Linac Measurements of the Fast Neutron Response of Organic Scintillators

The 20-MeV linac at EG&G, Santa Barbara, produces fast neutrons up to 11 MeV by the photoneutron reaction in a beryllium target. With a 10-nanosecond linac burst and a 25-meter evacuated flight path, neutron energy resolutions ranging from 7 percent at 10 MeV to 1.4 percent at 0.4 MeV are obtained. A plastic scintillator identical to that designed by Batchelor was placed at the end of the flight path, and its scintillation output was placed in coincidence with narrow time windows. These windows could be varied in width and position with respect to the linac burst. The scintillations passing through a window were the response of the detector to neutrons of a narrow band of energies. By scanning the window in time, the neutron pulse-height response of the detector was measured for neutrons from 1 MeV to 8 MeV. In a separate experiment the neutron detection efficiency of a 6-inch cube of MEL 150 C plastic scintillator was derived by comparison with Batchelor´s detector; that is, the same time-of-flight neutron spectrum was measured by both detectors. This is a new and convenient application of the linac time-of-flight technique for measuring the neutron response and efficiencies of organic scintillators.