Development of Scaling Factor Method for Activated Metal Wastes of Medical Cyclotrons

Medical cyclotrons, widely used to produce radiopharmaceuticals such as ¹⁸F-FDG for cancer diagnostics, cause the activation of the medical cyclotron itself and the surrounding structures by protons and neutrons produced mainly by the (p,n) reaction. Recently, there has been increased demand for indirect assessment methods to reduce the cost and time involved in analyzing the radioactivity of specific nuclides when cyclotrons are decommissioned. The scaling factor method, which is one of the indirect assessment methods, can be considered a promising candidate for evaluation of the radioactivity of activated waste, as it is based on correlations between the radioactivities of easily measurable gamma-emitting nuclides and difficult-to-measure nuclides. In this study, a scaling factor method was developed for the first time for activated metal wastes in medical cyclotrons. In an initial investigation, the radioactivity of Havar foils for various isotopes was evaluated by Monte Carlo simulation and experimental measurement. A correlation analysis indicated a high level of correlation for ⁵⁵Fe/⁵⁷Co, ⁵⁹Ni/⁵⁷Co, and ⁶³Ni/⁵⁴Mn. The scaling factor for ⁵⁵Fe/⁵⁷Co, which exhibited a relatively high correlation, was evaluated by linear regression. Meanwhile, those for ⁵⁹Ni/⁵⁷Co and ⁶³Ni/⁵⁴Mn, which exhibited comparatively low correlations, were evaluated by the geometric mean. As a first step, these results validate the preliminary applicability of the scaling factor method for characterization of activated metal waste in medical cyclotrons.