N.C. Tam, J. Bagi, E. Serf, L. Lakosi: Determination of the Pu content of Pu-Be neutron sources by non-destructive, passive methods


The Pu content of Pu-Be sources transported to Hungary is undeclared, the sole information provided by the supplier is the overall neutron output. Nominal Pu quantities are estimated from calculations using the neutron output and the specific neutron yield given for 239Pu by the supplier, while implying pure 239Pu content. Since the Pu-Be sources also contain other Pu (and Am) isotopes with much higher specific yields, the current estimates for the nominal Pu quantities can only be considered as an upper limit.

In our previous method the Pu content of the Pu-Be sources was estimated using a combination of the gamma-spectrometric determination of Pu isotopic abundances and total neutron counting. It was necessary to adopt specific (alpha,n) reaction yields for the other Pu (and Am) isotopes from the literature. Recently we adopted a new data set, by normalizing them to the value for 239Pu, used and provided by the manufacturer. This way a consistent data base was established, and now the data of previous measurements are re-evaluated.

In our recent study another method is developed as well, based solely on neutron measurements. In addition to total neutron counting, coincidences were also recorded. A correlation was found between the ratio of true coincidence rate (R) to the total count rate (T) and the Pu content. This correlation is used to construct a calibration curve – by using the Pu contents of the data base established in the first stage. Thus, the measurement of the sole R/T ratio may also provide a good assessment of the Pu content.