Activity standardization of two enriched ⁴⁰K solutions for the determination of decay scheme parameters and the half-life

In this paper we describe experiments on two enriched ⁴⁰K solutions to accurately determine decay data. The first solution was measured in 2004/2005 by means of a gamma-ray spectrometer with low background and a liquid scintillation (LS) counter to apply the CIEMAT/NIST efficiency tracing method. A combination of results yields an emission probability of the 1461 keV gamma-rays of P_γ = 0.1030(11) which is lower than current results of data evaluations. The activity concentration of the second solution was also determined by means of LS counting, but here, the CIEMAT/NIST efficiency tracing method as well as the TDCR method were applied. Again, the result was combined with that of independent gamma-ray spectrometry and the gamma-ray emission probability was found to be P_γ = 0.1029(9) in good agreement with the result obtained from the first solution. A combination of both experiments yields P_γ = 0.1029(9). The spectra of a TriCarb LS counter were carefully analyzed and a beta minus emission probability P_β⁻ = 0.8954(14) was determined. The new results for P_γ and P_β⁻ indicate that the overall probability of the decay via EC in recent data evaluations is overestimated. The LS counting efficiencies were computed with a stochastic model and up-to-date calculations of the beta spectrum and fractional EC probabilities were used. The final activity result of the second solution is combined with the outcome of a comprehensive isotopic analysis to determine the half-life of ⁴⁰K which is found to be 1.2536(27) ·10⁹ years. All above-stated uncertainties are standard uncertainties (k = 1).