The developments of nuclear medicine lead to an increasing demand for the production of radioisotopes with suitable nuclear and chemical properties. Furthermore, from the literature it is evident that the production of radioisotopes using charged-particle accelerators instead of nuclear reactors is gaining increasing popularity. The main advantages of producing medical isotopes with accelerators are carrier free radionuclides of short lived isotopes, improved handling, reduction of the radioactive waste, and lower cost of isotope fabrication. Proton-rich isotopes are the result of nuclear interactions between enriched stable isotopes and energetic protons. An interesting observation is that during the production of proton-rich isotopes, fast and intermediately fast neutrons from nuclear reactions such as (p,xn) are also produced as a by-product in the nuclear reactions. This observation suggests that it is perhaps possible to use these neutrons to activate secondary targets for the production of neutron-rich isotopes. The study of secondary radioisotope production with fast neutrons from (p,xn) reactions using a particle accelerator is the main goal of the research in this thesis.

There is a present and future need for the medical isotope Actinium-225, currently in short supply worldwide.  Only a couple manufacturers produce it in very low quantities.  In roughly the past 10 years the medical community has explored the use of Ac-225 and its daughter Bismuth-213 for targeting a number of differing cancers by way of Targeted Alpha Therapy (TAT). This method utilizes the alpha-decay of both Ac-225 (half-life 10 days) and Bi-213 (half-life 46 min) to kill cancerous cells on a localized basis.  Maximum energy is delivered to the cancer cells thereby greatly minimizing healthy tissue damage. This research proposes a production method using a high-energy photon spectrum (generated by a linear accelerator or LINAC) to irradiate a sample of Radium-226 (half-life 1600yrs).  The photo-neutron reaction liberates neutrons from Ra-226 atoms leaving behind Radium-225 (half-life 14.7 days).  Ra-225 decays naturally through beta emission to Ac-225.  Previous research demonstrated it is possible to produce Ac-225 using a LINAC; however, very low yields resulted which questioned the feasibility of this production method.  This research proposes a number of LINAC and radium sample modifications that could be greatly increase yield amounts for practical use. Additionally, photo-neutron cross-section data for Ra-226 was used, which led to improved yield calculations for Ra-225.  A MATLAB® model was also created, which enables users to perform quick yield estimates given several key model parameter inputs.  Obtaining a sufficient supply of radium material is also of critical importance to this research.  Therefore information was gathered regarding availability and inventory of Radium-226.  This production method would serve as a way to not only eliminate many hazardous radium sources destined for interim storage, but provide a substantial supply of Ac-225 for future cancer treatment. / Master of Science

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