Unconventional 67Cu production using high-energy bremsstrahlung and cross section evaluation

Background: The use of theranostic isotope pairs is an emerging field in contemporary medicine, yet their clinical availability is often constrained by production and supply chain challenges. Among them, 67Cu has gained significant interest as the therapeutic counterpart in the 67Cu / 64Cu theranostic pair. Traditional production methods rely on reactor-based or charged-particle accelerator routes, both of which face limitations in scalability and isotopic purity. Purpose: This work investigates the photonuclear production of 67Cu via the 68Zn(γ, p)67Cu reaction, using high-energy bremsstrahlung photons from an electron accelerator. In addition to quantifying production yields and radionuclidic purity, the work provides spectrum-weighted average cross sections for multiple emission channels, serving as a test of reaction modeling under broad excitation conditions. The results inform both the optimization of 67Cu production for theranostic applications and the improvement of nuclear reaction theory in an underexplored energy regime. Method: Natural zinc targets were irradiated using bremsstrahlung photons from an 855 MeV electron beam at the Mainzer Mikrotron (MAMI). The photon energy spectrum was measured via tagging and extended beyond the tagged range using Geant4 simulations. Reaction products were identified via gamma-ray spectroscopy, and spectrum-weighted average cross sections were derived. Theoretical cross sections were calculated using talys, incorporating giant dipole resonance and quasideuteron components of the photon absorption cross section. A model sensitivity analysis was performed to assess the influence of level density and photon strength function variations on the predicted cross sections. Results: A production yield of 0.410(16) MBq µA-1h-1g-1 was measured for 67Cu, with corresponding average cross sections determined from the bremsstrahlung spectrum. Yields and average cross sections were also measured for coproduced isotopes including 64Cu, 61Cu, 60Cu, 69mZn, 65Zn, 63Zn, and 62Zn. The relatively low production of nontarget isotopes indicates that the 68Zn(γ, p)67Cu reaction proceeds with favorable radionuclidic selectivity under the applied irradiation conditions. talys calculations showed good agreement with the (γ,p) channel, but neutron-emission and multi-particle channels were systematically overestimated. Model sensitivity points to assumptions in the photon strength function and level density, as well as the absence of explicit pion-resonance dynamics at high energies, as likely contributors to this behavior. Conclusions: Photonuclear reactions provide a promising approach for high-purity 67Cu production. Theoretical discrepancies highlight limitations in current reaction models, especially for (γ,xn) channels at high energy. Beyond this, the scalability of electron-driven methods, enabled by existing high-current infrastructure and compact low-energy linacs, offers flexibility for both centralized and decentralized isotope production. Continued development of accelerator technologies and nuclear models will be essential for expanding access to photonuclear-produced medical isotopes.