# A Review of Requirements for Gamma Radiation Detection in Space Using CubeSats
## 5 The Geant4 Simulation for the Detection of Short Burst of Gamma-Rays  
In order to evaluate the performance at regime operation of a detector orbiting for example at ISS orbit, a GEANT4 simulation (version 10.4 with \"FTFP_BERT\" physics list implemented) has been used to generate the particle events and the detector signals has been processed by means of a customized Mathematica algorithm based on the procedure presented in Section 3.4. The cross section of detector used in the GEANT4 simulation is reported in Figure 13.  
To determine the expected scientific and technological output of the mission, a critical task is to assess the ability of the instrument to detect high intensity and sub-millisecond bursts of gamma-rays (fluency of the order of 100 photons \(\times\) cm\({}^{-2}\)s\({}^{-1}\)) with energies up to tens of MeV. The major goal of the simulation is to demonstrate that it is possible to discriminate such featured events from the particle background. This is further evaluated by performing a simulation with SPENVIS [27], an open source tool developed by the European Space Agency (ESA) and optimized to model the particle background at a various orbits and solar activity conditions. Nowadays, a diverse software portfolio is available for modeling the expected particle background at given orbit. An example is the Satellite Tool Kit (STK) developed by AGI, which is a comprehensive software capable of modeling and simulating various mission components. The STK package dedicated to the particle background analysis is SEET (Space Environment Effects Tool) [28]. The background model proposed takes into account the effect of trapped protons and electrons (isotropically distributed) and extra-galactic photons (impinging only five out of six detector faces due to Earth shadowing), while the contributions of albedo photons and galactic cosmic rays are not considered. High radiation intensity areas (South Atlantic Anomaly and Polar regions) encountered along the orbit are not taken into account in the simulation as the instrument is in power off mode. The typical trapped proton and electron spectra for ISS-like orbit (\(\sim\)408 km, 51.6\({}^{\circ}\)) are reported in Figure 14.  
Figure 13: The cross section of the detector studied in the simulation. In yellow is the 4 CeBr\({}_{3}\) scintillating crystals [26] optically separated by means of a PTFE barrier (in black). The target is sealed by means of an aluminum enclosure (light gray). In blue is the polystyrene-based VETO for the rejection of charged particle induced events. In red is the Momentive RTV615 silicon used as electrical and thermal insulator. The whole assembly is ultimately enclosed in an outer aluminum box (dark gray). The isotropic extra-galactic photon spectrum [29] is reported in Figure 15.  
The procedure to create a more refined background model as a function of the position in the orbit is reported here [32]. A thorough paper that accurately discusses the particle background of a given orbit as a function of the satellite position is currently under review [33]. Charged particles and extra-galactic photons contributions have been used to build the average background environment corresponding to about 5000 s of detector operations (1,000,000 of events). The expected response of a detection channel during the first 2 s of mission is shown in Figure 16. The height of each spike (expressed in ADC channels) is proportional to the energy released in the interaction. The highest peaks correspond to the energy released by protons in the target, while the lower ones are mainly due to gamma-ray and electron interactions with the target. By measuring the number of spikes over the time, an average interaction rate of 30 Hz for CH1 and CH2 can be inferred (slightly less is the rate for CH3 and CH4 due to Earth shadowing against extra-galactic photons). Figure 14: Energy spectra for trapped protons (in red) and electrons (in blue) at the ISS altitude and orbit.