A new measurement of the half-life of (166m)Ho

The work presented here is a new and precise measurement of the half-life of (166m)Ho by determining the activity concentration, using an ionisation chamber calibrated for this nuclide, and measuring the number of (166m)Ho atoms using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Since the isotope (166)Er interferes with the mass spectrometric measurement, Er has to be eliminated from the (166m)Ho radioactive solution. The elimination was achieved using ion-exchange chromatography with the cation exchange resin Dowex AG 50W-X8 and 2-Hydroxybutanoic acid as the mobile phase. After a first transit through the chromatographic column, the purified (166m)Ho eluate was spiked with natural Er to get a resulting Er isotopic composition close to that of natural Er at better than 99.5%, and then it underwent two further separations to eliminate the Er. The activity concentration of this Er-free radioactive (166m)Ho solution was measured in our reference ionisation chamber calibrated for this nuclide by means of the 4πβ(PC)-γ and 4πβ(PS)-4πγ coincidence techniques and integral counting with a well-type NaI(Tl) detector and Monte Carlo efficiencies. An aliquot of this standardized solution was sent to the Paul Scherrer Institute (PSI) for mass concentration determination using an isotope dilution MC-ICP-MS approach. The mass concentration of (166m)Ho in this solution was determined with 0.25% relative standard uncertainty. This value was corroborated by two other independent measurements. The new half-life of (166m)Ho, 1132.6(39) years (k=1), is compatible with the value determined in 1965, but is 5.6% shorter and about 43 times more precise.