Preparation of the holmium-166m sealed source

The Ho-166m reference sources having a half-life of 1200 years, to be replaced with the conventional Ra-226, have been developed for the purpose of monitoring long term stability of ionization chambers used for the secondary standard instrument of radioactivity measurements. The powder of irradiated holmium oxide (Ho2O3) was dissolved to the holmium chloride solution (HoCl3) and subdivided into the small fused silica containers. The solutions were converted to the chemically stable holmium oxide again by drying and sintering process and they were contained in the stainless steel inner capsules which were further sealed in the titanium outer capsules completely by plasma welding. Ten sealed reference sources (15φx27mm) of radioactivity of around 1.2 MBq and radionuclidic purity of 99.4% for each were successfully prepared in this method. In the present report, the procedures of target analysis after reactor irradiation, dissolving of the target and preparation of the sources, sealing test of capsulation and radioactivity measurement are described. (author)