TOPAS Simulations for Estimating Cyclotron-Based Production Yield of Vanadium-48

Objectives: While many medical cyclotrons do not have a solid target system, they often have components that can be manipulated to allow for the irradiation of solid targets, such as placing thin foils in a beamstop. Previous research has used such a setup to produce vanadium-48for colorectal cancer imaging [1]. Due to its long half-life, ⁴⁸Valso has potential application in long-term monitoring and longitudinal studies of biological and physiological interest. [2] Due to the inexact nature of the makeshift solid target system, several parameters contributing to the activity yield are often unknown. In this study, we simulate the makeshift solid target system via TOPAS, a Geant4-based Monte Carlo program for proton and heavy ion simulations, to investigate how beam characteristics and target properties impact the activity produced. The simulated values are compared to experimental results of activity yield when irradiating thin, natural titanium foils. Methods: Experimentally, 12 µm natural titanium foils were irradiated with 18 MeV protons via an IBA 18/9 Cyclone at 10 or 20 µA for 1 hour; this was repeated with various target geometries. In simulation, beam properties were chosen using values from the literature for the IBA 18/9 Cyclone and 18 MeV protons [2]. Resulting beam width was estimated using a detector panel at the target position and measuring the energy deposited. Several physics lists were used to predict cross sections and the corresponding production rates for the reaction. To assess the cross section, a stack of natural titanium foils was simulated separately with thicknesses such that 1 MeV was lost per foil. The number of ⁴⁸V ions generated in each foil was measured and converted to cross section. To assess activity yielded, 12 µm natural titanium foils and their geometries were simulated being irradiated by 10⁸ 18 MeV protons. The number of ions generated in each foil was recorded and converted to radioactive yield. All simulations were repeated ten times with different seed numbers and summed. Results: Cross sections were obtained from: 1) TENDL (a theoretical calculation of the ⁴⁸Ti(p,n)⁴⁸V reaction [3]), 2) EXFOR (experimental results [4]), and those generated in simulation using TOPAS, 3) default physics list and the 4) QBBC physics list, chosen based on the literature. At lower energies, the default list (#3) produces results comparable to EXFOR, while the QBBC list (#4) overestimates the cross sections. At higher energies, TOPAS overestimates the cross section for both physics lists. In comparing radioactive yields generated in simulation and experimentally by irradiating flat foils at 18 MeV, the simulated activity for a single foil with 10 µA current was similar to experimental values, differing by 10% when using the TOPAS default physics list and by 17% for the QBBC list. Similarly, the experimental activity from irradiating a foil at 20 µA differed by 16% with the default physics list and 24% with the QBBC list. This difference is due to the differing cross sections for each of these lists at 18 MeV. Conclusion: TOPAS provides a reasonable estimate of radioactivity generated by cyclotron-based irradiation of natural titanium foils. While an apparent over-estimation of the cross section for commonly used physics lists limits the possibility of construing an exact radioactivity measurement, this tool can be used to estimate the radioactivity in order to guide experiments and inform experimental outcomes.