Neutron detectors for reactor instrumentation

The paper discusses the principles involved in the detection of thermal neutrons and describes three types of detector used in the operation of natural-uranium graphite-moderated reactors, namely flux-scanning detectors, installed flux monitors and detectors used in association with automatic power-control plants. Flux-scanning detectors consist of linear assemblies of Chromel-¿Alumel thermo-couples with alternate junctions coated with boron. Installed flux monitors employ discs of boron steel in place of the boron beads. Ionization chambers may be used as installed flux monitors, and they are also used for the starting-up and control of nuclear reactors; designs and characteristics of the standard chambers are given. The problem of the ¿-ray sensitivity of such chambers is considered, and a design of a compensated chamber is described in which the sensitivity to ¿-rays is reduced by a factor of 160. It is thought that compensated chambers of this type will be much used for the measurement of power level in future nuclear reactors, particularly in automatic power-control apparatus.