Relative intensities of 2.5- and 14-MeV source neutrons from comparative responses of U-238 and U-235 detectors

The response of polyethylene-moderated U-235 fission counters is only weakly dependent on incident neutron energy, while the response of unmoderated U-238 or Th-232 fission counters increases strongly with energy. A given concentration of D–T neutrons in a mixed DT–DD source results in a unique relative detector response that depends on the parameters R14 and R2.5, where R14 is the ratio of the unmoderated U-238 and moderated U-235 detector efficiencies for a pure 14-MeV neutron source, and R2.5 is the corresponding ratio for a pure 2.5-MeV source. We have recently determined R14 and R2.5 using D–D and D–T neutron generators inside the TFTR vacuum vessel. The results indicate that, for our detector geometry, the ratio of U-238 to U-235 count rates should increase by a factor of about 3 when the fusion neutron source changes from pure D–D to pure D–T. This calibration is being applied to recent TFTR ‘‘supershot’’ data, where the uncollided neutron flux in the postbeam phase contains a high proportion of D–T neutrons from the burnup of D–D tritons.