Speaker
Description
Medical cyclotrons, which are used to produce radiopharmaceuticals such as 18F-FDG for cancer diagnosis, cause the activation of the medical cyclotron itself and surrounding structures through various nuclear reactions. In 2024, approximately 30% of the cyclotrons are effectively out of operation in the Republic of Korea. Therefore, there is a growing need for indirect assessment methods to reduce the costs and time involved in analyzing the activities of radionuclides when cyclotrons are decommissioned or dismantled. In the case of radioactive waste from nuclear power plants, the scaling factor method is used for waste management based on technical documents from international standards such as the International Organization for Standardization (ISO) and the International Atomic Energy Agency (IAEA). It is used to indirectly estimate the activities of difficult-to-measure (DTM) nuclides, such as alpha- and beta-emitting nuclides, based on the activities of key nuclides, such as gamma-emitting nuclides, by using correlation between nuclides. However, the applicability of the scaling factor methods to activated metal waste from medical cyclotrons has not been fully verified and implemented. In this study, activation scenarios were constructed on the basis of technical documents for nuclear power plants and CERN technical documents for high-energy accelerators in order to verify the applicability of the scaling factor method for medical cyclotrons. These activation scenarios take into account the operating conditions of medical cyclotrons in Korea. The Monte Carlo simulation code, PHITS-DCHAIN, was used to evaluate the activation of materials. PHITS is a particle transport code and DCHAIN is a radiation analysis code, using default libraries for cross-section data. Based on the results for various activation scenarios, SPSS software was used to conduct scatter plots and Pearson correlation analyses to determine the relationship between key nuclides and DTM nuclides. The target foil, known as Havar foil, used to maintain a vacuum in the target system, was chosen for evaluation. The target foil is located close to the target material and thus activated by both primary protons and secondary neutrons, and the H218O liquid target was considered, as it is the most commonly used target material. The activation scenarios included variations in beam energy, material composition, irradiation time, and decay time. These variables were integrated into discrete scenarios. The target foil’s irradiation conditions, which is proton beam energy and current, were fixed at 16.5 MeV and 50 μA, respectively, matching the specifications of the GE PETtrace 880. Material composition was determined based on XRF analysis and literature review: cobalt (40–44.2%), chromium (18.8–22.2%), iron (18–19%), nickel (11.6–13%), manganese (1.3–2%), molybdenum (1.4–2%), tungsten (0.9–3%), carbon (0–2%), and beryllium (0–0.3%). The irradiation and decay times were chosen to 2–30 months and 1–6 years, respectively. These parameters were chosen to reflect the replacement cycle of target foils and typical domestic operation intervals for medical cyclotrons. In the absence of complete historical data on cyclotron operation, decay time was uniformly assumed across the irradiation duration to conservatively approximate the actual operational history with irradiation patterns. Ultimately, 40 variable combinations were randomly selected to produce 40 activation scenarios. Simulation results identified a range of nuclides, including 3H, 14C, 54Mn, 55Fe, 58Co, 60Co, 59Ni, and 63Ni, which exceeded clearance levels. 54Mn, 56Co, 58Co, and 60Co are considered as key nuclides, while 55Fe and 63Ni are considered as DTM nuclides. Scatter plots and Pearson correlation analyses indicated a generally linear relationship between most key and DTM nuclides, except for 56Co, which emerged as an outlier. The correlation coefficients for 54Mn, 58Co, and 60Co were above 0.80 with DTM nuclides, suggesting that these nuclides are reliable for estimating the activities of DTM nuclides. However, 56Co showed weak and statistically insignificant correlations, with correlation coefficients of -0.09 and -0.07, respectively. The low correlation of 56Co with DTM nuclides was attributed to differences in production mechanisms and its shorter half-life, making it unsuitable as a key nuclide. Nuclear reactions of the type (p, n) or (n, p) are responsible for the production of 58Co and 60Co from natural occurring isotopes of iron or manganese. Unlike other nuclides, 56Co production involves intermediate nickel isotopes. Future work will include verifying the nuclide correlations observed in this study through the analysis of actual irradiated samples from decommissioned cyclotrons. Through regression analysis, specific scaling factors for the target foil will be derived, further refining the indirect estimation method. This verification step will help assess the reliability and accuracy of the scaling factor method for medical cyclotrons, supporting sustainable and cost-effective decommissioning practices.
