Emission probabilities of γ-rays associated with the decay of 229Th and its daughters

Abstract

As an aid in the quantitative assay of 229Th by means of γ-ray spectrometry, we have made precise measurements of the emission probabilities of several γ rays from the decay of 229Th and seven from the decay of its daughters in the energy range from 40 to 808 keV. The decay-rate calibration of an isotopically separated source was obtained from the total α-emission rate as determined with an Au-Si surface-barrier detector gated to accept only the α particles from 229Th. The geometry of this detector was measured with a 249Cf source calibrated on a low-geometry proportional counter. The γ-ray emission rates were measured on calibrated Ge detectors for this calibrated 229Th source. The relative γ intensities were also measured for a chemically separated 229Th source, which allowed an assessment of the contributions from the daughter activities. The percentage uncertainties in the measured γ-emission probabilities range from 1.3 to 4.0%. Our results differ significantly from those of two recently published measurements. A comparison of these three sets of results is given.