Thermal separation of volatile rare isotopes from irradiated target materials

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Very promising research on radio-therapeutic methods like radio-immunotherapy, targeted alpha-particle therapy and associated imaging techniques is hampered by the limited availability of extremely rare and relatively short-lived isotopes like 209,211At, 225,226Ac, 223,224Ra , 213Bi or 212Pb. A suitable production method, in particular for isotopes that can't be easily obtained via traditional sources like nuclear reactors or low-energy medical cyclotrons, is provided by ISOL (Isotope Separation OnLine) facilities like TRIUMF/ISAC (Isotope Separation and ACceleration). Even though the main purpose of ISAC is to supply rare isotope beams for research in the fields of nuclear physics and material science, recent efforts to provide samples of 209,211At and 225,226Ac for nuclear medicine research have yielded excellent results. The advantage of the ISOL method is that via the implantation of mass-separated ion beams isotopically and chemically pure samples can be produced. Subsequent, time consuming chemical separation and purification steps are either relatively straightforward or unnecessary. The disadvantage of the method is that the quantities that can be extracted from an ISOL target are limited by target size, low extraction efficiencies and beamtime availability. Therefore, nuclear medicine experiments relying on isotopes from an ISOL source are essentially restricted to basic pre-clinical studies. A major difference between a production target for isotopes with subsequent chemical separation and an ISOL target is their operating temperature. The former is actively cooled, enabling iradiations with high beam intensities, retaining the entire isotope production inventory inside the target. In contrast, ISOL targets are operated at very high temperatures up to 2300 °C to facilitate the fast and efficient release of sufficiently volatile chemical species while the target is irradiated. This proposal aims at research into a combination of both methods, in which a production target made from refractory materials is transferred to a high-temperature vacuum furnace from where fractions of volatile species are extracted from the irradiated target material by diffusion and effusion. It requires the investigation, understanding and subsequent optimization of the thermodynamics and chemistry driving the release and thermo-chromatographic separation of volatile species from a target. This includes development, testing and optimization of suitable experimental setups. One of the two main objectives of the proposal is to find a fast and efficient method for the extraction of the relatively volatile but also short-lived theranostic system 209,211At and its precursor 211Rn. The second main objective is the investigation of thermal separation of Ra and Ac isotopes which are released from actinde carbide targets at much higher temperatures. In the long run, any isotope of interest for harvesting could be investigated.