New measurement of the emission probability of the 63.290 keV 234Th γ-ray from 238U α decay

The measurement of nanogram and microgram quantities of 241Am can be carried out by comparing its rate of 59.541 keV γ emission to that of known amount of 238U emitting a 63.290 keV γ-ray following α decay to 234Th. As the accuracy of the method appears to be limited by the relatively high uncertainty of the literature value for this emission probability, we decided to re-measure with improved accuracy the emission probability in terms of the emission probability of the 59.541 keV γ-ray from 241Am. After correction for the small difference in attenuation and detection efficiency between 59.541 and 63.290 keV, the experimentally determined emission probability is in good agreement with the value obtained by other authors. The experimental procedure used to measure the emission probability of the 63.290 keV γ-ray will be described together with the procedure to quantify and validate the relatively small correction for the difference in attenuation between 59.541 and 63.290 keV. This is followed by a discussion on the traceability of the result and a discussion on the main sources of error determining the overall uncertainty. Finally, our value is compared to those reported by others.