114Inm as a Long Half-Life Exposure Indicator for Criticality Accident Dosimetry

Indium is activated, principally by epithermal neutrons, to give 114Inm with a half-life of about 50 d. Indium foils exposed in the UK criticality dosemeter, at an International Intercomparison Experiment at the SILENE reactor, were measured three months later both by gamma ray spectrometry and beta ray counting. The specific activity measured was in good agreement with that estimated independently from gold foil measurements of the thermal and intermediate energy neutron fluences. Beta ray counting with a low background end window Geiger-Muller counter was the more sensitive method of measurement, giving a detection efficiency of about 0.14 count.disintegration-1. This gave a practical lower limit of measurement for 114Inm of 0.5 Bq.g-1, which corresponded to total neutron doses of about 16mGy and 21mGy for the lead shielded and bare SILENE neutron spectra respectively. Such measurements demonstrate the value of indium as a long-term exposure indicator, in addition to its usually accepted uses as a short-term exposure indicator (116Inm) and a fast neutron detector (115Inm).