Experimental re-evaluation of the gamma -ray energy and emission probability for the 159 keV transition in super(238)U following the alpha -decay of super(242)Pu

Because of the very low specific activity of super(242)Pu, the non-destructive assay of this isotope by means of conventional high-resolution gamma-spectrometry (HRGS) is possible only for Pu samples highly rich in super(242)Pu. For bulk samples suffering from the gamma self-attenuation and self-fluorescence effects, the only practical choice for the quantitative analysis of super(242)Pu is the weak gamma -line emitted in the 159 keV transition of its alpha -decay daughter super(238)U. A recent study revealed a significant disagreement between the super(242)Pu mass values in a 99.72% enriched super(242)PuO sub(2) sample as reported by HRGS and neutron coincidence counting. This fact motivated the present study on the experimental re-evaluation of the gamma -emission probability for the 159 keV transition using a combination of alpha -, gamma - and mass-spectrometry techniques. The obtained new emission probability P sub(2)=(2.20+/-0.08)10 super(-6) turned out to be [asymptotic to]35% smaller than the currently adopted value. The study also suggested a new value E sub(2)=159.018+/-0.016 keV for the energy of the respective gamma -ray.