Study of the screening survey using an ambient dose equivalent rate survey meter in criticality accidents

When a criticality accident occurs, a screening survey to triage high-dose radiation exposed persons is performed. We established a rapid method for the screening survey by measuring the gamma dose rate mainly from 24Na in the victims’ body with a conventional NaI(Tl) scintillation survey meter. In this study, a water-filled slab phantom containing NaCl was irradiated with neutrons from a 252Cf neutron source. The radioactivity concentration of 24Na produced in the phantom was determined by means of gamma-ray spectrometry and simulations using the Monte Carlo N-Particle Transport Code. The ambient dose equivalent rates at the phantom’s outer surface were simulated by the Monte Carlo N-Particle Transport Code and were directly measured with a NaI(Tl) scintillation survey meter. From the experiments and calculations, we obtained the results that 1 Gy (neutron absorbed dose) corresponds to 18–76 μSv/h (ambient dose equivalent rate) at the surface of the victim’s body, which can be distinguished from normal background levels. Therefore, this method allows us to rapidly screen high-dose radiation-exposed victims.