The Japanese 5-yen coin is about 22 mm in diameter and 1.5 mm thick, weighs 3.75 g and has a central hole 5 mm wide. We chose this coin for monitoring neutron exposure because it is widely circulated, the zinc content is precisely controlled, and the 65Zn generated has a convenient half-life (244.1 days) and γ-ray energy emission (1,115.5 keV). To obtain a record of the dosage of neutrons released as a result of the accident, we collected exposed coins from people's houses at distances 100–550 m from the facility.

Two types of detector were used to determine the concentration of 65Zn in these coins: both are pure germanium detectors with a low background contribution, one of which (University of Tokyo) is covered with an inner copper shield 5 cm thick and a 5-cm layer of iron, with an outer lead casing of 10 cm — in this system, radon in the air is purged by using nitrogen gas; the other (Kyoto University) is covered with an inner copper shield of 10 cm and outer lead layer of 10 cm. Using these detectors, we tested 119 coins for a maximum of 1,000,000 seconds; some samples were measured in both systems to crosscheck for consistency. The scattering and self-absorption of 1,115.5-keV γ-rays in the coin samples were calculated by using the Monte Carlo method in three dimensions.

Table 1 shows 65Zn concentrations measured in coins taken from houses in the neutron-affected areas around the nuclear-fuel processing facility. 65Zn concentrations at distances ranging from 100 to 550 m from the plant dropped from 39.6 to 0.76 Bq per kg zinc, decreasing exponentially with increasing distance.

Table 1 65Zn in 5-yen coins from houses around the accident site Full size table

65Zn concentrations and neutron dosage at various distances from the accident site have also been estimated by considering the neutron energy spectrum in relation to distance (T. Imanaka, personal communication); values were calculated with the neutron transport programs DOT and MORSE (ref. 1). Using these data, we were able to estimate approximate neutron doses from the 65Zn concentrations shown in Table 1.

Ambient dose equivalent values (in millisieverts at 1 cm depth) were calculated as about 220 mSv at 100 m, 6 mSv at 350 m and 1.8 mSv at 550 m. These estimates agree well with the early official estimates2, which were revised to about half some weeks later3. Our estimated exposures indicate that those living 350 m away from the accident — the distance to which people were advised to evacuate — were also irradiated by neutron doses of over 1 mSv.

Our dose estimates include a bias because the neutron field in the environment might differ from that assumed in the calculation. If the neutron field is unimpeded, the 65Zn in 5-yen coins can serve as an indicator of neutron dosage at various positions. Our results potentially offer information not only about the total neutron effect during the accident4 but also about shielding by modern Japanese houses, given that the coins were recovered from indoors.