Determination of the age of uranium-bearing items of arbitrary shape

A practical application of a method developed for determining the age of highly enriched uranium is presented. The age of uranium samples is an important piece of information relevant both in combating illicit trafficking and in nuclear safeguards. It can help in determining the origin of seized or found nuclear materials, and, on the other hand, knowing the date of production of the material can help the safeguards expert to verify the correctness of the declaration of safeguarded material. It has already been demonstrated that both mass spectroscopy and gamma spectroscopy can be reliably used to determine the age of uranium, provided that a suitable sample is available. Items from which it is not possible to take out a sample, however, may frequently occur in safeguards verification or in combating illicit trafficking of nuclear materials. The purpose of the present research is to demonstrate that gamma spectroscopy can also be used to determine the age of such items, i.e., items that can only be investigated non-destructively, as whole. The initial methodology for gamma-spectroscopic age dating was derived during a 'Round Robin' exercise, in which the properties of a highly-enriched uranium (HEU) powder were assessed by several laboratories, from the point of view of nuclear forensic science. The uranium-age obtained by this gamma-spectrometric method was in agreement with the results reported by other participating laboratories, which used mass-spectrometry. In the present measurement setup (with a 150 cm3 coaxial HPGe detector) the lower bound for age determination of 90% enriched HEU was estimated to be around 5 years. If the presently best available well-type HPGe detector could be used, the sensitivity of the method could be improved, so that the lower bound for the applicability of the method would be approximately 1 year for 90% enriched uranium and 20 years for natural uranium. The non-destructive method for age dating presented here relies on measuring the daughter/parent activity ratio 214Bi/234U by low-background, high-resolution gamma-spectrometry. In order to eliminate the geometrical effects relative, efficiency calibration is used, so that the the age of uranium samples of arbitrary physical form and shape can be determined. In particular, the age of uranium hermetically sealed in two fission chambers was determined. Each fission chamber contained approximately 1 g of hermetically sealed uranium with nearly 100% of 235U. It has been shown that the activity ratio 214Bi/234U can be determined without destroying the compactness of the chambers