Investigation of Concrete Radioactivation in Cyclotron Type Proton Therapy Facilities using in situ ²⁴Na Measurement Method

Abstract

 It is essential to clarify the characteristics of the material activation in cyclotron-type proton therapy facilities for future decommissioning. In particular, concrete occupies the largest volume among object materials, but its activation characteristics of activity level and distribution are unknown. We investigated concrete radioactivation in cyclotron-type proton therapy facilities at the National Cancer Center Hospital East and Aizawa Hospital. The in situ ²⁴Na measurement method, which is easy and high sensitivity, was applied for the investigation. Using the in situ ²⁴Na measurement method, we obtained the thermal neutron fluence rates in concrete with high sensitivity. Based on these fluence rates, the sum of the ¹⁵²Eu and ⁶⁰Co specific activities in the concrete were predicted for an operation duration of 30 years. The study aims to predict general ¹⁵²Eu and ⁶⁰Co (which are major radionuclides) specific activities for cyclotron-type proton therapy facilities, and we found the following common characteristics in the investigation results. In the accelerator room, neutrons were mainly produced at the degrader and/or objects surrounding it and diffused over the room. As a result, the sum of the ratios of the concrete floor's specific activities of ¹⁵²Eu and ⁶⁰Co for their respective clearance level (0.1 Bq/g) was estimated to be close to 1. On the other hand, the production of neutrons both in the treatment and beam transport line rooms was low, and the sum of the ratios of the ¹⁵²Eu and ⁶⁰Co specific activities for their respective clearance level was estimated to be below 1.