Role of shielding in modulating the effects of Solar Particle Events: Monte Carlo calculation of physical and “biological” dose in different organs.

Distributions of absorbed dose and DNA clustered damage yields in various organs and tissues following the October 1989 solar
particle event (SPE) were calculated by coupling the FLUKA Monte Carlo transport code with two anthropomorphic phantoms (a
mathematical model and a voxel model), with the main aim of quantifying the role of the shielding features in modulating organ
doses. The phantoms, which were assumed to be in deep space, were inserted into a shielding box of variable thickness and material
and were irradiated with the proton spectra of the October 1989 event. Average numbers of DNA lesions per cell in different organs
were calculated by adopting a technique already tested in previous works, consisting of integrating into ‘‘condensed-history’’ Monte
Carlo transport codes – such as FLUKA – yields of radiobiological damage, either calculated with ‘‘event-by-event’’ track structure
simulations, or taken from experimental works available in the literature. More specifically, the yields of ‘‘Complex Lesions’’ (or
‘‘CL’’, defined and calculated as a clustered DNA damage in a previous work) per unit dose and DNA mass (CL Gy1 Da1) due to
the various beam components, including those derived from nuclear interactions with the shielding and the human body, were
integrated in FLUKA. This provided spatial distributions of CL/cell yields in different organs, as well as distributions of absorbed
doses. The contributions of primary protons and secondary hadrons were calculated separately, and the simulations were repeated
for values of Al shielding thickness ranging between 1 and 20 g/cm2. Slight differences were found between the two phantom types.
Skin and eye lenses were found to receive larger doses with respect to internal organs; however, shielding was more effective for skin
and lenses. Secondary particles arising from nuclear interactions were found to have a minor role, although their relative contribution
was found to be larger for the Complex Lesions than for the absorbed dose, due to their higher LET and thus higher
biological effectiveness.