Analogies between neutron and gamma-ray imaging

Although the physics describing the interactions of neutrons with matter is quite different from that appropriate for hard x-rays and gamma rays, there are a number of similarities that allow analogous instruments to be developed for both types of ionizing radiation. A pinhole camera, for example, requires that the radiation obeys some form of geometrical optics, that a material can be found to absorb some of the radiation, and that a suitable position-sensitive detector can be built to record the spatial distribution of the incident radiation. Such conditions are met for photons and neutrons, even though the materials used are quite different. Neutron analogues of the coded-aperture gamma camera and the Compton camera have been demonstrated. Even though the Compton effect applies only to photons, neutrons undergo protonrecoil scattering that can provide similar directional information. There is also an analogy in the existence of an energy spectrum for the radiation used to produce the images, and which may allow different types of sources to be distinguished from each other and from background.