Energy calibration of GTM on ground

TL;DR

This paper details the energy calibration process of the GTM instrument on the FS-8B satellite, including calibration methods, challenges faced, and the instrument's energy resolution and linearity in detecting gamma-ray bursts.

Contribution

It introduces a calibration approach for GTM's energy measurement, addressing crosstalk and saturation issues, and provides performance metrics like energy resolution and gain behavior.

Findings

Energy resolution at 662 keV is approximately 16%.

GTM's energy response is linear with photon energy after calibration.

Increasing voltage or decreasing temperature improves gain.

Abstract

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 2+2 CITIROC chips to manipulate 4+4 detectors, which are composed of GAGG(Ce) scintillators coupled with Silicon Photomultipliers (SiPMs) and oriented in various directions to achieve all-sky coverage, the GRB saturation fluences of GTM in the 50 keV to 1 MeV range for Short GRBs (SGRBs) and Long GRBs (LGRBs) were estimated to be about $3.1 \times 10^{-4}$ and $5.0 \times 10^{-3}\ {\rm erg/cm^2}$, respectively, based on simulations. To precisely interpret the GTM readout signal in terms of energy, several measurements for isotope and gain calibration were conducted. Despite encountering issues with crosstalk and SiPM saturation effect in the data, the energy spectrum can still be recovered by appropriately discarding channel noise and mapping with the correct ADC-to-energy relation. This paper summarizes the energy resolution of GTM and the linear variations in the relationship between photon energy and readout signal. At 662 keV, the energy resolution is about 16 %. Also, it demonstrates that greater gain is achieved by increasing voltage or decreasing temperature.