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Understanding radionuclide production in cyclotron target systems Zacchia, Nicholas Alexander

Abstract

Diagnostic scans in nuclear medicine often make use of radionuclides produced on-demand using fluid target systems on medical cyclotrons. The prevailing theoretical model of radionuclide production applies adequately to solid target systems but overlooks a host of behaviour unique to fluid targets and poorly describes the radioactivity recovered from them. The design of more efficient and reliable target systems for nuclear medicine demands an understanding of these distinct phenomena. This thesis augments the foundational work of Krasnov [1974] in describing radionuclide production by providing theoretical models and experimental investigations elucidating fluid effects in target systems. Examining gas target systems, a new production model is created which accounts for radioactivity adsorbing to the walls of the target body. The equations of this model are simplified by solving for leading-order terms and used to describe a variety of data sets. This adsorption model is expanded to incorporate in-target chemical kinetics by examining ¹¹C production. The expanded model is simplified by neglecting contributions from short lived species and the results are generalized into an augmented production equation that is able to describe experimental data with great accuracy. The model provides expressions for the initial production rate and saturation activity in these target systems and indicates that inefficiencies in [¹¹C]CH₄ production result primarily from adsorption of precursor radioactive species. These new models show that adsorption is the primary driver of low activity yields and paves the way for new target designs that limit adsorption, favour chemical conversion and induce desorption for better radioactivity recovery. In liquid solution targets the formation of radiolytic gas products is experimentally shown to be sensitive to initial solution composition. Radiolysis is reduced by over 60% by introducing nitrite to low pH solutions prior to irradiation. Finally, foil degradation in solution targets is studied with experiments indicating that electrochemical etching is responsible for premature target failure. This work lays the foundation for a fundamental understanding of radionuclide production in fluid target systems and is a necessary step in making radionuclides more easily available for nuclear medicine.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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