Evaluation of radioactivity induced in the accelerator building and its application to decontamination work

For decommissioning of the cyclotron and electron synchrotron facilities, the residual radioactivity in surface and core samples of concrete, collected from various parts of buildings, was determined by gamma-ray spectrometry. It was found that the concrete samples were activated mainly by neutrons and that the major radioisotopes were ¹⁵²Eu, ⁶⁰Co, ¹³⁴Cs, ²²Na and ⁵⁴Mn. The maximum activity induced by thermal neutron capture was observed at the depth of 10 cm in the concrete wall near the deflector of the cyclotron. Tritium was also produced by the neutron reaction, because its concentration was proportional to the activities of ¹⁵²Eu and ⁶⁰Co. The surface dose rates inside the accelerator room were also monitored to define the decontamination area. The surface dose rate was proportional to the residual radioactivity, such as ⁶⁰Co. A careful evaluation was very useful in order to minimize the radioactive waste during decontamination.