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 i n determining the Pa-234m and Th-234 content of environmental samples. These radionuclides are daughters of U-238 and should rapidly reach s ecular equilibrium. Several measurements of the absolute emission prob ability 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, r ecent gamma-ray studies are in good agreement to give a newly recommen ded 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 spe ctrometric methods adopted for low-energy gamma-ray measurements as ap plied to Th-234. Such quantitative studies are relatively difficult, a nd therefore a suitable procedure has been recommended, together with experimental and computer-modelling validations. Measurements were und ertaken on four silt samples from the Ribble Estuary using this recomm ended procedure and the newly evaluated emission probability data: suc h a combination of improvements resulted in good agreement between the Th-234 and Pa-234m activities in secular equilibrium. (C) 1998 Elsevi er Science Ltd. All rights reserved.