Maximizing the radionuclide collection efficiency at CERN-MEDICIS: A case study on terbium collections and sputtering

This thesis is focussed on maximizing the efficiency of medical radionuclide collections at CERN-MEDICIS. More specifically, a case study on terbium was performed.$\\$ In particular, radionuclide collections begin with the production of the radionuclide. Here, a target is irradiated with a beam (often protons) in order to induce nuclear reactions. In that way, the radionuclide of interest is produced amongst others. Separation of the isotope of interest from other nuclei is required to ensure the purity of medical samples. Here, both elemental selection (with laser resonance ionization) and mass selection (by using a mass separator) are combined in order to obtain a high specific activity. Thereafter, the purified radionuclide beam is implanted in a foil to collect the radionuclides and to be able to transport them to a chemical facility or hospitals. During this implantation process, some isotopes are lost due to self-sputtering. All the above-mentioned steps contribute to the total efficiency of a collection, which should be maximized. Therefore, the main aim of this thesis was to investigate terbium collections and sputtering in order to maximize the collection efficiency. $\\ \\$ The first crucial contribution is the efficiency of the mass separating method. To gain insight into this process, the terbium collections that were performed in 2022 at MEDICIS are analyzed. The targets for these collections were irradiated at an external facility and chemical separation was performed prior to mass separation to purify the samples. Using chemical separation, terbium can be separated from other elements, but no purification within the terbium isotopes can be performed. Hence, mass separation is needed and its efficiency should be maximized. $\\ \\$ The next part focuses on implantation of the radionuclides in the collection foil, more specifically, on the self-sputtering that occurs during these implantations. In particular, self-sputtering takes place when the momentum of an impinging ion is transferred to nuclei in the collection foil, causing a nuclear collision cascade that can eject surface nuclei, i.e., these nuclei are sputtered out of the foil. As the collection proceeds, more foil material nuclei will be sputtered out of the foil, hence the original surface of the foil will approach the earlier implanted isotopes. Consequently, these earlier implanted isotopes eventually reach the surface, so that they can be ${self}$-sputtered from the foil. An analysis on the self-sputtering behavior has been performed by the use of TRIDYN, a Monte Carlo-based package for implantation simulations. By performing these simulations, insight in the self-sputtering process is obtained, which can, eventually, be used to reduce the amount of self-sputtering that occurs during a collection. Moreover, an experimental protocol is put forward to verify the reliability of the simulations. $\\ \\$ The last part entails the additional time-aspect for $\alpha$-decaying nuclei. In $\alpha$-decaying nuclei there is an additional decay-related loss of isotopes, namely that after the decay, the daughter nucleus obtains a large recoil momentum. Hence, the daughter nucleus travels through the foil and can sputter away other implanted mother nuclei. A first approximation was obtained by combining TRIDYN and TRIM simulations. $\\ \\$ Ultimately, this work aims to contribute to the improvement of radionuclide collections, which is crucial for the production of radiopharmaceuticals for diagnostic and therapeutic applications.