An optimized method for neutron dose evaluation based on24Na activity distribution in the human body

Abstract

The neutron dose resulting from external irradiation can be evaluated by measuring the counts of characteristicγrays produced by²⁴Na in the human body. The detection geometry with the highest detection efficiency for measuring the whole-body²⁴Na activity has not been studied. In this work, the MCNP code is used to calculate the spatial distribution of²⁴Na in the human body irradiated by neutrons with different energies in different irradiation geometries. The fluence distribution of²⁴Na characteristicγrays on the body surface is calculated. The counts of²⁴Na characteristicγrays induced by monochromatic neutron irradiation are simulated to fit the scenarios of neutron irradiation by a continuous energy spectrum neutron. When the spontaneous neutrons from²⁵²Cf with 1Gy dose irradiate the human body, (3.63-4.35) × 10^{10 24}Na atoms are produced. The lower detection limit for the neutron absorption dose is reduced from ∼100 to less than 1 mGy when the radiation detector is placed over the back of the human body close to the liver. The relative error between the measured counts of²⁴Na characteristic γ rays caused by²⁵²Cf neutron irradiation and the counts fitted by monochromatic neutron irradiation data is less than 5.7%. The neutron dose received from a continuous energy spectrum neutron can be acquired quickly and accurately by weighted summing of the data for monochromatic neutron irradiations calculated in this paper, which is more convenient and practical than the previous method.

Keywords: 24Na distribution; absorbed dose; characteristic γ ray; computational phantom; γ fluence.