# Simulation and Evaluation of the Radiation Dose Deposited in Human Tissues by Atmospheric Neutrons
## 2 Materials and Methods: Setup of the Simulation  
Excel-Based Program for Calculating Atmospheric Cosmic-Ray Spectrum (EXPACS) calculate cosmic ray fluxes of neutrons, protons, muons, electrons, positrons, and photons nearly anytime and anywhere in the Earth's atmosphere [11].  
In Figure 1, we show the atmospheric neutron spectra for various altitudes obtained with EXPACS for geographical points with a latitude of 0\({}^{\circ}\) and a longitude of 0\({}^{\circ}\). These data were calculated for the year 2020, period of minimum solar activity.  
Figure 1: Atmospheric neutron spectra for various altitudes at geographical points with a latitude of 0\({}^{\circ}\) and a longitude of 0\({}^{\circ}\). See text for more details.  
In Figure 2, we show the atmospheric neutron spectra for various latitudes obtained with EXPACS for geographical points with an altitude of 10 km and a longitude of 0\({}^{\circ}\). These data were calculated for the year 2020, period of minimum solar activity.  
The Geant4 software is a toolkit for the simulation of the passage of particles through matter. It is used for a variety of applications domains, including high-energy physics, astrophysics and space science, medical physics and radiation protection [12]. Geant4 is used extensively in medical physics applications such as particles beam therapy, microdosimetry and radioprotection. The basic extensibility of the toolkit has facilitated its expansion into new user domains, such as biochemistry, material science and non-destructive scanning.  
For this simulation we used as source, fluxes of 5 \(\times\) 10\({}^{5}\) neutrons with kinetic energies between 1 and 1000 MeV. We also used tissues of blood (G4_BLOOD_ICRP, \(\rho\) = 1.06 g/cm\({}^{3}\)), adipose (G4_ADIPOSE_TISSUE_ICRP, \(\rho\) = 0.95 g/cm\({}^{3}\)), bone (G4_BONE_CORTICAL_ICRP, \(\rho\) = 1.92 g/cm\({}^{3}\)) and brain (G4_BRAIN_ICRP, \(\rho\) = 1.04 g/cm\({}^{3}\)) as samples obtained from Geant4 material's database, with a spherical geometry with a radius of 5 cm. These samples are tissues equivalent reported by The International Commission on Radiological Protection (ICRP). The ICRP is the primary body in protection against ionising radiation, it is a registered charity and is thus an independent non-governmental organisation created by the 1928 International Congress of Radiology to advance for the public benefit the science of radiological protection [13].  
With the help of Geant4 software, we got the RD that neutrons deposit in the different sample tissues as a function of neutron's energy, the neutrons were injected isotropically to each of the samples. We arbitrarily choose geographic points, all of them with the same longitude (0\({}^{\circ}\)), at these points, we calculated the RD that neutrons deposited in the samples for different altitudes and different latitudes.  
To obtain the RD in the tissues, we used the spectra shown in Figures 1 and 2 to calculate the neutron flux for each energy of interest, in addition, these fluxes were calculated for periods of one hour over the spherical area of the samples, later we multiplied the neutron flux and the RD deposited per neutron, for each energy, finally we added all products to obtain the RD deposited in the tissues.