Moderating Sphere Research Articles

The basis of a suggested instrumental approach to the surveying of neutron fields and the measurement of personal dose from neutrons

The basis of a two-component neutron monitoring system that could be applied to both survey instruments and personal dosemeters is considered. Neutrons with energies below 10 keV are detected by a thermal neutron detector which, in the survey instrument, is located inside a small moderating sphere, whereas in the personal dosemeter it is positioned behind a thermal neutron shield located on an individual's body. Neutrons above 10 keV are detected by counting pulses produced by protons ejected from a thin hydrogenous foil. By judicious choice of the characteristics of the detection elements, it is possible to ensure that the sum of the sensitivities of the elements of both survey instrument and personal dosemeter vary with neutron energy in a way close to that required for radiological protection purposes through the range thermal-10 MeV. The realisation of the elements sensitive below 10 keV presents few fundamental problems and appropriate devices have been reported in the literature. Those sensitive above 10 keV have proved to be more difficult to realise in practice.

Fluence and dose equivalent determination in a neutron field by means of moderating spheres containing fission tract recorders.

A description is given of the determination of neutron fluence and dose equivalent by means of registration of tracks in plastic foils from thin layers of fissile material surrounded by moderator spheres.

A new type of neutron spectrometer

Neutrons are detected in a small Li 6I(Eu) scintillator placed at the center of polyethylene moderating spheres with sizes ranging from 2 to 12 inches in diameter. The efficiency of this neutron counter has been experimentally determined using monoenergetic neutrons from thermal energies to 15 MeV. The counter has excellent energy sensitivity from 0.1 to 2 MeV and is particularly useful for determining the shapes of continuous neutron spectra. The pronounced difference in the efficiencies for the five sizes of spheres which have been calibrated provides a basis for accurate neutron energy determination. The good γ ray discrimination of the counter allows it to be used with a radium-beryllium neutron source. Neutron spectra from a variety of sources have been determined with this counter. These include the two groups of neutrons from the C 14(p,n)N 14 reaction, the evaporation spectrum of the neutrons from the reaction Rh 103(p,n)Pd 103, the energy spectra of inelastically scattered neutrons, and the neutron spectrum from the scattering of fast neutrons by the floor and walls of a building.