Theoretical optimal232Th target thicknesses for225Ac production at proton energies of 70-200 MeV

Objective. Conventional232Th target thicknesses used for225Ac production are substantially smaller than the proton ranges, underutilizing the225Ac production potential of proton beams. This study explores theoretical optimal thicknesses of232Th targets at widely used proton energies, 70-200 MeV.Approach. Yields of225Ac and impurity levels of227Ac were calculated for proton energies of 70-200 MeV and232Th target thicknesses of 0.05-24 mm using Monte Carlo simulations. Ranges of optimal target thicknesses were defined for each proton energy based on the relative rates of change (RROCs) of225Ac yields per target thickness of 0.25 mm and on energy-adjusted RROC thresholds. Expected225Ac yield gains from using the optimal thicknesses were also estimated.Main results. The ranges of theoretical optimal thicknesses for representative energies, 70, 100, 160, and 200 MeV, were found to be 2-2.25 mm, 4.5-6.25 mm, 7.75-14.5 mm, and 14.25-21.75 mm, respectively. All these thicknesses are markedly larger than those of the conventional thin targets used for medium- to large-scale225Ac production, 0.25 mm and 0.5 mm. By using these optimal target thicknesses,225Ac yields are expected to increase by factors of up to 8.8 at 70 MeV and 63.8 at 200 MeV. Actinium-227 impurity levels were unaffected by target thickness optimization at all proton energies.Significance. Optimizing232Th target thicknesses can provide proton accelerator facilities operating in the 70-200 MeV range with a straightforward means of increasing their225Ac production capacity. Optimal target thicknesses offer greater225Ac yield scalability at higher energies near 200 MeV, while enabling large-scale225Ac production at lower energies close to 70 MeV.