Production of Th229 for medical applications: Excitation functions of low-energy protons on Th232 targets

Abstract

As a part of a general program to evaluate production routes for $^{229}\mathrm{Th}$, we studied production of $^{229}\mathrm{Th}$ via proton-induced reactions on $^{232}\mathrm{Th}$ targets bombarded with low-energy protons, ${E}_{\mathrm{p}}\ensuremath{\le}40\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. The reported excitation functions include those for proton-induced reactions on natural thorium yielding to $^{228,229,230&232}\mathrm{Pa}$ isotopes; $^{232}\mathrm{Th}$(p,xn) reactions, where $x=1$, 3, 4, and 5, at proton energy ranges of 12--40 MeV. Although the data for $^{232}\mathrm{Th}(p,n)^{228}\mathrm{Pa}$, $^{232}\mathrm{Th}(p,3n)^{230}\mathrm{Pa}$, and $^{232}\mathrm{Th}(p,5n)^{232}\mathrm{Pa}$ reactions were deduced by direct analysis of the thorium foils after irradiation, the data for $^{232}\mathrm{Th}(p,4n)^{229}\mathrm{Pa}$ were obtained by radiochemical techniques. The half-life of $^{229}\mathrm{Pa}$ was evaluated and determined to be 1.55 \ifmmode\pm\else\textpm\fi{} 0.01 d. Further, the \ensuremath{\alpha}-branching ratio, \ensuremath{\alpha}/(\ensuremath{\alpha} + EC) of $^{229}\mathrm{Pa}$ was evaluated to be 0.53 \ifmmode\pm\else\textpm\fi{} 0.10% by allowing $^{229}\mathrm{Pa}$ to decay for $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{d}$, then chemically extracting and quantifying the $^{225}\mathrm{Ac}$ (${t}_{1/2}=10.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{d}$) from $^{229}\mathrm{Pa}$ samples. In addition, we report the effective production cross section of $^{229}\mathrm{Th}$ in a thick $^{232}\mathrm{Th}$ target in the proton energy range of 23--33 MeV. The peak of the excitation function for the $^{232}\mathrm{Th}(p,4n)^{229}\mathrm{Pa}$ reaction occurs at 162 \ifmmode\pm\else\textpm\fi{} 14 mb and ${E}_{p}=29.7\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. This is only slightly larger than the effective cross section for the $^{232}\mathrm{Th}(p,x)^{229}\mathrm{Th}$ reaction (obtained from a thick target experiment). This data indicates that the $^{232}\mathrm{Th}(p,4n)^{229}\mathrm{Pa}$ reaction is the major reaction pathway for the cumulative $^{232}\mathrm{Th}(p,x)^{229}\mathrm{Th}$ 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 $^{229}\mathrm{Th}$ 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.