Neutron flux intercomparison at NBS

National Bureau of Standards (NBS) participation in an International Bureau of Weights and Measures (BIPM) sponsored neutron flux intercomparison is described. The efficiencies of two transfer instruments, a gas-filled /sup 3/He proportional counter and a BF/sub 3/ counter imbedded in a polyethylene sphere, were determined at neutron energies of 250 keV and 560 keV. The efficiency of the /sup 3/He detector was determined by placing it in the flux monitored by the NBS black detector. Since the polyethylene sphere detector shadows the black detector, a secondary BF/sub 3/ counter was calibrated and it normalized the sphere and black detector runs. The efficiencies are reported and compared to the quantities determined at other participating standards laboratories.