Neutron dose estimates from 5-yen coins

Abstract

After a criticality accident at a nuclear-fuel processing facility in Tokaimura, Japan, at 10:35 Japanese standard time on 30 September 1999, neutron emission continued until early the next morning. To estimate the neutron dose to the surrounding population, we collected 5-yen coins (37 weight per cent zinc) from places likely to have been exposed to radiation as a result of the accident and determined the amount of ⁶⁵Zn generated in them in the nuclear reaction ⁶⁴Zn+n→⁶⁵Zn by spectrometrically measuring the photons emitted by ⁶⁵Zn. Our results indicate that people evacuated to 350 m outside the irradiated area still received a significant neutron dose.