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.