Effects of neutron energy spectrum on the efficiency calibration of epithermal neutron detectors

The 235U epithermal neutron fission detector system at TFTR has been calibrated in situ, using three different neutron sources: 252Cf, a D‐D, and a D‐T neutron generator. These sources produced a continuous spectrum, a line spectrum at ∼2.5 MeV, and a line spectrum at ∼14 MeV, respectively. The toroidally averaged detection efficiencies are 4.61×10−9 (252Cf), 4.80×10−9 (D‐D), and 5.42×10−9 (D‐T) counts/neutron for a moderated 1.3‐g 235U detector placed ∼4 m from the torus centerline. The effects of (n,2n) and (n,n’γ) reactions are discussed.