Measurements of 60Co in spoons activated by neutrons during the JCO criticality accident at Tokai-mura in 1999

Neutron activated items from the vicinity of the place where the JCO criticality accident occurred have been used to determine the fluence of neutrons around the facility and in nearby residential areas. By using underground laboratories for measuring the activation products, it is possible to extend the study to also cover radionuclides with very low activities from long-lived radionuclides. The present study describes γ-ray spectrometry measurements undertaken in a range of underground laboratories for the purpose of measuring 60Co more than 2 years after the criticality event. The measurements show that neutron fluence determined from 60Co activity is in agreement with previous measurements using the short-lived radionuclides 51Cr and 59Fe. Limits on contamination of the samples with 60Co are evaluated and shown to not greatly affect the utility of neutron fluence determinations using 60Co activation.