Year
2009
Abstract
Quantification and interpretation of isotopic ratios is generally acknowledged as a powerful method in nuclear forensic analyses to identify the origin of nuclear and radiological materials or the contamination source of environmental media. Nuclear spectrometric measurements provide a straightforward, relatively easy and fast technique for such analyses, which can even be performed on site for gamma emitting radioisotopes. However, radioisotopes relevant for nuclear forensic analyses often show only a weak signal, e.g. due to their low abundance or small gamma-ray emission probability. Obviously, any conclusion on nature or origin of a nuclear or radiological material is considerably limited, if the measurement merely gives a detection limit for a relevant isotope. This paper discusses a new approach which aims at exploiting maximum information from the measurement. Using Bayesian statistics, a method is presented which allows statistical inference on nuclide ratios taking into account both prior knowledge and the information gained by the measurements. It is shown that our method allows quantitative conclusions to be drawn, including the statistical significance of isotopic ratios, if counts of single isotopes are low or become even negative after background subtraction. Differences to the traditional statistical approach of specifying decision thresholds or detection limits are highlighted. Application of this new method is illustrated by a number of examples, including spectra of a shielded uranium source, measurements of polonium in air filters performed during the radiological deployment after the poisoning of Litvinenko, and spectra of radioxenon isotopes in ambient air measured in the context of the verification of the Comprehensive Nuclear Test Ban Treaty (CTBT).