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.