Purification and activity standardisation of a 166mHo solution

Abstract

As part of a project to use the long-lived ( T 1/2=1200 a) 166mHo as reference source in its reference ionisation chamber, IRA standardised a commercially acquired solution of this nuclide using the 4 πβ– γ coincidence and 4 πγ (NaI) methods. The 166mHo solution supplied by Isotope Product Laboratories was measured to have about 5% Europium impurities (3% 154Eu, 0.94% 152Eu and 0.9% 155Eu). Holmium had therefore to be separated from europium, and this was carried out by means of ion-exchange chromatography. The holmium fractions were collected without europium contamination: 162 h long HPGe gamma measurements indicated no europium impurity (detection limits of 0.01% for 152Eu and 154Eu, and 0.03% for 155Eu). The primary measurement of the purified 166mHo solution with the 4 π (PC) β– γ coincidence technique was carried out at three gamma energy settings: a window around the 184.4 keV peak and gamma thresholds at 121.8 and 637.3 keV. The results show very good self-consistency, and the activity concentration of the solution was evaluated to be 45.640±0.098 kBq/g (0.21% with k=1). The activity concentration of this solution was also measured by integral counting with a well-type 5″×5″ NaI(Tl) detector and efficiencies computed by Monte Carlo simulations using the GEANT code. These measurements were mutually consistent, while the resulting weighted average of the 4 π NaI(Tl) method was found to agree within 0.15% with the result of the 4 πβ– γ coincidence technique. An ampoule of this solution and the measured value of the concentration were submitted to the BIPM as a contribution to the Système International de Référence.