National Physical Laboratory

Neutron Fluence Metrology

We have a manganese sulphate bath, which is used to measure absolutely the neutron emission from radionuclide sources, and we also keep a set of calibrated sources so that we can produce neutron fields standardised in terms of neutron fluence.

Manganese BathThe manganese bath, used to measure the neutron emission from radionuclide sources Van de Graaff AcceleratorMost neutron measurements, including those made with the Van de Graaff accelerator, are carried out in a large low-scatter experimental area


In addition we use our Van de Graaff accelerator to generate ISO standard monoenergetic neutron fields from 144 keV to 5 MeV, and other well-characterised fields, for testing and calibrating area survey instruments, dose meters and spectrometers.

Most neutron measurements, including those made with the Van de Graaff accelerator, are carried out in a large low-scatter experimental area.

The primary objective of this work is to ensure the continued availability of national standards of neutron fluence and dose equivalent quantities. These standards are essential for calibrating and checking radiological protection instruments and dose meters. Our work during the present programme includes:

  • Re-locating the manganese sulphate bath to a new purpose-built site, and updating its data acquisition electronics and source handling equipment.
  • Carrying out regular checks of the neutron emission rate from our sources, and keeping the energy calibration of the Van de Graaff accelerator up-to-date.
  • Investigating the production of monoenergetic neutron fields at high energies (above 15 MeV).
  • Improving the neutron-producing targets, for example by reducing the number of gamma rays that accompany the neutrons, and investigating how the target thickness is best measured.
  • Studying how neutron-producing targets can change over time (for example, by slow migration of the neutron-producing nuclide into the target backing), and the effect such changes have on the neutron fields.
  • Taking part in international intercomparisons of neutron fluence measurements. A comparison of radionuclide source emission rates is presently underway and a comparison of thermal neutron standard fields is being planned.
Long CounterOur fluence-measuring instruments, e.g. the long counter shown here, are subjected to international inter-comparisons Neutron SourcesStandard radionuclide neutron sources
Last Updated: 15 Aug 2012
Created: 17 Oct 2007

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