# Geant4 Simulations of a Scintillator Cosmic-Ray Detector
## 2 Geant4 Software
### Detector Geometry and Composition  
The considered detector is based on the Cosmic Watch design which comprises a plastic scintillator with the dimensions 5 cm \(\times\) 5 cm \(\times\) 1 cm and a Silicon Photomultiplier (SiPM) attached in the middle of the largest side of the scintillator. Such a device is sufficient to detect most muons and the majority of electrons from cascades initiated by cosmic-ray particles. The geometrical description of the detector in the simulation also includes reflective coating, light isolation, and shielding, as presented in Figure 1. The idea of adding shielding is to reduce the background of low-energy particles that do not originate from EASs. Those can be single cosmic-ray particles reaching the ground without producing an extensive cascade or particles from radioactive decays in the air.  
Figure 1: A schematic presenting the detector’s geometry. Shielding of the thickness \(z\) is placed at a distance \(d\) above the scintillator.  
Everything was simulated in a standard air environment and all elements had sizes, shapes, and compositions similar to those in the Cosmic Watch design [9]. The main component of the detector is a plastic scintillator of the BC408 type, with a wavelength of maximum emission around 425 nm. On the bottom side, in its centre, a 6 mm \(\times\) 6 mm \(\times\) 1 mm SiPM (C-series, MICROFC-60035, Onsemi, Scottsdale, AZ, USA) is mounted. This model has the largest surface among single SiPMs in this price range and it is the most sensitive to light in the emission range of BC408. The scintillator is wrapped in aluminium foil with reflectivity equal to 88% serving as a mirror that keeps photons inside. Then it is covered by a layer of a black tape for additional isolation from external light. Scintillation photons are emitted in all directions along the path of particle that passes through it and then they scatter from the reflective coating, filling more or less the whole volume of the scintillator. There is also some Cherenkov light emitted in a cone around the passage of charged particles but in the case of plastic, it is negligible in comparison to that from scintillation [16]. The photomultiplier is separated from the scintillator by a layer of Vaseline (petroleum jelly) which serves as a medium helping photons to reach the SiPM. Without petroleum jelly, a layer of air with a significantly different refractive index would make the internal scattering of those photons more probable and that would reduce the detector's sensitivity. Vaseline, on the other hand, has a refractive index similar both to that of the scintillator and that of the material of the SiPM. The photomultiplier registers only photons within certain range of wavelengths: from 300 nm to 950 nm. For most elements of the detector, the value of the refractive index is approximately constant in this wavelength range. Only aluminium and iron have refractive indexes that vary significantly within this regime (Figure 2).  
The values of all important parameters used to characterise detector elements in the simulations are listed in Table 1.  
Figure 2: Values of refractive index of iron and aluminium as function of photons’ wavelength \(\lambda\)[17]. Refractive index of iron is applied to stainless steel as it is its main component.  
The sensitivity of the results to variations in these parameters is discussed in Section 4. Several different configurations of shields were also tested. The following materials that can be used in a realistic design were considered: aluminium, iron, glass, and steel. In the simulations, shielding material of the thickness \(z\) was placed between the detector and the source of particles at the distance \(d\) from the scintillator. Dependence on the thickness of those shields as well as the distance between them and the scintillator were also studied to find an optimal setup. The values of the studied parameters are presented in Table 2.