Cross sections of Sm(Li,x) reactions for the production of Tb for targeted alpha therapy

Terbium-149g (= 4.12 h) is of particular interest for targeted alpha therapy cancer treatment due to its ability to decay via both alpha and positron emission, making it a potential theranostic nuclide. Due to many challenges facing its production, there are limited facilities worldwide that have demonstrated the ability to produce this nuclide in quantities sufficient for medical research. Since the Cyclotron Institute at Texas A&M University is a specialized accelerator facility capable of accelerating a wide variety of ions, we are investigating production pathway options. One of the major challenges facing its production is the known co-production of the excited isomeric state, Tb (= 4.1 min). However, this state does not decay to the ground state of Tb, negating any potential contribution to its yield. Due to its short-half life, the cross section for the population of this state has never been measured. After calculating several potential reaction yields using predictive models, the reactions of Sm(Li,xn)Tb were identified as candidates. Lithium-6 beams of varied energies between 45-65 MeV were impinged on enriched Sm, Sm, and Sm targets at the Cyclotron Institute at Texas A&M University, and the reaction products were measured immediately following irradiation using high-purity germanium detectors, enabling detection of both Tb and Tb. Cross sections for all nuclides produced in sufficient activity in these reactions were also measured and reported here. We conclude that the population of Tb is much preferred over population of the ground state for these Li-induced reactions, and it is necessary to explore other options for Tb production.