HEU age determination by the activity ratio 227Th/235U

It is important to measure the age of a highly enriched uranium (HEU) assembly for authentication of the material in the frame of arms control inspections. A new non-destructive gamma spectrometric method for HEU age-dating is reported. This method relies on measuring the daughter/parent activity ratio 227Th/235U by high-resolution gamma spectrometry. Only a narrow gamma range of energy of uranium from 230 keV to 242 keV will be used for analysis. The relative efficiency of every characteristic gamma ray changes in a small range because it has a near energy, which makes the results more accurate in theory. It provides a quick and reliable method for HEU age determination. Several gamma spectra of the same HEU assembly have been measured with different conditions (gain settings, distance and measurement time). When a branching ratio of 12.6% was chosen for the 235.96 keV line of 227Th, we obtained the activity ratios of (5.61 ± 0.40) × 10−4, (5.17 ± 0.39) × 10−4, (5.26 ± 0.39) × 10−4, (5.10 ± 0.35) × 10−4, (5.50 ± 0.44) × 10−4 and (5.47 ± 0.42) × 10−4, respectively. These ratios correspond to ages of 52.2 ± 2.4 years, 49.7 ± 2.3 years, 50.1 ± 2.3 years, 49.3 ± 2.2 years, 51.6 ± 2.5 years and 51.5 ± 2.4 years, respectively, which are consistent with the known age of this material and the results of the U-Bi method.