Passive shielding of space radiation for human exploration missions - Simulations and Radiation Tests

Space radiation is one of the main showstoppers for human exploration of deep space. When leaving the protection provided by Earth’s atmosphere and magnetic field, the astronaut crew find themselves immersed into a complex radiation field, originated by the interaction of different high-energy radiation sources with the spacecraft’s walls, and characterized by many particle species with a broad range of energies. The biological effects of the long-term radiation exposure is largely uncertain and could give rise not only to late solid cancers and leukemia, but also to early effects to cardiac and nervous tissues, possibly undermining mission success. An available countermeasure to defend the astronauts from radiation is passive shielding, i.e. the interposition of shielding materials between the radiation sources and the exposed subjects. However, the majority of space radiation is practically impossible to completely stop: the high energetic particles constituting the space environment have the capability to penetrate several meters of materials, generating a harmful component of secondary particles, further contributing to the radiation dose. The ability of a material to attenuate the incoming space radiation and the nature of the generated secondary particles largely depends on the traversed material itself, in particular on the ratio between its charge and mass atomic numbers, Z/A. The lower is this ratio, the higher the material’s capability to attenuate the incoming radiation will be, both through electromagnetic and nuclear interactions. While the radiobiology community is focusing on the biological effects, radiation physics is trying to lower uncertainties characterizing the radiation interactions with materials, performing radiation measurements of various nature. In this framework I focused my PhD activity on the study of materials which could be used in space as shielding layers and multipurpose structures have been evaluated and selected under different criteria. At first, their ability to shield different kinds of space radiation were calculated with the aid of 1D Monte Carlo simulations, also followed by an evaluation of their structural and thermal proprieties, cost, availability and compatibility with the space environment. Simulations, in particular, were performed both to support the material selection process both to produce guidelines for design. The selected materials were then procured to be tested under different radiation beams and different set-ups, in single and multi-layers configurations, in an attempt to reproduce space exposure conditions. At the same time, the radiation tests have been reproduced by means of Monte Carlo simulations, to compare the experimental results and the simulations’ outputs, confirming the codes’ ability to reproduce radiation measurements involving High Z-number and high Energy (HZE) particles. For some materials, suggestions were provided on which nuclear model was better reproducing the data. The performed experimental campaign suggested that a candidate shielding material suitable for Galactic Cosmic Rays (GCR) should be tested with at least two beams with different characteristics, since the results indicated that some materials good at shielding 972 MeV/nuc 56Fe ions performed very poorly when irradiated with high energetic alphas. Furthermore, among the material types included in this investigation work, Lithium Hydride resulted the best option to stop space radiation, when only radiation shielding properties are considered. At the end of the experimental campaigns, on the basis of the test results, a 3D simulation activity has started and is still on-going and a modular space habitat model has been created. Monte Carlo simulations have been carried out, reproducing different Moon exposure scenarios with the goal of calculating crew radiation exposure during a Moon surface mission. This work reports results only for a standard aluminum habitat, with only Moon soil used as shielding material. However, future simulations will include Lithium Hydride and possibly others materials as shielding layers, to evaluate their effectiveness in reducing the dose in a realistic exposure scenario. Preliminary results show that even with a heavily shielded spacecraft (the habitat taken in consideration in this work is providing from every direction at least 30 g/cm2 of aluminum equivalent) radiation exposure approaches values close to the existing annual radiation exposure limits. Part of this thesis’ work was done at Thales Alenia Space, using Thales Alenia Space infrastructures and in the framework of the ROSSINI2 study. The ROSSINI2 study has been supported by European Space Agency (ESA) under the contract RFP IPLPTE/LF/mo/942.2014 and with the generous support of NASA and BNL, providing beam time at the NSRL facility.