Distribution of residual long-lived radioactivity in the inner concrete walls of a compact medical cyclotron vault room.

Objective: Compact medical cyclotrons have been set up to generate the nuclides necessary for positron emission tomography. In accelerator facilities, neutrons activate the concrete used to construct the vault room; this activation increases with the use of an accelerator. The activation causes a substantial radioactive waste management problem when facilities are decommissioned. In the present study, several concrete cores from the walls, ceiling and floor of a compact medical cyclotron vault room were samples 2 years after the termination of operations, and the radioactivity concentrations of radionuclides were estimated.

Methods: Cylindrical concrete cores 5 cm in diameter and 10 cm in length were bored from the concrete wall, ceiling and floor. Core boring was performed at 18 points. The gamma-ray spectrum of each sample was measured using a high-purity germanium detector. The degree of activation of the concrete in the cyclotron vault room was analyzed, and the range and tendency toward activation in the vault room were examined.

Results: (60)Co and (152)Eu were identified by gamma-ray spectrometry of the concrete samples. (152)Eu and (60)Co are produced principally from the stable isotopes of europium and cobalt by neutron capture reactions. The radioactivity concentration did not vary much between the surface of the concrete and at a depth of 10 cm. Although the radioactivity concentration near the target was higher than the clearance level for radioactive waste indicated in IAEA RS-G-1.7, the mean radioactivity concentration in the walls and floor was lower than the clearance level.

Conclusion: The radioactivity concentration of the inner concrete wall of the medical cyclotron vault room was not uniform. The areas exceeding the clearance level were in the vicinity of the target, but most of the building did not exceed the clearance levels.