Radiometric characterization for the decommissioning of the Scanditronix MC17 medical cyclotron of the Italian National Cancer Institute of Milano

Aim and background. This work aims at presenting the first radiometric characterization of the non-self-shielded Scanditronix MC17 medical positive ions cyclotron of the National Cancer Institute of Milano, conceived for producing ¹¹C and ¹⁸F for PET. This study allowed to carry out one of the very first decommissioning of a medical cyclotron in Italy. Constituent metals of the cyclotron, the walls of the vault and all the auxiliary structures inside (pumps, water and gas pipes) have been exposed for 20 years to intense and mixed radiation fields mainly given by fast neutrons (energy up to about 10 MeV) produced by the 17 MeV ¹⁸O(p,n)¹⁸F nuclear reaction. Metals inside the acceleration chamber were exposed also to direct and scattered proton beam. The consequent activation, not crucial during normal machine operation, became a fundamental issue to face for a safe management of the accelerator decommissioning. For this reason, after the cyclotron definitive shutdown, a qualitative and quantitative characterization of the radioactivity concentration of its main components was performed to define and optimize decommissioning. Materials and methods. Several representative samples of different compositions (bolts, screws, slabs, connectors) were removed from the external surface and from acceleration chamber. The experimental characterization involved high resolution gamma spectrometry by means of the ISOCS detector (In-situ Object Counting System, Mirion Technologies). This was followed by Monte Carlo simulations performed with the FLUKA code, in order to compare experimental and simulated results. Results. This study highlighted the presence of long-lived isotopes, in particular ⁶⁰Co, ⁵⁴Mn and ⁶⁵Zn, whose activity concentration was always above the reference clearance levels, equal to 0.1 Bg g^-1 (IAEA RS-G-1.7, 2004). These values resulted to be strongly dependent on the material composition and on the position with respect to the cyclotron target, leading to very different time scenarios. Components with low amount of Nickel showed the predominance of ⁵⁴Mn, produced by the ⁵⁴Fe(n,p) reaction, whose relatively short half-life of 0.86 y allows to bring the activity concentration below the clearance levels in few years. On the contrary, on materials with non-negligible Nickel content, the major contribution is due to the ⁶⁰Ni(n,p)⁶⁰Co reaction (⁶⁰Co half-life 5.28 y), which keeps the activity concentration above the clearance levels for tens of years. Very different activity concentration levels were measured, even for similar components, especially for samples removed from the acceleration chamber. This is due to the strong dependence of the activation on the primary beam trajectory and on the spatial distribution of beam losses, which produce a large amount of ⁶⁵Zn from the 65Cu(p,n) reaction. (author)