High-Accuracy Relative Biological Effectiveness Values Following Low-Dose Thermal Neutron Exposures Support Bimodal Quality Factor Response with Neutron Energy

Abstract

Theoretical evaluations indicate the radiation weighting factor for thermal neutrons differs from the current International Commission on Radiological Protection (ICRP) recommended value of 2.5, which has radiation protection implications for high-energy radiotherapy, inside spacecraft, on the lunar or Martian surface, and in nuclear reactor workplaces. We examined the relative biological effectiveness (RBE) of DNA damage generated by thermal neutrons compared to gamma radiation. Whole blood was irradiated by 64 meV thermal neutrons from the National Research Universal reactor. DNA damage and erroneous DNA double-strand break repair was evaluated by dicentric chromosome assay (DCA) and cytokinesis-block micronucleus (CBMN) assay with low doses ranging 6–85 mGy. Linear dose responses were observed. Significant DNA aberration clustering was found indicative of high ionizing density radiation. When the dose contribution of both the 14N(n,p)14C and 1H(n,γ)2H capture reactions were considered, the DCA and the CBMN assays generated similar maximum RBE values of 11.3 ± 1.6 and 9.0 ± 1.1, respectively. Consequently, thermal neutron RBE is approximately four times higher than the current ICRP radiation weighting factor value of 2.5. This lends support to bimodal peaks in the quality factor for RBE neutron energy response, underlining the importance of radiological protection against thermal neutron exposures.

Neutron biological effectiveness in inducing DNA damage: a full mapping as a function of incident neutron energy and depth in a human-sized phantom

“…The approach behind our RBE model explicitly considers the specific geometry of the target and the change in the neutron-induced charged particle field, both with target size and with penetration in a large-size target. Of interest, other models 33,34 indicate an increase of quality factors below 10 −3 MeV, and in vitro experimental studies 35 have reported high RBE for DNA damage (dicentric chromosome assay and cytokinesis-block micronucleus assay) for thermal neutrons. It has to be recalled however that such increase is particularly manifested only considering a small target size, as thermal neutron dose to a large size receptor is always largely dominated by the photon component, hence leading to an overall low effectiveness in inducing damage.…”

Space Environmental Effects on Multifunctional Radiation Shielding Materials

“…The Lunar Neutron Probe Measurements conducted during the Apollo 17 showed a significant increase in the flux of thermal and epithermal neutrons up to 1 m below the Lunar surface (Woolum, 1975). Recent high accuracy measurements have indicated that the relative biological effectiveness (RBE) values for thermal neutrons can be 4 times higher than the previous recommended value of 2.5 (Paterson, 2022). Given their high RBE values the thermal neutron component should be considered for radiation shielding countermeasures.…”