Non-destructive assay of plutonium in absence of gamma-ray spectrometry

Gamma-ray spectrometry-based isotopic composition analysis is a prerequisite for absolute non-destructive assay of Pu by neutron counting or calorimetry. However, gamma-ray spectrometry can be complicated in samples that are heavily shielded or have a high 241Am fraction. Herein, for the absolute non-destructive assay of Pu, a simple yet potent empirical methodology combining neutron multiplicity counting and calorimetry is proposed, which obviates the need for gamma-ray spectrometry-based isotopic composition analysis. The approach includes stepwise correlation between the parameters obtained from neutron multiplicity counting, i.e., α ((α, n) neutron to spontaneous fission neutron ratio), effective mass of 240Pu, and D (doubles neutron rate) with the thermal power output (W Pu) due to radioactive decay (measured by calorimetry). Because D and W Pu both are governed by the Pu isotopic composition, their correlation should be sensitive to the isotopic information in a given sample. Mandatory correction for the 241Am contribution in W Pu was done using the linear plot between its weight fraction and the corresponding α. The proposed methodology was validated by analyzing several PuO2 standards containing high 241Am fraction (0.02–0.09). The measured Pu amounts were within 8 % of the actual values with measurement uncertainties between 8 and 10 %, which shows the potential of the proposed methodology for the assay of samples having high 241Am content or for the quantification of Pu in heavily shielded samples.