Study on the effect of body size on²⁴Na activity-based neutron dose assessment

Neutron dose assessment in radiation accidents relies on measuring induced²⁴Na activity via the²³Na(n,γ)²⁴Na reaction. However, the effect of body size on neutron moderation, gamma self-absorption, and organ distribution-remains poorly quantified, with conflicting literature reports. To systematically evaluate the body size effect on neutron absorbed dose and²⁴Na yield, four human voxel phantoms of different body sizes (5-, 10-, 15-, and 38 year-old individuals) were irradiated in MCNP simulations with²⁵²Cf and 28 monoenergetic neutron sources (10^-9-20 MeV) under six geometries. The neutron absorbed dose, induced²⁴Na activity per unit mass, and the conversion coefficients between²⁴Na activity per unit mass and neutron absorbed dose were calculated. The results show that body size significantly affects the neutron dose. When the human body is irradiated with²⁵²Cf neutrons under different irradiation geometries, the conversion coefficients between²⁴Na activity per unit mass and neutron absorbed dose are at least 48.3% larger in adults than in 5 year-old children. When the human body is irradiated with monoenergetic neutrons under isotropic irradiation geometry, these conversion coefficients decrease with increasing body size for neutron energies <0.1 MeV, and increase with increasing body size for neutron energies >0.1 MeV. For 10^-9MeV neutron irradiation, the coefficient for adults is 26.5% smaller than that of 5 year-old children. Conversely, for 20 MeV neutron irradiation, the coefficient for adults is 114.2% larger than that for 5 year-old children.