## 3 Results  
### Shielding Material Composition  
The USRBDX card in FLUKA was used to score the fluences of secondary particles passing through a vacuum sphere (radius = 10 cm) with the geometric center of the target surface. The source-term fluence distribution of neutrons and photons crossing the sphere are shown in Figure 7.  
Three shielding materials for both neutrons and photons were considered: Portland concrete without and with boron carbide (B\({}_{4}\)C) (0.2% in mass), and dense concrete as defined in the literature [9]. Their elemental components are listed in Table 11. As the dense concrete contains a higher amount of iron, some reaction products are enhanced, such as \({}^{54}\)Fe(n, \(\gamma\))\({}^{55}\)Fe, \({}^{58}\)Fe(n, \(\gamma\))\({}^{59}\)Fe, and \({}^{54}\)Fe(n, p)\({}^{54}\)Mn. These reactions produce \({}^{55}\)Fe (T\({}_{1/2}\) = 2.74 y), \({}^{59}\)Fe (T\({}_{1/2}\) = 44.5 d), and \({}^{54}\)Mn (T\({}_{1/2}\) = 312.1 d), respectively, and therefore may increase the residual dose levels. Thus, the dense concrete should not be used on the inner wall surface inside the terminal room. Furthermore, the commonly used Portland concrete contains elements such as Na (1.6% in mass) and Mg and Al, which undergo \({}^{23}\)Na(n, \(\gamma\))\({}^{24}\)Na, \({}^{27}\)Al(n, \(\alpha\))\({}^{24}\)Na, and \({}^{24}\)Mg(n, p)\({}^{24}\)Na reactions, resulting in \({}^{24}\)Na (T\({}_{1/2}\) = 15 h) [9; 20]. Therefore, 40 cm low-sodium borated concrete with Na reduced to 0.2% in mass was used at the inner side of the terminal room to potentially mitigate the residual dose rate. As illustrated in Figure 8, the specific activity of \({}^{24}\)Na in the top shielding at the  
Figure 6: Prompt ambient dose equivalent (in \(\upmu\)Sv/h) at beam level with and without biasing. The usage of biasing techniques improves the statistics, especially in the area far from the source term, even with fewer primaries.  
end of beam (EOB = 0 s) is reduced by about two orders of magnitude when low-sodium borated concrete is used instead of Portland concrete.  
Figure 8: Decline in \({}^{24}\)Na specific activity in Portland concrete and borated low-sodium concrete throughout the top shielding.  
Figure 7: Neutron and photon fluences generated during the irradiation of \({}^{100}\)Mo target using a 24 MeV proton beam with intensity of 500 \(\mu\)A.  
Since the dose rate limit on the top of the experimental terminal is relatively low, the use of homogeneous shielding may result in the specification for a thicker terminal roof. Alternatively, a combination of different shielding materials can be used to improve the shielding efficiency. The USRBIN card in FLUKA was utilized to score the average ambient dose equivalent rates for five different testing combinations, as shown in Figure 9. Owing to Group 4's large volume of dense concrete and its economic impact, the Group 5 solution was finally selected. As illustrated in Figure 10, the top shielding thickness of Group 5 was reduced by 1.1 m compared to Group 1, and the average ambient dose equivalent rate outside the top shielding was \(\sim\)0.15 \(\mu\)Sv/h.  
Figure 10: Average dose rate attenuation in different configurations with the elevation of the top shielding. The bullets (1, 2, 3, 4, 5) correspond to the number of five different combinations of shielding materials illustrated in Figure 9.  
Figure 9: Five combinations of shielding materials (Portland concrete, borated low-sodium concrete, borated concrete, and dense concrete) in the top shielding, which are filled from L1 to L4. Distances are given in cm.