Conceptual design of sandwich walls for shielding against secondary neutrons using MC simulations with FLUKA

FLUKA Monte-Carlo transport code was employed to evaluate the secondary neutron spectra emerging from spherical sandwich shielding configurations composed of concrete and soil, similar to that used at the particle therapy facility MedAustron. This study provides a comparative analysis of neutron spectra attenuated by a concrete-soil-concrete (CSC) sandwich wall shielding configuration versus a full concrete wall design (CCC). Furthermore, we enhanced the shielding performance of the CSC configuration by adding an additional concrete layer (CCSC) to achieve results comparable to the CCC shielding. Two scenarios were tested for shielding performance: (1) primary protons at 100 MeV, and (2) primary carbon ions (C-ions) at 190 MeV/u. Our simulations with primary protons of 100 MeV showed that adding additional internal concrete wall to the CSC configuration, therefore designing the CCSC configuration, the RP performance becomes slightly improved - the HE-peak drops from (1.43 ± 0.11)10^-11 to (5.62 ± 0.3)10^-12, about 2.5 times. Still, the HE-peak of the exiting neutron spectrum from CCC -(6.29 ± 1.87) 10^-13 is about 9 times lower than that exiting CCSC - (5.62 ± 0.3) 10^-12. Our simulations with primary C-ions showed that by placing an additional internal concrete wall to the CSC configuration (CCSC) the RP performance becomes slightly improved - the exiting HE peak can be further attenuated from (6.92 ± 0.40)10^-9 for CSC to (3.79 ± 0.15)10^-9, becoming comparable to the one exiting the CCC configuration, (0.92 ± 0.04)10^-9, only 4 times higher. Future research should be focused on improvements of the RP performance of the CCSC, by increasing the soil layer thickness and taking into consideration the humidity (water content) in the soil and concrete and also improve the number of primaries to 10⁹ or even 10¹⁰ for better statistical outcome.