Energetic neutron spectrometry. Final report, December 1, 1967--March 31, 1972

Abstract

A research program in energetic-neutron spectrometry was carried out under Contracts AT(30-1)-3914 and AT(11-1)-3258 with Clarkson College of Technology during the period from 1 December. 1967 through 31 March 1972. Accomplishments under this program include the development of a self-contained time-of-flight spectrometer for neutrons from 1 to 500 MeV and the measurement of neutron spectra from a thick target. Early developmental work was conducted with 14 MeV neutrons at the National Research Council, Ottawa, Canada. Later, measurements at higher energies were made at the 184-inch cyclotron of the Lawrence Berkeley Laboratory. These include the measurement of 220 MeV neutrons from stripping 4 447 MeV deuterons on beryllium and the measurement of neutron spectra from 740 MeV proton bombardment of a thick uranium target. Basically, the spectrometer measures the time-of-flight of a neutron scattered between two scintillation counters. The intrinsic time dispersion of the system is 2.2 nanoseconds (fwhm) with a 2* inch' diameter by 2* inch high first detector and a 9 inch diameter by 8 inch thick second detector. For a flight path of.4 meters, the energy resolution of the spectrometer varies from about 2 x loTs at 10 MeV to 8 x 10-7 at 500 MeV. Background from neutron-carbon interactions is determined from separate spectral measurements with first detectors differing iii iV significantly in their relative hydrogen-carbon composition. Measurements with 14 MeV neutrons confirm the resolution and efficiency of the spectrometer at low energies. Measurements at 220 MeV verify the resolution of the spectrometer at this energy and demonstrate the technique for subtracting:.the. carbon background. Neutron spectra from 740 MeV proton bombardment of a 30-cm thick uranium target were measured from 20 to 500 MeV at 50 degrees and from 5 to 140 MeV at 130 degrees with respect to the proton beam at LBL. The results are compared with a Monte Carlo calculation of the neutron cascadeevaporation &5 spectrum. While the shapes of the measured and calculated V spectra are in general agreement, this comparison shows that the intranuclear cascade model underestimates the production of cas cade neutrons at wide angles; furthermore, this comparison shows that this discrepancy increases with increasing angle of ernission. :