SPECTROMETER FOR NEUTRONS FROM 10 TO 100 MeV.

A new radiation detector instrument system has been designed to measure the spectra of neutrons in the energy region from about 10 to 100 MeV, The principle of the energetic-neutron spectrometer requires first that the incident neutron lose a small fraction of its energy in a collision with a proton in an hydrogenous scintillator and then that the scattered neutron interacts in another scintillator spaced a suitable distance from the first scintillator.The time interval between the two scintillation pulses provides a measure of the neutron energy.Means for discriminating against background events include an anti-coincidence arrangement for rejecting charged particles and a pulse-shape discrimination scheme for rejecting neutron interactions with carbon nuclei.Gamma rays are rejected by time-of-flight and also by pulse-shape discrimination circuitry.The energetic-neutron spectrometer may be used to measure (1) the fluxes and spectra of the energetic-neutron component of stray radiation fields around high-energy particle accelerators, ( 2) the energy and angular distributions of energetic-neutrons produced in the bombardment of both thin and thick targets, and (3) the energy of energetic-neutrons in nuclear reactions.For the case of an isotropic radiation flux of the order of 30 neutrons /cm^-sec in the energy interval from 10 to 100 MeV, the expected counting rate at 10 MeV is about 16 per hour.This estimate is for a first scintillator with a volume of 785 cc spaced at a distance of 1. 91 meters from a second cylindrical scintillator with a diameter of 20 cm and a height of 15 cm.For this case, the energy resolution for a system with a time dispersion (fwhm) of 3 nanoseconds varies from 16 percent at 10 MeV to 27 percent at 100 MeV.In experiments with higher fluxes, the energy resolution can be improved substantially bv utilizing" Pi^oiier detectors.