Distinguishing fissions of $^{239}$Pu and $^{235}$U with low-resolution detectors*

When $^{239}$Pu and $^{235}$U undergo thermal neutron-induced fission, both produce significant numbers of $\beta $-delayed gamma rays with energies in the several MeV range. Experiments using high energy-resolution germanium detectors\footnote{R. E. Marrs \textit{et al.,} Nucl. Instr. {\&} Meth. A (in press).} have shown that it is possible to distinguish the fission of $^{239}$Pu from that of $^{235}$U. Using differences in the temporal behavior and in the shapes of the gamma-ray energy spectra, we show that these two isotopes can also be differentiated using low-resolution plastic or liquid scintillators. It is likely this method could be extended to homeland security applications, such as screening of cargo containers for $^{235}$U and $^{239}$Pu, using a neutron source and such scintillators.