ABSTRACT
A common method of detecting fast neutrons is through neutron moderation. Neutrons scatter in the moderator material and lose energy with each scatter, until, ultimately, the lose enough energy such that absorption in the thermal-neutron detector becomes likely. A small thermal-neutron detector is placed in the center of a sphere of high density polyethylene (HDPE). Fast neutrons entering the HDPE sphere lose energy through scattering and, thus, increase the probability of interacting in the thermal-neutron detector. The differential scattering cross section quantifies how likely a neutron is to scatter into some new direction from its initial direction. Neutron scattering from hydrogen is nearly isotropic in the center-of-mass coordinate system. One of the earliest methods for determining the energy dependence of the neutron field was the placement of different foils in the neutron field, and from the activity produced in the irradiated foils, the energy spectrum of the neutrons could be inferred.
