U-238 decay chain: resolution of observed anomalies in the measured secular equilibrium between Th-234 and daughter Pa-234m

Recent gamma-ray spectrometry measurements have highlighted problems in determining the Pa-234m and Th-234 content of environmental samples. These radionuclides are daughters of U-238 and should rapidly reach secular equilibrium. Several measurements of the absolute emission probability of the 1001 keV gamma ray of Pa-234m have resulted in doubts concerning earlier recommended decay data for this radionuclide. While a value of 0.00590±0.00010 was adopted in previous evaluations, recent gamma-ray studies are in good agreement to give a newly recommended value of 0.00835±0.00011. A detailed evaluation has also been made of the decay data for Th-234 to generate a recommended value for the emission probability of the 63.3 keV gamma ray of 0.037±0.002, compared with values between 0.040 and 0.048 from other evaluations. A detailed experimental examination has been made of the gamma-ray spectrometric methods adopted for low-energy gamma-ray measurements as applied to Th-234. Such quantitative studies are relatively difficult, and therefore a suitable procedure has been recommended, together with experimental and computer-modelling validations. Measurements were undertaken on four silt samples from the Ribble Estuary using this recommended procedure and the newly evaluated emission probability data: such a combination of improvements resulted in good agreement between the Th-234 and Pa-234m activities in secular equilibrium.