Simplified Method for Determining Residual Specific Activity in Activated Concrete of a PET-cyclotron Room Using a Survey Meter

An in situ method to determine the specific activity in concrete structures using a survey meter was established in this study for decommissioning accelerator rooms. This method is much simpler than the conventional sampling method that requires core boring. A survey meter (T-SP2, TAC Inc., Japan) was customised and equipped with a small detector probe, whose size allowed the design of a light-weight Pb shield. For dose rate measurement, a Pb shield thickness of 4.5 cm was enough to shield ambient γ-rays, and its weight (16 kg) was low enough for the device to be carried by a human. A calibration curve was obtained to quickly and easily convert the net contact dose rate on concrete into the specific activity in a PET cyclotron room. The specific activity detection limit for this device was lower than the clearance limit of ¹⁵²Eu and ⁶⁰Co in activated materials. Therefore, this method can be used for separating nonactivated concrete from activated concrete during decommissioning. A general correction factor for the slope of the calibration curve was also obtained for use at other accelerator facilities.