On-ground energy calibration of the Gamma-ray Transients Monitor (GTM) of the Formosat-8B satellite

The Gamma-ray Transients Monitor (GTM) on board the Formosat-8B (FS-8B) satellite is designed to detect and localize Gamma-Ray Bursts (GRBs). By utilizing two identical detector units with four sensor modules each, which are composed of GAGG(Ce) scintillators coupled with Silicon Photomultipliers (SiPMs) and oriented in various directions, the all-sky coverage is achieved. The GRB saturation fluences of GTM in the 50 keV to 1 MeV range for Short GRBs (SGRBs) and Long GRBs (LGRBs) are estimated to be about $3.1\times 10^{-4}$ and $5.0\times 10^{-3}\mathrm{erg}/{\mathrm{cm}}^2$, respectively, based on simulations. To precisely interpret the GTM readout signal in terms of energy, several measurements with isotopes and gain calibration were conducted to calibrate the ADC-to-energy relation of all readout channels. With careful analyses for crosstalk and SiPM saturation effects in the data, the energy spectrum can be mapped with a good readout ADC-to-energy relation. An approximate 16% energy resolution is achieved at the 662 keV gamma-ray energy.