Activation detection using indium foils for simultaneous monitoring neutron and photon intensities in a reactor core

In this work, the time-sequential changes in neutron and photon intensities in the core of a research reactor were monitored for 330 days following the time of shutdown. Indium foils, used as activation detectors, were irradiated during different time periods in the core. The activated nuclides 116m1In and 115mIn were measured and their activities were used as indices of the changes in neutron and photon intensities during the period of interest. Photon and neutron sources that may have been responsible for the activation reactions with indium foils were identified. The activities of both 116m1In and 115mIn rapidly decreased in the first 20 h (Phase I), and subsequently their activity levels were proportionally varied with 140La activity between 20 and 3000 h (Phases II and III). After 3000 h (Phase IV), neutron fluence rate slowly decreased to levels of less than 10 cm−2 s−1. Then, the main neutron sources were identified as the neutron-emitting transuranic nuclides and the most important photon source that activated indium to form 115mIn was identified as 60Co. The activation detectors based on indium foils were found to be effective for simultaneously monitoring the variations of neutron and photon levels in reactor cores under sub-critical conditions.