Tb radioisotopes for medical applications: Spallation cross section measurements and first isotope delivery from CERN-MEDICIS

Tb has four different isotopes that are useful in the context of nuclear medicine, i.e. $^{149}$Tb, $^{152}$Tb, $^{155}$Tb and $^{161}$Tb. In nuclear medicine, radiopharmaceuticals are applied for the diagnostics and treatment of certain deceases. Their chemical equivalence means that Tb-labelled radiopharmaceuticals for diagnostics or therapy will have identical pharmacological properties. This is an important advantage for so-called theranostic applications, where therapy and diagnostics are combined to improve the results of the therapy. $^{161}$Tb is efficiently produced by irradiating $^{160}$Gd with thermal neutrons to form $^{161}$Gd, which quickly decays into $^{161}$Tb. For the neutron deficient isotopes mentioned above, i.e. $^{149}$Tb, $^{152}$Tb and $^{155}$Tb, one of the most promising production methods is high-energy proton-induced spallation of Ta targets, coupled with isotope separation on-line or off- line. It is however unavoidable to collect isobaric contaminants. These include so-called pseudo-isobars, which are molecules with the same total mass, such as oxides or fluorides. An example is the presence of $^{139}$Ce impurities in the collection of $^{155}$Tb, as $^{139}$Ce$^{16}$O. These contaminant often need to be chemically removed before the Tb isotopes can be used in nuclear medicine. Therefore, it is beneficial to optimise the production protocol such that these isobaric contaminants are minimised. This work describes two methods to achieve this. One way is to select the most appropriate proton energy for the production of the isotopes of interest, while minimising isobaric contaminants. Another way is to choose the target temperature to optimise the extraction purity of the isotope of interest. To analyse the effect of the proton energy on the in-target production, Ta foils are irra- diated with protons of an energy between 300 and 2500 MeV at the COoler SYnchrotron (COSY) in the Forschungszentrum Jülich in Germany. Subsequently, γ-ray energy spec- tra are analysed for some of these foils in order to calculate cumulative cross sections. Because of a decay time of at least a month between the irradiation of the foils and the measurement of the spectra, cumulative cross sections are only calculated for longer-lived isotopes. Some of these isotopes are present as a contaminant in the collection of Tb isotopes. This analysis shows that a lower proton energy reduces the number of nucleons evaporated in the spallation process. Therefore, for the collection of $^{155}$Tb, protons with an energy below 800 MeV should be used to minimise the contamination with $^{139}$Ce$^{16}$O. This work also presents the cumulative cross section data of $^{149}$Tb that was obtained from an $\alpha$-analysis, done by other people, of the same foils. The goal of this analysis is to solve the discrepancy between different data sets in literature. The results presented here, show that the values of Winsberg and Miranov are preferred. Another way to minimise the collection of isobaric contaminants is by optimising the target temperature for the extraction. For this, the results of the first isotope delivery from CERN-MEDICIS are analysed. Here, $^{155}$Tb is collected after it is produced by the irradiation of a Ta target with 1400 MeV protons. Different collections are executed, each with the target at a different temperature. The results show that temperatures below 2000 ◦C can be used to extract CeO, while a higher temperature is needed to extract Tb. Because this is the first isotope collection at the CERN-MEDICIS facility, it is a great opportunity to compare FLUKA simulations to estimate the ambient dose equivalent rates in the experimental hall with measurements. The results show that these kind of simulations can give a qualitative idea of the dose distribution and of the main contributions to the dose. However, quantitative results should be handled with care.