The major uncertainties in space radiation risk estimates for personnel are related to the incomplete knowledge of the radiobiology of energetic heavy charged particles. To a smaller degree they are related to the characterization of the space radiation environment and its primary interactions with the shielding structures and the human body. In order to increase the reliability of risk coefficients, modelling and transport through matter of the initial galactic cosmic ray (GCR) and solar particle event (SPE) charged-particle spectra has to be improved. Since it is practically impossible to assess experimentally all primary and secondary particles encountered in the variety of possible position-projectile-target-energy combinations, modern one- or three-dimensional particle transport codes are used. The particle collisions and transport can be evaluated by using either deterministic or Monte Carlo (MC) methods.
Deterministic computer codes are based on different approximations of the Boltzmann transport equation. Many existing codes are tailored to a specific application and take advantage of this fact by applying relevant simplifications. Two examples of well-known deterministic computer codes are HZETRN and HIBRAC. However, the interaction of a heavy ion with matter is a complex process including a variety of processes such as ionization, excitation, nuclear fragmentation, production of positron-emitting nuclei, and de-excitation by gamma-ray emission. These processes are not fully accommodated within deterministic models and their complexity requires the use of numerical methods for solving the probabilities of different events, e.g. a MC method. MC codes are also to be employed for neutron dose estimation, since the transport of neutrons cannot be handled by one-dimensional codes. One of the best-known MC transport codes is the MCNP package. Many other codes have been developed for fundamental high-energy particle physics, partly in response to NASA’s call for development of a transport code to be used for radiation protection in space. These algorithms include FLUKA, GEANT, SHIELDHIT, HETC-HEDS, MARS, and PHITS.
The models and transport codes have to be carefully benchmarked and validated to make sure they fulfil preset accuracy criteria, e.g., are able to predict particle fluence, dose and energy distributions within a certain degree of uncertainty. For a proper validation, both space and ground-based accelerator experiments are required. A number of accelerators is currently available for performing the necessary certification experiments, including GSI, JINR, NIRS-HIMAC, NSRL, TSL, etc. Experiments with neutron beams can be performed, e.g., at iThemba, LANSCE, or TSL.
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