PhD Defense | Lisa Gubbels | Development of thorium-based target materials for the production and release of 225Ac at the ISOL facility of MYRRHA

Development of thorium-based target materials for the production and release of 225Ac at the ISOL facility of MYRRHA 

Thorium-based target materials were developed for isotope separation on-line (ISOL) applications at the future ISOL@MYRRHA facility at SCK CEN, with the aim of supporting the production of medical radionuclides such as Actinium-225 (²²⁵Ac). Efficient release of refractory isotopes requires target materials that remain stable at temperatures above 2000 °C while retaining sufficient open porosity and a fine microstructure to facilitate isotope diffusion and release.

The study first investigated the preparation and sintering of thoria (ThO₂) powders produced by different precipitation routes. The resulting powders exhibited distinct morphologies, which allowed the influence of the starting powder characteristics on compaction, densification and microstructural evolution to be investigated. Although all powders produced highly dense pellets after prolonged sintering, room-temperature oxalate-derived powders offered the most suitable balance between controlled sintering behavior and scalability for the development of porous targets.

To preserve open porosity at high temperature, graphitic carbon nitride (GCN) was subsequently introduced as a pore-forming additive. A ThO₂ with GCN composition containing 50 vol% GCN retained up to 37 % open porosity after sintering at 1750 °C and 29 % after 24 h at 2000 °C, while maintaining a grain size below 10 µm. These results demonstrated that both the pore-former composition and sintering temperature strongly influence the high-temperature stability of the porous structure.

In parallel, thorium carbide (ThC₂) materials were investigated as an alternative target material through carbothermal reduction of ThO₂ using different carbon precursors. The optimized process achieved complete conversion to ThC₂ in the presence of excess carbon.

The optimized porous ThO₂ with GCN composition was subsequently scaled up to a 50 g target and irradiated at CERN-MEDICIS with a 1.4 GeV proton beam. After irradiation and heating to 2000 °C, the material retained 43% open porosity and a mean grain size of 5.3 µm. ²²⁵Ra release efficiencies of 33.7% and 31.4% were measured in two irradiation campaigns. ²²⁵Ac release started at 2222 °C, with 1.2 MBq collected over 8.5 h at temperatures up to 2431 °C. These results demonstrate the potential of porous ThO₂ targets for high temperature ²²⁵Acrelease in future ISOL applications.

Promoter(s):

Prof. Jozef Vleugels, KU Leuven

SCK CEN mentor(s):

Dr. João Pedro Ramos, SCK CEN

Dr. Beatriz Acevedo Muñoz, SCK CEN