Year
2002
Abstract
Neutron counting and calorimetry are usually considered separately in analyzing special nuclear materials (SNM). The results from each of these techniques are typically transformed into SNM mass values using isotopic information from gamma-ray spectroscopy. This paper describes an approach that integrates the results from neutron and calorimetry techniques to solve assay problems that could not be solved using either technique separately. This new approach has been developed for the assay of nuclear materials partly composed of isotopes with no measurable gamma-ray signatures. This methodology uses a combination of neutron counting, calorimetry, and gamma measurements of other associated isotopes with measurable gamma-ray signatures. This allows for assay of such isotopes as 244Cm mixed with Pu, 236U in U highly enriched in 236U, and 242Pu in Pu highly enriched in 236Pu. Another potential use of this technique would be to determine low concentrations of Pu with a known isotopic composition mixed in high-enriched uranium. In this case, the uranium isotopic gamma radiation shields the less intense Pu isotopic spectral peaks. The methodology to be presented uses the ratio of measured spontaneous-fission (SF) neutron singles rate to the measured thermal power, k, to estimate the quantity of the unknown isotope. For the two component case, where two elements or isotopes emit neutrons and thermal power, but cannot be related by relative gamma-ray intensity, the mass ratio is (G1 / G2) = (n2-kP2)/(kP1-n1), where n1,n2 and P1, P2 are the SF neutron emission activities, and effective specific powers, respectively. Once the relative ratio of the two components is known, each of their masses can be calculated. The overall performance of the technique is dependent on the performance of the neutron counting, calorimetry, and gamma-ray isotopic measurement components. The uncertainties of these components propagate additively into the relative uncertainty of the unknown component. The technique’s performance for the 236U and 242Pu cases is not suitable for low mass fractions, but improves with increasing mass fractions. Results for the measurement of Pu enriched in 242Pu will be presented.