Investigating $^{225}$Ac production at CERN-MEDICIS: Extraction yield estimation and sample purity

This thesis is focused on the separation process of actinium-225, a necessary step in the production method of high energy proton irradiation of a thorium-based target. $^{225}$Ac is a promising candidate in targeted alpha therapy, which provides an alternative and selective way to fight the devastating disease of cancer. However, clinical research is rendered difficult due to insufficient supply of this medical isotope. Alternative pathways are considered to upscale the production of $^{225}$Ac, but further research is needed to overcome the challenges associated with it. In the production method of high energy proton irradiation, many different isotopes are created in the target, including the long-lived isotope $^{227}$Ac, which necessitates a separation process. This can happen via radiochemistry, but this method does not provide isotopic selectivity. The typical activity of the long-lived $^{227}$Ac in the medical samples would cause problems for hospitals in terms of facility licensing and decommissioning, product licensing and waste disposal. This disadvantage requires the need of a mass separation process, where isotopes are filtered based on their mass-to-charge ratio. It is this particular step in the production process that is investigated in this work. Two separate analyses were performed in order to investigate the feasibility of the production method. The first part of the thesis revolves around the ion beam current data of four experimental runs performed at CERN-MEDICIS. This current was measured at different parts in the experimental setup and can be retrieved in real-time during collection. The data are used to estimate the collected activity on the foils, which could serve as a real-time assessment of the activity during experiment. On top of this, the different sources of the collected $^{225}$Ac could be separately identified, which can come from either the decay of the parent nucleus $^{225}$Ra on the foil or directly collected $^{225}$Ac from the ion beam. This analysis showed good agreement between the measured and estimated values, but showed a consistent underestimation of the activity. On top of these estimations, the influence of the laser ionization was investigated. Lasers are directed at the extracted isotopes of the target, which selectively ionize the element of interest. This results in a higher extracted amount of $^{225}$Ac, which results in a higher collected ion beam current. By regularly blocking the laser, a drop in the collected current could be observed. This allowed to investigate the influence of the ionizing lasers by assessing the laser enhancement and the gain in activity due to the lasers alone. The laser enhancement is defined as the ratio of the collected activity of $^{225}$Ac when the laser was on and blocked. The results of this work suggested a factor of approximately 2. The second part of the thesis is a purity analysis on the collected activities of two runs performed at CERN-MEDICIS. In particular, the activity of $^{227}$Ac served to calculate the separation enhancement factor. This factor signifies the ability to separate the long-lived isotope $^{227}$Ac, which is defined as the ratio of $^{225}$Ac to $^{227}$Ac before and after separation. A separation factor of 150 ± 14 and 646 ± 59 were found for the two runs, where the large difference is due to experimental conditions. Still, both runs enabled to suppress the activity of $^{227}$Ac enough for medical purposes. In conclusion, the two experimental runs performed at CERN-MEDICIS fulfilled the purpose of suppressing $^{227}$Ac, although a large variability is seen due to experimental conditions. This makes the production method suitable for $^{225}$Ac production, but the extraction efficiency should increase to reach global demand. The extraction efficiencies analyzed in this work were approximately 1% for $^{225}$Ac. For its parent nucleus, $^{225}$Ra, the efficiencies were approximately 20-40%. The typical higher efficiency of the latter is due to the lower boiling point and more efficient surface ionization. These values emphasize the need for further research and a deeper understanding of the extraction process is needed. By investigating the ion beam current data measured in the setup, an attempt is made to extract information which could benefit the insight in the conditions met of collection runs. Good agreement of the results are found w.r.t. the measured values of gamma spectroscopy, but a systematic underestimation is made.