A Spectrometer for Energetic Neutrons

A time-of-flight spectrometer has been designed to detect neutrons in the energy region from 10 to 100 MeV (and possibly also to higher energies). The principle of the spectrometer requires an incident neutron to lose a small fraction of its energy in a collision with a proton in a scintillator. The scattered neutron must then travel over a suitable flight path before interacting in a second scintillator. The time interval between the two scintillation pulses is a measure of the neutron energy. The small energy-transfer condition on the scattering process is imposed by restricting the kinetic energy of the recoil proton to be less than a predetermined value. Means for discriminating against background events include an anticoincidence detector 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 the pulse-shape discrimination circuitry. The spectrometer may be designed either for low counting rate applications such as measurements of fluxes and spectra of the energetic-neutron component of stray radiation fields around particle accelerators, or for higher counting rate experiments such as angular and energy distribution measurements of energetic-neutrons produced in the bombardment of both thin and thick targets. For the case of 14 MeV neutrons emerging isotropically from a target, a typical detector configuration consists of a first scintillator with a volume of 103 cc, a second cylindrical scintillator with a diameter of 10 cm, and a mean spacing between the two scintillators of 1.91 meters.