Cross-section measurement of thulium radioisotopes with an 18 MeV medical PET cyclotron for an optimized 165Er production

¹⁶⁵Er is a pure Auger-electron emitter with promising characteristics for therapeutic applications in nuclear medicine. The short penetration path and high Linear Energy Transfer (LET) of the emitted Auger electrons make ¹⁶⁵Er particularly suitable for treating small tumor metastases. Several production methods based on the irradiation with charged particles of Er and Ho targets can be found in the literature. In this paper, we report on the study of ¹⁶⁵Er indirect production performed via the ¹⁶⁶Er(p,2n)¹⁶⁵Tm →¹⁶⁵Er reaction at the 18 MeV Bern medical cyclotron. Despite the use of highly enriched ¹⁶⁶Er₂O₃ targets, several Tm radioisotopes are produced during the irradiation, making the knowledge of the cross sections involved crucial. For this reason, a precise investigation of the cross sections of the relevant nuclear reactions in the energy range of interest was performed by irradiating Er₂O₃ targets with different isotopic enrichment levels and using a method based on the inversion of a linear system of equations. For the reactions ¹⁶⁴Er(p, γ)¹⁶⁵Tm, ¹⁶⁶Er(p,n)¹⁶⁶Tm, ¹⁶⁶Er(p, γ)¹⁶⁷Tm, ¹⁶⁷Er(p,3n)¹⁶⁵Tm, ¹⁶⁷Er(p, γ)¹⁶⁸Tm, ¹⁶⁸Er(p,2n)¹⁶⁷Tm and ¹⁷⁰Er(p,3n)¹⁶⁸Tm, the nuclear cross section was measured for the first time. From the results obtained, the production yield and purity of the parent radioisotope ¹⁶⁵Tm were calculated to assess the optimal irradiation conditions. Several production tests with solid targets were performed to confirm these findings.