Production of ⁸⁹Zr via the ⁸⁹Y(p,n)⁸⁹Zr reaction in aqueous solution: Effect of solution composition on in-target chemistry

Objective: The existing solid target production method of radiometals requires high capital and operational expenditures, which limit the production of radiometals to the small fraction of cyclotron facilities that are equipped with solid target systems. Our objective is to develop a robust solution target method, which can be applicable to a wide array of radiometals and would be simply and easily adopted by existing cyclotron facility for the routine production of radiometals. Method: We have developed a simplified, solution target approach for production of ⁸⁹Zr using a niobium target by 14 MeV energy proton bombardment of aqueous solutions of yttrium salts via the ⁸⁹Y(p,n)⁸⁹Zr nuclear reaction. The production conditions were optimized, following a detailed mechanistic study of the gas evolution. Results: Although the solution target approach avoided the expense and complication of solid target processing, rapid radiolytic formation of gases in the target represents a major impediment in the success of solution target. To address this challenge we performed a systematic mechanistic study of gas evolution. Gas evolution was found to be predominantly due to decomposition of water to molecular hydrogen and oxygen. The rate of gas evolutions varied >40-fold depending on solution composition even under the same irradiation condition. With chloride salts, the rate of gas evolution increased in the order rank Na<Ca<Y. However, the trend was reversed with the corresponding nitrate salts, and further addition of nitric acid to the irradiating solution minimized gas evolution. At optimized condition, ⁸⁹Zr was produced in moderate yield (4.36±0.48 MBq/μA_•h) and high effective specific activity (464±215 MBq/μg) using the solution target approach (2.75M yttrium nitrate, 1.5N HNO₃, 2h irradiation at 20μA). Conclusion: The novel findings on substrate dependent, radiation-induced water decomposition provide fundamental data for the development and optimization of conditions for solution targets. The developed methodology of irradiation of nitrate salts in dilute nitric acid solutions can be translated to the production of a wide array of radiometals like ⁶⁴Cu, ⁶⁸Ga and ⁸⁶Y, and is well suited for short-lived isotopes.