## 7 Characterisation Simulations
### Simulations and Comparison with Physical Measurements  
Two groups of simulations were performed to compare with the GMOD FM physical measurements and investigate the accuracy of the EIRSAT-1 MEGAlib model. Initially, the 12 h integrations were simulated using MEGAlib. The activity of the source is required as a simulation input; therefore, the response of GMOD to the five sources with known activities from Table 1 were simulated, i.e., \({}^{241}\)Am, \({}^{137}\)Cs, \({}^{133}\)Ba, \({}^{60}\)Co and \({}^{22}\)Na. The measurement duration, source activity and position relative to the centre of the spacecraft were input into the Cosima source file. Cosima was run to generate all the particle interactions. Revan was then used to process the Cosima output file, along with the spacecraft and instrument geometry, to produce a spectrum recorded by the simulated detector. Figure 19 presents a comparison of the simulated spectra (teal) and the spectra produced by the physical FM (grey) for each source. Both spectra are displayed in counts/keV/hour. As expected, the number of counts in the simulated photopeaks does not exactly match that of the physical measurements; these differences can be accounted for with uncertainties in the activities and positions of the sources. As most evident in the \({}^{137}\)Cs spectrum, the additional low energy counts in the region of \(\sim\) 50 keV to 250 keV in the physical measurements can be attributed to scattering from the surrounding environment. The MEGAlib simulation assumes that the spacecraft is in a vacuum, so there are no testing mounts or walls to scatter the gamma rays. The photopeak of the gamma-ray line at 59.4 keV in the measured \({}^{241}\)Am spectrum (grey spectrum in the last panel of Figure 19) has an unusual profile, as this peak is at the energy threshold of the physical flight model of GMOD. Prior to FM vibration, GMOD was capable of detecting the full peak at 59.4 keV. After FM vibration testing, a shift in energies down in the channel number was observed in the GMOD spectra due to a lower current being produced by the SiPMs. A lower current could be caused by a lower light collection or an electronic issue with the SiPM bias voltage. As the bias voltage has been measured and confirmed to be configured correctly, the issue is attributed to the lower light collection. An optical coupling issue would cause lower light collection, e.g., trapped air between the scintillator and SiPMs. However, as the spacecraft had been qualified after the FM environmental testing, it could not be disassembled to confirm this possible cause or rectify it. Figure 18: The GMOD GTRB modelled in MEGAlib Geomega included above the detector assembly (LHS) and then within the full spacecraft model (RHS) where the position of the detector assembly has also been updated to reflect the flight model. The peaks of the simulated spectra were fitted using the same method as followed for the measured spectra to determine the relationship between \(\sigma\) and energy, as shown in Figure 20. The relationship described by Equation (5) was fitted to the simulated data points (teal) with the fit coefficients \(b=0.6810\) and \(c=9.6791\times 10^{-3}\). The model is in good agreement with the relationship fit to the measured spectra (grey solid line). The energy resolution, in terms of FWHM (%), of the simulated detector was then determined using these \(\sigma\) values and Equation (3). The energy resolutions of the simulated spectra were fitted with the relationship described by Equation (6) and fit coefficients \(b=0.6274\) and \(c=1.3940\times 10^{-2}\), as in Figure 21. The MEGAlib model results (teal) reflect the measured energy resolution of GMOD FM (grey solid line). Figure 19: Spectra (counts/keV/hour) produced by the simulated MEGAlib GMOD detector (teal) for the five sources with known activities in Table 1. In each panel, the spectra produced by the physical GMOD FM are plotted for comparison.