Simulations of neutron response and background rejection for a scintillating-fiber detector

We address the problem of detection of fast neutrons at energies of 5-30 MeV in the presence of strong backgrounds of low-energy neutrons and gammas. In an organic scintillator, the neutrons are detected mainly as recoil protons from n-p scattering; the gammas produce electrons primarily by Compton scattering. The recoil protons and electrons can be distinguished because the light output per unit distance near the end of a proton track is much higher than that for an electron. This difference could be used to reject gamma backgrounds such as those in a reactor environment by constructing a segmented detector from a bundle of scintillating fibers connected to a readout device. We present the results of extensive simulations for a module composed of 6400 fibers, each 250 μm square, arranged into a 2x2x2 cm3 bundle. Considered were fast neutrons with energies of 3, 7, 14 and 28 MeV and backgrounds composed of neutrons and gammas with energy distributions characteristic of delayed emission from a fission source. The simulations indicate that the detector can obtain a good compromise between outstanding background rejection and reasonable detection efficiency for the neutrons of interest. (orig.)