## 4 Radiation Analysis
### Radiation Simulations Results
#### 4.2.2 SPE Gy-equivalent Doses to BFO on the Moon  
Only multilayer structures were tested for habitat protection against SPEs. Figure 5 gives the calculated BFO dose as a function of regolith areal density, showing that low areal densities of regolith lead to high doses in BFO per event, particularly for the February 1956 SPE.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Effective Dose Equivalent in CAF/180d (mSv)** \\ \hline
1.88 m deep & 6.25 m deep \\
102.8 & 4.76 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Effective dose equivalent in CAF in simulated lava tubes of highlands regolith for the solar minimum GCR BON 2014 spectrum after 180 days.  
Figure 4: Effective dose equivalent in CAF behind multilayer shielding (regolith followed by 3 mm of aluminium and 5 cm of polyethylene) as a function of highlands regolith areal density for the solar minimum GCR BON 2014 spectrum after 180 days.  
The largest areal densities in Figure 5 correspond to two metres of 2.7 g/cm\({}^{3}\) compressed regolith. Then the doses are attenuated down to a few mGy-Eq on average. Thickening of the polyethylene layer up to 10 cm from the original 5 cm allows the dose to be further attenuated, as Table 3 indicates. The shelter's walls can be thickened with polyethylene as the mission progresses, where plastic waste can become the source of polyethylene. Refs. [45; 46; 47] studied the utilisation of trash for radiation protection and [45] suggested that condensed plastic waste be used for the SPE protection in deep space to the extent that it could even replace an initial water-based protection.  
As for the GCR, lava tubes are safer against the SPE. Table 4 summarises the doses in BFO and demonstrates how the dose reduces by three orders of magnitude in a 6.25 m deep tube. Therefore, if any lava tubes are present in the habitation zone, they may be considered as potential candidates for temporary radiation shelters.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**SPE Spectrum**} & \multicolumn{2}{c}{**BFO Dose in CAF (mGy-Eq)**} \\ \cline{2-3}  & **300 g/cm\({}^{2}\) of Regolith** & **1000 g/cm\({}^{2}\) of Regolith** \\ \hline August 1972 & 1.22 & 2.44 \(\times\) 10\({}^{-3}\) \\ February 1956 & 29.3 & 0.374 \\ October 1989 & 1.47 & 6.46 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: BFO dose in CAF in simulated lava tubes for different historical SPE spectra.  
Figure 5: BFO dose in CAF behind multilayer shielding (regolith followed by 3 mm of aluminium and 5 cm of polyethylene) as a function of highlands regolith areal density for different historical SPE spectra.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**BFO Dose in CAF**} & \multicolumn{2}{c}{**Polyethylene Shielding Thickness (cm)**} & \multirow{2}{*}{**Relative Change**} \\ \cline{2-3} **(mGy-Eq)** & **5** & **10** & **(\%)** \\ \hline August 1972 & & & \\
135 g/cm\({}^{2}\) of regolith & 5.01 & 3.98 & 20.6 \\ February 1956 & & & \\
270 g/cm\({}^{2}\) of regolith & 29.11 & 13.19 & 54.7 \\ \hline \hline \end{tabular}
\end{table}
Table 3: BFO dose reduction due to polyethylene layer thickening in a multilayer material for SPE protection.