## 4 Radiation Analysis
### Radiation Simulations Results  
#### 4.2.1 GCR Effective Dose Equivalent on the Moon  
Figure 1 illustrates the results of GCR simulations for a lunar habitat made of highlands regolith. Figure 1a shows results against the areal thickness in g/cm\({}^{2}\) to demonstrate the local maximum. Figure 1b presents the same results against the thickness of the regolith sphere in cm. It illustrates vividly how compacting regolith is an effective technique for dose reductions if the same wall thickness is kept, which may have important implications for the architectural freedom and choices for the habitat. Beyond the point of the local dose maximum, the dose has a general trend to attenuate. It can be argued that the dose build-up at around 100 g/cm\({}^{2}\) in Figure 1a of regolith shielding is linked to the production of slow and fast secondary neutrons which are most dangerous to the human body, as reflected in the radiation weighting factors presented in Figure 2. The authors of [6] argue that beyond 100 g/cm\({}^{2}\) of aluminium or lunar regolith shielding, the dose is mainly due to secondary neutrons and secondary radiation from their reactions in the body. Protons of energies up to several hundred MeV produce secondary protons, neutrons, alpha particles, etc. [6]. Protons of higher energies are stopped further into the material, beyond the local maximum. However, this phenomenon is not only dependent on the incoming radiation energy and the thickness of shielding but also on the nature of the shielding, as polyethylene does not appear to have any local maximum, as demonstrated in Figure 3.  
Polyethylene CH2 is hydrogen-rich which leads to lower production of secondary neutrons [9]. Furthermore, carbon and hydrogen do not have significant resonances in their nuclear structure to interact with secondary neutrons in the structure as the heavier atoms do [29], and carbon tends to disintegrate into helium nuclei and produce no neutrons [9]. These beneficial properties of polyethylene make it a favourable radiation protection material. However, scattering can occur, and a highly energetic neutron can produce a proton of the same energy which will lead to a dose build up. Furthermore, some particles will have high enough energies that they will create ballistic collisions and create  
Figure 1: Effective dose equivalent in CAF as a function of highlands regolith areal density (**a**) and regolith sphere thickness (**b**) for different volumetric densities for the solar minimum GCR BON 2014 spectrum for 180 days on the Moon. secondary neutrons. Nevertheless, polyethylene will generate fewer secondary neutrons than aluminium. Based on these considerations, a multilayer structure with regolith as the main thick layer, followed by 3 mm of aluminium for further structural rigidity and finally, 5 cm of polyethylene has been modelled. Multilayer simulations' results for regolith/aluminium/ polyethylene shields are shown in Figure 4. In contrast to pure regolith, there is no more local maximum in the effective dose equivalent curve, which suggests that the polyethylene layer helps attenuate the dose build-up. Instead, the region of the build-up is marked with a slow dose decline when compared to low areal densities. The overall trend up to high regolith densities is declining. The dose attenuation at thick areal densities follows the trend dominated by the thick regolith layer and the general decline suggests that the dose may be even lower in deep lava tubes. Figure 3: Dose equivalent in polyethylene for the GCR BON 2010 spectrum in free space at 1 AU. Figure 2: Radiation weighting factor for neutrons vs. neutron energy, figure taken from [1].