Development of a segmented fast neutron spectrometer

We discuss the development of a spectrometer based on full energy absorption using liquid scintillator doped with enriched Li-6. Of specific interest, the spectrometer is expected to have good pulse height resolution, estimated to lie in the range 10 - 15% for 14-MeV neutrons. It should be sensitive to flux rates from 10(-6) cm(-2) s(-1) to 10(6) cm(-2) s(-1) above a threshold of 500 keV in an uncorrelated gamma background of up to 10(4) s(-1). We have constructed a pilot version of the detector using undoped liquid scintillator, and we report its present status. The detector's efficiency is determined by the volume of the scintillator (similar to 1.2 l) and is estimated to be 0.2 - 0.5% for 3-MeV neutrons. The good pulse height resolution is achieved by compensation of the nonlinear light yield of the scintillator due to the use of optically separated segments, which collect scintillations from each recoil proton separately. We demonstrate here the response of the detector to neutrons from a Pu - alpha - Be source, whose energies range up to 10 MeV. Initial testing indicates a low threshold (approximate to 600 keV) and good spectral response after requiring a multiplicity of three segments. Such a spectrometer has applications for low-background experiments in fundamental physics research, characterizations of neutron flux in space, and the health physics community.