Increasing Specific Activity of ¹⁶⁶Ho through Szilard-Chalmers Effect in ¹⁶⁵Ho-loaded Zeolite A

Abstract

Holmium-166 is a promising radionuclide for therapeutic and imaging applications due to its advantageous decay characteristics. However, these applications require or would benefit from a higher specific activity of 166Ho than that obtained via a 165Ho(n,γ)166Ho reaction by irradiating a pure 165Ho target. The specific activity is limited, because only a minor fraction of the 165Ho target nuclides is converted into 166Ho during irradiation and these isotopes are inseparable using conventional chemical methods, since they are of the same element. The Szilard-Chalmers effect facilitates this separation by changing the chemical state of the produced radionuclide due to the recoil induced by neutron absorption. Nevertheless, currently applied target materials, mainly ligands, are unsuitable for commercial scale production with sufficient yield and specific activity, because of: radiolysis, isotopic interchange and recombination. It is proposed that these processes can be significantly reduced by exploiting zeolites, which are materials with interconnected cavities, to bind 165Ho target nuclides in the closed cages prior to irradiation. The concept behind is, that upon recoil the 166Ho nuclides relocate to the open cages, from which they are extracted after irradiation, while the 165Ho target nuclides remain locked in the closed cages. This results in an increased specific activity. Therefore, the aim of this thesis is to examine the increase in the specific activity of 166Ho produced via a (n,γ) reaction by applying the Szilard-Chalmers effect using zeolite A.

A procedure to achieve the optimal loading of holmium into the open cages (18.3 wt% Ho) and closed cages (93.3% of total holmium) of zeolite A was investigated, as well as the efficiency of its extraction from the open cages (between 39 and 55%). The obtained specific activity and yield were analysed upon irradiation of these zeolite A samples. Irradiating zeolite A with 165Ho exclusively in the open cages resulted in only a slight increase of the specific activity (maximal 3.3 GBq/g), but a relatively good yield (maximal 53%). In contrast, irradiation of zeolite A with 165Ho loaded in both the closed and the open cages, resulted in a considerably lower yield (maximal 14.4%), but a substantially increased specific activity (maximal 7.3 GBq/g), which is a factor 5.2 higher than without the Szilard-Chalmers effect. 
Nevertheless, further improvement should be considered as a higher specific activity is key to successful radionuclide therapy. This could potentially be achieved by increasing the extraction efficiency by exploring other types of zeolites, in search of more suitable functional characteristics.