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Human missions in the Solar System. assessment of radiation hazards with Monte Carlo simulation tools

Universitat Politècnica de Catalunya, June 14th. 2007

The human exploration of the Solar System has gained a lot of interest by space agencies and many efforts are currently under development to solve various problems for human adaptation to the space environment. One of the key points in the hazards of the space environment, especially outside the influence of the Earth’s magnetic field is the radiation environment.
The space radiation environment is a mixed field of high energy particles driven by the solar activity. There are
two sources of particle radiation: solar particles events (SPE) and galactic cosmic rays (GCR). Radiation protection assessment for this environment involves calculating the protection effects provided by shielding materials covering the habitable parts of a space vehicle or habitat, and the health risks to astronauts due to exposure to this radiation.
This work uses the MULASSIS computer simulation code developed for the European Space Agency to calculate the dose received by astronauts under several radiation source conditions, shielding material, and shielding thickness for a simplified geometry. MULASSIS is based on Geant4, a general purpose system for radiation-matter interaction using Monte Carlo algorithms.
SPE sources used correspond to a statistical model for worst-case situations, and proton fluence spectra from selected solar events that are typically used as a reference for radiation protection calculations. GCR sources used in our calculations correspond to the CREME96 model, calculated for the worst GCR conditions that happen during solar minimum.
Two different scenarios are examined: deep space and the Lunar surface. For the deep space scenario, the geometry of the simulation consists of a set of planar slab layers on which the incoming particles impinge. One slab represents the shielding material, and behind it a reference volume consisting of a 30 cm thick slab of water represents a human body. The dose deposited on the reference volume by both the primary and secondary particles is calculated and compared to currently established limits.
Four materials have been considered for shielding: Aluminium, water, polyethylene, and lithium hydroxide. Al is the most common structural material used in the space industry, while the other materials represent different hydrogen-rich compounds. Low Z and hydrogen-rich materials are found to have better shielding properties for the extremely high energetic particles present in space.
The effect of shielding thickness is studied varying the simulation volumes between 2 g cm−2 to 100 g cm−2 areal density; from nominal spacecraft wall thicknesses to considerably thick shielding.
The results show that exposure to most SPE can be reduced to acceptable levels with medium shielding (about 20 g cm−2), with no clear differences among the materials. The only noticeable difference is that Al shielding is shown to yield the lowest protection compared to the other three materials in all cases.
High energy SPE and GCR particles show a higher penetration through the shield, and ultimately the intensity of these particles fluence will determine the limiting conditions for deep space operations.
The Lunar surface radiation environment is analyzed, considering the backscattered fluence of secondary particles generated by the interaction of the primary radiation with the regolith that covers the Lunar surface. The effect of this interaction in the proton and neutron fluences within the top 1 m of the Lunar surface is studied for three different regolith materials representative of both Lunar highlands and maria.
Protons depth profile is shown to decrease, while there is an increased production of secondary neutrons at depths between 20 cm and 40 cm into the regolith. This is the depth at which most of the spallation processes take place, producing recoil protons, nuclear fragments, and high energy neutrons.
Preliminary results on the radiation shielding properties of Lunar regolith are given for a typical Lunar highlands material, with shielding thicknesses varying between 10 cm and 1 m. These results indicate that a minimum of 50 cm of regolith is needed, but doses are still close to current limits.