Deuterium-deuterium and deuterium-tritium reaction rates may be compared to determine plasma temperatures in the 10- to 200-eV range. Distinguishing neutrons from these two reactions is difficult when yields are low or unpredictable. Time-of-flight (TOF) 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 O(102)  neutrons per sterad even when TOF methods fail, define a figure of merit, and calculate its performance over a broad range of parameters.