A Krypton-81 half-life determination using a mass separator

Abstract

The half-life of 81Kr was determined with the aid of a mass separator equipped with a current integrator. In each experiment a natural krypton sample was irradiated with neutrons to produce 81Kr. After determining the 81–82 atom ratio with a conventional mass spectrometer, the sample was processed in the mass separator, the mass 81 beam being collected in aluminium foil while the mass 82 fraction was collected simultaneously in the Faraday cup of the current integrator. The number of atoms of 81Kr collected in the aluminium foil was calculated from the 81–82 atom ratio and the integrated current of 82Kr. The disintegration rate of these atoms was determined by counting the bromine K X-rays produced in the electron-capture decay of 81Kr using a NaI scintillation spectrometer and thus the half-life was deduced. Tests using natural krypton mixed with 11 y 85Kr showed that ±3% accuracy can be achieved with the current integrator. The average of six determinations of the half-life was 2.13 × 10 5 y with an estimated experimental error ±5%. However, when the uncertainty in the 81Kr decay energy (250±90 keV) was included the error was increased, giving T 1 2 ( 81 Kr) = (2.13 −0.26 +0.16)10 5 y , a value that agrees with the only previous measurement and is more accurate.