Thorium-229 quantified in historical Thorium-228 capsules

Thorium-229 is a valuable, but scarce, radionuclide for nuclear clock applications or targeted alpha therapy. While it is mostly produced by the decay of ²³³U, ²²⁹Th can also be produced by neutron irradiation of ²²⁶Ra. At SCK• CEN, capsules containing mainly ²²⁸Th (by-product of ²²⁶Ra irradiation) were characterized to quantify the present amounts of ²²⁹Th, ²²⁸Th, ²²⁷Ac, ²²⁶Ra with high resolution gamma spectroscopy, after a decay period of 40 years in which ²²⁸Th has decayed. High purity ²²⁹Th was quantified, and after recovery using radiochemical separation processes, it can be used to support ongoing research. - Highlights: • Presence of ²²⁹Th of high radioisotopic purity in historical capsules was proven. • Amounts of ²²⁹Th, ²²⁸Th, ²²⁷Ac and ²²⁶Ra were quantified using in-situ gamma-ray spectroscopy. • Thorium-229 found in the historical capsules can support ongoing research after a recovery campaign.