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Production of 229Th for medical applications: Excitation functions of low-energy protons on 232Th targets...

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Physical Review C
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As a part of a general program to evaluate production routes for 229Th, we studied production of 229Th via proton-induced reactions on 232Th targets bombarded with low-energy protons, Ep≤40MeV. The reported excitation functions include those for proton-induced reactions on natural thorium yielding to 228,229,230&232Pa isotopes; 232Th(p,xn) reactions, where x=1, 3, 4, and 5, at proton energy ranges of 12–40 MeV. Although the data for 232Th(p,n)228Pa, 232Th(p,3n)230Pa, and 232Th(p,5n)232Pa reactions were deduced by direct analysis of the thorium foils after irradiation, the data for 232Th(p,4n)229Pa were obtained by radiochemical techniques. The half-life of 229Pa was evaluated and determined to be 1.55 ± 0.01 d. Further, the α-branching ratio, α/(α + EC) of 229Pa was evaluated to be 0.53 ± 0.10% by allowing 229Pa to decay for ∼7d, then chemically extracting and quantifying the 225Ac (t1/2=10.0±0.1d) from 229Pa samples. In addition, we report the effective production cross section of 229Th in a thick 232Th target in the proton energy range of 23–33 MeV. The peak of the excitation function for the 232Th(p,4n)229Pa reaction occurs at 162 ± 14 mb and Ep=29.7±0.5MeV. This is only slightly larger than the effective cross section for the 232Th(p,x)229Th reaction (obtained from a thick target experiment). This data indicates that the 232Th(p,4n)229Pa reaction is the major reaction pathway for the cumulative 232Th(p,x)229Th reaction cross section in this energy range. The measured cross sections were compared with theoretical cross sections using the simulation codes Particle and Heavy Ion Transport code System (PHITS) and Monte Carlo Neutral Particle 6 (MCNP6). At proton energy ranges of 12–33 MeV, the cumulative excitation function predicted by PHITS for the reactions leading to 229Th was in close agreement with the experimental function, whereas the function predicted by MCNP6 was a factor of two higher at the peak of the excitation function.