Calibration of the NBS black neutron detector at 2.3 MeV using the time-correlated associated-particle method

Abstract A time-correlated associated-particle measurement capability using the D(d, n) 3 He source reaction has been developed at the National Bureau of Standards 3-MV positive-ion Van de Graaff Accelerator Laboratory. The facility has been used to measure the efficiency of the NBS Black Neutron Detector at a neutron energy of 2.3 MeV. The associated 3 He particles are detected at an angle of 45° with respect to the beam axis which is a more forward angle than conventionally employed. The kinematically more energetic 3 He particles detected at the forward angle are readily separated from scattered deuterons at an incident beam energy of 250 keV. The time-correlated coincidence requirement on events detected in the Black Neutron and associated-particle detectors virtually eliminates the need for background corrections to the Black Neutron Detector rate. A result for the efficiency of the Black Neutron Detector at 2.3 MeV has been obtained with an accuracy of about ± 1% and agrees well with a Monte Carlo calculated value. The measurement extends the usefulness of the Black Neutron Detector as an absolute neutron flux monitor to the higher energy region.