SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy\nrange 50ââ?¬â??500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray\nburst prompt emission through polarisation measurements. Optimising the signal-to-background\nratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is\nused in this work. From the simulation, the total background outside the South Atlantic Anomaly\n(SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma\nrays, which contribute âË?¼60% and âË?¼35% of the total background, respectively. The background from\nalbedo neutrons and primary and secondary cosmic rays is negligible. The delayed background\ninduced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one\nyear. The resulting total background level of âË?¼513 counts/s allows the polarisation of âË?¼50 GRBs with\nminimum detectable polarisation less than 30% to be determined during the two-year mission lifetime
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