Neutron Metrology - Comprehensive Introduction
Measuring neutrons, however, is not straightforward. Being uncharged, neutrons have to be detected indirectly. Furthermore, neutron fields can often cover a wide range of energies, which is critically important when considering potential health effects as the risk is strongly dependent on neutron energy.
The NPL Neutron Metrology (NM) section provides measurement standards and services for calibrating and testing instruments designed to measure neutron fluence (the number of neutrons per unit area) or neutron dose equivalent (a measure of the risk to human health), as well as for characterising less direct effects such as errors induced in memory chips. The NM section also hosts a world-leading facility for measuring the neutron emissions from radionuclide sources.
To support these services extensive research is carried out to improve not only the standards but also the basic techniques used to measure neutron fluence and dose equivalent. An increasingly powerful tool in this research is the use of computer codes to simulate neutron sources, detectors and even whole facilities.
NM section is particularly well placed in being able to provide a variety of neutron fields, covering a wide range of energies and intensities. Several radionuclide sources (with broad energy spectra) are available, as are monoenergetic neutrons derived from the section's 3.5 MV Van de Graaff accelerator. The accelerator is also used to produce low energy (thermal) neutrons, and neutron fields that are representative of those found in workplaces around nuclear reactors.
Another key area of the NM section’s capabilities is the range of field instruments available for use. These include active and passive systems to measure neutron fluence rates and energy spectra, as well as instruments for performing neutron dosimetry, including cosmic ray dosimetry for air crew.