The application of scanning electron microscopy (SEM) and secondary
ion mass spectrometry (SIMS) for characterization of mixed plutonium and
uranium particles from nuclear weapons material is presented. The
particles originated from the so-called Thule accident in Greenland in
1968. Morphological properties have been studied by SEM and two groups
were identified: a “popcorn” structure and a spongy structure.
The same technique, coupled with an energy-dispersive X-ray (EDX)
spectrometer, showed a heterogeneous composition of Pu and U in the
surface layers of the particles. The SIMS depth profiles revealed a
varying isotopic composition indicating a heterogeneous mixture of Pu and
U in the original nuclear weapons material itself. The depth distributions
agree with synchrotron-radiation-based μ-XRF (X-ray fluorescence
microprobe) measurements on the particle (Eriksson, M., Wegryzynek, D.,
Simon, R., & Chinea-Cano, E., in prep.) when
a SIMS relative sensitivity factor for Pu to U of 6 is assumed. Different
SIMS identified isotopic ratio groups are presented, and the influence of
interferences in the Pu and U mass range are estimated. The study found
that the materials are a mixture of highly enriched 235U
(235U:238U ratio from 0.96 to 1.4) and so-called
weapons grade Pu (240Pu:239Pu ratio from 0.028 to
0.059) and confirms earlier work reported in the literature.