Measurement of thermal neutron fluence distribution with use of ²³Na radioactivation around a medical compact cyclotron

A medical compact cyclotron produces about 10¹⁵ neutrons per day along with 100 GBq of ¹⁸F. Therefore, it is important to establish radiation safety guidelines on residual radioactivity for routine operation, maintenance work, and decommissioning. Thus, we developed a simple method for measuring the thermal neutrons in a cyclotron room. In order to verify the feasibility of our proposed method, we measured the thermal neutron distribution around a cyclotron by using the activation of ²³Na in salt. We installed 78 salt dosimeters in the cyclotron room with a 50 cm mesh. The photopeak of ²⁴Na was measured, and the neutron flux distribution was estimated. Monitoring the neutron flux distribution in a cyclotron room appears to be useful for not only obtaining an accurate estimate of the distribution of induced radioactivity, but also optimizing the shield design for radiation safety in preparation for the decommissioning process.