  DD and DT reaction rates may be compared to determine plasma temperatures in
the 10--200 eV range. Distinguishing neutrons from these two reactions is
difficult when yields are low or unpredictable. Time of flight methods fail if
the source is extended in time. These neutrons may be distinguished because
inelastic scattering of more energetic neutrons by carbon produces a 4.44 Mev
gamma-ray, and because hydrogenous material preferentially attenuates lower
energy neutrons. We describe a detector system that can discriminate between
lower and higher energy neutrons for fluences as low as $\mathcal{O}(10^2)$
neutrons/sterad even when time of flight methods fail, define a figure of merit
and calculate its performance over a broad range of parameters.
