Calibration of monitors used for surveillance of radioactivity in effluent water from CERNʹs accelerator installations

Water released into the environment from CERNʹs accelerator installations may contain both long-lived (7Be, 22Na) and short-lived (11C, 13N, 24Na) gamma radioactivity. Each potential release point is equipped with an on-line monitor for short-lived radionuclides, which consists of a scintillation probe immersed in a tank filled with monitored water. Whilst calibration standards are available for long-lived radioactivity, computer simulations are the only feasible way to determine the monitor efficiency for the short-lived radionuclides. The paper describes computer simulations using the Monte Carlo code GEANT 3.21. An excellent agreement between measured and computed efficiencies was obtained for the long-lived radionuclides, validating the computer model. A calibration method is proposed for light positron emitters, which combines an experimental calibration for 7Be and correction factors obtained in the simulations.