## 4 Radiation Analysis  
### Radiation Models  
The shielding and radiation environments are simulated with the On-Line Tool for the Assessment of Radiation In Space (OLTARIS, TARIS 4.01) developed by NASA [35], TARIS version 4.01. The tool runs a deterministic High \(Z\) and Energy TRanNsport (HZETRN) 3D code [36; 37] which solves the Boltzmann equation using continuous slowing down approximation. The BFO dose in grey-equivalent and effective dose equivalent in mSv are calculated for the CAF model [11; 12], the latter one using the ICRP 60 quality factors. The doses are calculated as a function of shielding measured in g/cm\({}^{2}\). To convert from aerial densities presented in graphs to walls or regolith thicknesses in cm, it is required to divide the aerial density by the nominal volumetric density of regolith. Symmetrical spherical geometries of the shielding are used to approximate all habitats in the middle of which the CAF phantom is found vertically oriented, head upwards. Concentric spheres of appropriate thicknesses are used to define the different layers of the habitat wall, such as the outer regolith shell and the aluminium and polyethylene shells. Thickness distribution files were produced for the spheres, and the reader is encouraged to use the OLTARIS user guide for their utilisation details. Unless stated otherwise, the GCR conditions are simulated for 180 days at the solar minimum parametrised with the solar modulation parameter of 475 MV and using the Badhwar-O'Neill (BON) 2014 GCR model [38]. The solar modulation is a temporal parameter related to the solar activity through its cycle, which modulates the total flux and differential energy spectrum of GCR in the heliosphere. Here, 475 MV were chosen to roughly represent a period of solar minimum based on observations in [39]. The model is implemented in OLTARIS, which reports accuracy of the GCR environment at 1 astronomical unit (AU) within 10% or less. Free space galactic rays and solar particles at 1 AU follow the specified ray distributions from free space to the target point on the habitat. Ray distribution files can be downloaded and modified from OLTARIS database; they are necessary for ray-by-ray calculations which are used for effective dose equivalent and can also be used for BFO doses. This work uses the ray-by-ray option throughout. OLTARIS' GCR models for the lunar surface consider the albedo. The program uses thickness distributions files written to represent a habitat to apply either the free space environment at 1 AU for rays emanating from space to a point on the habitat or neutron albedo for surface-pointing rays. Lunar albedo is calculated for GCR models by applying the free-space GCR to the lunar regolith [35; 40]. Simplistic spherical habitats on the surface of the Moon were simulated for the following SPEs:  
* August 1972 King [41], which is widely regarded as the largest of the space age;
* February 1956 LaRC [42], which is marked particularly for the elevated flux of protons of energies above 500 MeV;
* October 1989 [43], which is categorised in the energy band of 5-100 MeV. Ref. [44] explains that the SPE of August 1972 dominated the cumulative solar energetic particle contribution of solar cycle 20 and that this was the largest event in space exploration times in its energy range and for energies relevant to spacecraft components. The October 1989 event is the largest of the past 40 years in its energy range [44]. The largest uncertainties in SPE models are in the high energy tails as exponential models are used to curve fit data. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Oxide Quantity (\%)**} \\ \hline SiO\({}_{2}\) & Al\({}_{2}\)O\({}_{3}\) & TiO\({}_{2}\) & FeO & MnO & MgO & CaO \\
45 & 27 & 0.7 & 5 & 0.3 & 6 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Highlands regolith composition averaged from the Apollo missions’ sample return [34]. Lava tubes are simulated in a simplistic way as spheres.