Open Access Publications from the University of California

## Next-Generation Isotope Production via Deuteron Breakup

Abstract

As part of the next generation of cancer treatment, targeted alpha therapy (TAT) has demonstrated the potential to improve patient outcomes by targeting cancer cells with highly cytotoxic $\alpha$-emitters. These are prized for their ability to efficiently kill cancer cells, while dealing minimal damage to nearby healthy tissue. When properly paired with a positron-emitting radionuclide, a rapid bio-assay of the therapeutic agent can be performed using positron emission tomography (PET), enabling micro-dosimetry and personalized treatment planning. One of the main challenges associated with these treatments is the production of the $\alpha$-emitting radionuclides in quantities which are relevant to widespread clinical use.

In this work, we seek to improve the understanding of the isotope production pathways for \ce{^{225}Ac}, one promising candidate for TAT, and \ce{^{134}Ce}, which is a positron-emitting analog of actinium. The main pathway which we explored was through the \ce{^{226}Ra}(n,2n) reaction, which produces the \ce{^{225}Ac} generator isotope \ce{^{225}Ra}. Thick target deuteron breakup was utilized as a high intensity, variable energy neutron source for inducing this reaction.

A parameterized hybrid model for the double differential neutron spectra from thick target deuteron breakup was developed, and the optimal parameters for the model were determined through a fit to selected literature data. Measurements of the spectrum were performed on a beryllium target at 33 and 40 MeV deuteron energies, using the activation and time-of-flight techniques, and these measurements were compared to the hybrid model and literature data. Additionally, production cross sections for several other emerging radionuclides were measured using the activation technique, with the deuteron breakup neutrons.

The \ce{^{226}Ra}(n,2n) reaction cross section was measured with the breakup spectra from 33 and 40 MeV deuterons on beryllium, using the activation method and $\alpha$ spectroscopy. These measurements were compared to the predictions of the TENDL-2015 and ENDF/B-VII.1 evaluations. A chemical separation of the produced \ce{^{225}Ac} was performed following both irradiations, to evaluate the separation capabilities of the DGA and AG50 cation exchange resins, as well as to search for any co-produced impurities such as \ce{^{227}Ac} or fission products.

Additionally, measurements of the \ce{^{139}La}(p,6n)\ce{^{134}Ce} cross sections from 35--60 MeV were performed using the stacked-foil activation technique. Thick target yield calculations showed this to be a promising pathway for \ce{^{134}Ce} production, which is a positron-emitting chemical analog to \ce{^{225}Ac}. Several other \ce{^{139}La}(p,x) reaction channels were also measured, which are useful for quantifying impurities and for evaluating nuclear reaction models. These measurements were compared to the predictions of the TALYS, EMPIRE and ALICE nuclear reaction codes, which showed that the default pre-equilibrium models were insufficient, providing motivation for further study of these processes.

Finally, a comparison study was performed between several of the most promising \ce{^{225}Ac} production routes. The \ce{^{232}Th}(p,x)\ce{^{225}Ac} pathway showed generally good yields, but was shown to contain significant \ce{^{227}Ac}, which may have negative long term side-effects, depending on the bio-chemistry of the complexed actinium molecule. The \ce{^{232}Th}(p,4n)\ce{^{229}Pa} pathway had a high purity, but poor yields. Pathways involving directly irradiating \ce{^{226}Ra} targets with charged particles or photons were shown to have engineering issues regarding target heating, that must be overcome if they are to be pursued further. And the \ce{^{226}Ra}(n,2n) pathway using secondary neutrons from deuteron breakup was shown to have high purity, and had yields comparable to \ce{^{232}Th}(p,x) if a radium target of 10--50 grams is used.

The primary goals of this thesis are to improve the understanding of deuteron breakup, both through measurements and modeling, and to highlight its potential applicability for isotope production purposes. It is the hope that this will inspire a more widespread adoption of technology utilizing deuteron breakup as a neutron source, and will lead to an improvement in the supply of \ce{^{225}Ac} available for research and clinical applications.