Life on Mars? Beware of neutrons! Modeling and measurements of the neutron component of space radiation and its biological effects

Neutrons and their associated biological effects have always represented a charged issue in radiation protection and estimation of health risk. In the space radiation environment, neutrons are not present as primary radiation component but produced following nuclear reactions in space vehicle or habitat walls, in the passage through the planetary atmosphere or when radiation gets to the planetary surface, and, finally, in the traversal of body tissues themselves. Though the associated physical dose to astronauts might be small, the biological effectiveness of neutrons varies a lot with their energy, and their quality factor might be as high as 20, in comparison to 1 or 2 for that of other light particles. Neutrons elusiveness when it comes to experimental measurements adds up with severe practical limitations (e.g. selection of energy and dose-rates) concerning radiobiological measurements with neutron sources, also leading to data that are much varied among different experiments. Importantly, even considering possible ground-based simulators of mixed-field for space radiobiology, it has to be kept in mind that the neutron component will be hard if not impossible to reproduce. Modeling neutron production (correctly assessing associated uncertainties) with radiation transport codes and neutron interactions leading to biological consequences at the subcellular/cellular level is therefore of fundamental importance. In this talk we will review modeling approaches and measurements of the neutron component of space radiation and predictions and data on neutron biological effects. In view of manned missions in deep-space, we will particularly address possible neutron spectra of interest for astronauts' exposures inside a space vehicle or habitat, and on the surface of Mars, in the effort of integrating the available knowledge and achieving a correct risk estimation.