Quantitative assay of Plutonium-Beryllium sources.

As containing nuclear (fissile) material, PuBe sources represent a nuclear safety and safeguards issue. A large number of such sources - mostly out of use - are stored in Hungary (and this is the case in several neighbouring countries as well). The Pu-content of these sources is to be accounted for, regularly reported to and inspected by IAEA, and from 2004 on, also by EURATOM safeguards. Since the late Soviet supplier did not declare the Pu-content of the sources, the values for the Pu content presently on record are based on rough overestimates assuming that the sources contain pure 239Pu. Recently, the isotopic composition of some of the sources was determined by gamma-spectrometry and more precise estimates for their Pu-content were given. These estimates, however, still included the high uncertainty associated with the specific neutron yields needed for the calculation of the Pu-content. In parallel with the gamma-spectrometric measurement of the isotopic composition, a high-resolution gamma-spectrometric method which does not require the knowledge of the specific neutron yields was developed for the experimental determination of the Pu-content of these sources. Furthermore, combining the results of the gamma-spectrometric measurement with the measurement of the total neutron counts the specific neutron yields of the sources can also be determined, thus providing additional information about the sources. The gamma-spectrometric method made it possible to prepare a set of calibrating sources which were used in developing an independent neutron coincidence counting method for the determination of the Pu-content of the sources. A neutron coincidence counter was built and a correlation was obtained between the Pu-content and the ratio of the number of real coincidences to the total number of counts. This method provides another alternative to determine the Pu-content and also to derive additional information about the sources. This research was accepted by the IAEA as part of the Hungarian support programme to the IAEA. Recently, in collaboration with JRC Ispra, the results of the Pu-mass measurements of several sources were confirmed by calorimetric measurements.