruction errors of 3 % to 8 %. The effective dynamic range was confirmed to meet the BSU design requirement. The energy resolution of individual detector arrays at each end of the BGO crystal reaches approximately 32% a 1 MeV. However, combining the signals from both crystal end-faces substantially improves the resolution to approximately 21% at 1 MeV, closely approaching the COMCUBE-S spectroscopic requirement of 20% at 1 MeV. The dead time of the readout chain was characterised independently through a pulsed LED method and inter-arrival time analysis of 137Cs source data, with both approaches yielding compatible results of τ = (192 ± 5) μs per event. These results identify gain non-uniformity in the SiPM readout as the principal obstacle to meeting the mission requirements, and provide a quantitative baseline to guide the next stage of BSU development, where improvements are expected to yield compatibility with all design requirements.