Tuning of ThO2 target materials properties towards highly porous structure to produce radioisotopes for medical applications

Isotope Separation On-Line (ISOL) technique relies on a series of successive steps starting with the irradiation of a target material by a proton beam where the impact of the protons induces nuclear reactions through which the radioisotopes are produced. The produced radioisotopes diffuse out of the bulk and are evaporated from the target at temperatures in the range of ~2000 °C and then, selectively ionized, extracted and accelerated towards a magnetic mass separator for further purification before being delivered to the end-users. ISOL target materials shall be designed to enhancehave the main purpose of releaseing of the produced isotopes with as high efficiency as achievable, ideally 100% of the produced isotopes of interest. However, slow diffusion, chemical reactions within the target and high boiling point of the element to be released, together with the isotopes limited half-life can drastically reduce this efficiency. All the mentioned effects can be mitigated by designing and engineering the target material to produce and release a specific isotope. The latter developments are mostly focused on tailoring the microstructure of these materials, which has been done by reducing its grain size, controlling porosity volume and size [21]. Furthermore, the high power operation of such target materials comes as well with challenges since the target disks have to be mechanically stable and be designed to remove heat through radiation and/or conduction. In the recent years, the ISOL method is getting more and more attention for the production of medical research isotopes, particularly Ac-225 [2,3]. At SCK CEN, thorium base materials are of interest to produce Ac-225 amongst other isotopes at the ISOL facility of MYRRHA [4]. [4] There is long-standing experience in the research of ThO2 at SCK CEN but mainly focused on nuclear fuel material. Several ThO2 powder production processes have been explored at laboratory scale, including dry metallurgy routes and various aqueous conversion processes [5-8]; and also covering the scrap recycling [9,11].