Realization of a neutron Bonner sphere spectrometer system

Of the many types of neutron spectrometer that have been developed, the system known as the multisphere, or more commonly the Bonner sphere spectrometer, has been built and extensively used by more laboratories than any other instrument. This is chiefly because it has an almost isotropic response, can cover the energy range from thermal to several hundred MeV neutrons, and is easy to operate. This spectrometer consists of a thermal neutron sensor which is used at the centre of a number of moderating spheres of different diameter. These are almost invariably made of polyethylene (C2H4)n and are usually made to exact inch, or half-inch, diameters. The convention of labeling each sphere by its diameter in inches has long been adopted over many years and will also be conserved. The combination of a thermal sensor plus moderating sphere has sensitivity to neutrons over a broad energy range. However, the sensitivity of each sphere peaks at particular neutron energy depends on the sphere diameter. From the measured readings of a set of spheres, information can be derived about the spectrum of the neutron field in which the measurements are made. In the following subsections, an overview of the Bonner sphere spectrometers using various types of thermal neutron detector will be considered, and then the setup and characterization of the DIMNP Bonner sphere spectrometer is described.