Development of intense gamma-ray source monitoring system in water for radiation safety

A gamma-ray dose measurement system with a small-sized cesium iodide scintillator and a long-optical-fiber combination is developed to monitor an intense cobalt-60 gamma-ray radiation source in a deep-water pool, for radiation safety. The compact sensor consists of cesium iodide doped with thallium scintillator and a light focusing lens component. The luminescence of the scintillator, as the gamma-ray response, is collected using the lens and introduced into the 10 m-long optical fiber. Finally, the light output from the sensor is input to a photomultiplier and converted to an output voltage corresponding to the luminescence intensity. A 940 TBq cobalt-60 gamma-ray source is placed at the pool bottom and used as a target for the monitoring experiment. The dose rate to water is measured with two sensors located at different depth positions. By moving the sensors horizontally, the dose rates at several positions within the pool are measured and compared. The distance to the source can be estimated from the dose rate measured at the sensor position. Observations of the dose rate changes by moving both the sensors vertically against the pool bottom indicate that the motion of the source in an upper direction can be detected using two sensors located at different depths.