A time-of-flight spectrometer for neutrons from 1 to 500 MeV

Abstract

A self-contained time-of-flight spectrometer has been developed for neutrons from 1 to about 500 MeV. The spectrometer consists basically of two scintillation counters. The principle of the spectrometer requires an incident neutron to scatter elastically from a proton in the first scintillator, travel over a known flight-path, and interact in the second scintillator. The time interval between the scintillation pulses in the two detectors is the time-of-flight of the scattered neutron. Kinematically, the kinetic energy of the incident neutron is specified in terms of the scattering angle, the flight-path, and the flight-time of the scattered neutron. The intrinsic time dispersion of the system is 2.2 ns (fwhm) with a 2 1 2 ″ diam. by 2 1 2 ″ high first scintillator and a 9″ diam. by 8″ thick second scintillator. For a mean flight-path of 4 m between these two scintillators, the energy resolution of the spectrometer varies from about 7 to 20% and the spectrometer efficiency varies from about 2×10 −5 to 8×10 −7 over the range of energies from 10 to 500 MeV. The background from interactions of neutrons above about 15 MeV with carbon nuclei in the first scintillator is subtracted by measuring the spectrum twice with first scintillators differing significantly in their relative hydrogen-carbon composition. Measurements with monoenergetic neutrons at 14 MeV confirm the resolution and efficiency of the spectrometer at low energies. Measurements with monoenergetic neutrons at 220 MeV verify the resolution of the spectrometer at this energy and demonstrate the technique for subtracting the carbon background. Recently, we have measured neutron spectra from 740 MeV proton bombardment of a 30 cm thick uranium target over the energy range from 1 to 500 MeV. In each of these measurements, the count rate of real events was typically a count/s, and the chance coincidence count rate was of the same order of magnitude.