  In this work, Rossi-alpha measurements were simultaneously performed with a
$^3$He-based detection system and an organic scintillator-based detection
system. The assembly is 15 kg of plutonium (93 wt$\%$ $^{239}$Pu) reflected by
copper and moderated by lead. The goal of Rossi-alpha measurements is to
estimate the prompt neutron decay constant, alpha. Simulations estimate
$k_\text{eff}$ = 0.624 and $\alpha$ = 52.3 $\pm$ 2.5 ns for the measured
assembly. The organic scintillator system estimated $\alpha$ = 47.4 $\pm$ 2.0
ns, having a 9.37$\%$ error (though the 1.09 standard deviation confidence
intervals overlapped). The $^3$He system estimated $\alpha$ = 37 $\mu$s. The
known slowing down time of the $^3$He system is 35-40 $\mu$s, which means the
slowing down time dominates and obscures the prompt neutron decay constant.
Subsequently, the organic scintillator system should be used for assemblies
with alpha much less than 35 $\mu$s.
