Monoenergetic neutron production

Calibration and irradiation

At NPL well-characterised mono-energetic neutron fields covering the greater part of the energy range 50 keV to 5 MeV are routinely available for the calibration of neutron sensitive devices or for irradiation purposes.

Energies just above and below this range can also be produced for special applications.

• Neutrons within the above range of energies are produced, via a variety of nuclear reactions, by employing beams of protons or deuterons from a 3.5 MV Van de Graaff accelerator to bombard appropriate targets. The charged particle beam energy and energy spread are determined using a calibrated analysing magnet and the mean energy is defined to within ± 2 keV.
• The neutron-producing targets are located at the centre of a low scatter facility, at a point at least 6 m from the walls, floor or ceiling of the room. This arrangement minimises corrections for scattered neutrons when performing calibrations.
• Lightly constructed supports are provided for neutron detectors, which may be positioned automatically at the desired angle to the incident charged particle beam (this angle determines the neutrons energy) and at the required distance from the neutron producing target.
• The neutron fluence rate depends upon factors such as target thickness, charged particle beam current, geometry and required neutron energy resolution. These factors may be varied within certain limits to meet the requirements of individual customers.
• The various standard fields are characterised in terms of neutron fluence rate, neutron energy and energy resolution.
• Neutron fluences are measured using a carefully calibrated long counter. Fluence uncertainties depend on the particular arrangements but are typically in the range 3 to 5%.
• The instrumentation used to measure the neutron fluence is subject to periodic international comparisons.
• Values for quantities of interest for dosimetry, e.g. ambient or personal dose equivalent, can be derived by making use of internationally agreed conversion coefficients.

ISO recommended energies

• Neutrons can be produced with energies almost anywhere within the range 50 keV to 5 MeV, and to some extent outside this range for special applications. However, certain energies are recommended by the International Organisation for Standardization, ISO, as being particularly appropriate for performing calibrations. For example, at 565 keV a relatively high fluence of neutrons can be produced because this energy corresponds to a peak in the ⁷Li(p,n)⁷Be reaction cross sections. The relevant ISO energies are listed in the table below together with indications of the maximum fluence and ambient dose equivalent rates achievable at 1m from the target. Personal dose equivalent rates are very similar.

Please note that:

• In the determination of neutron fluence rate, corrections are applied for in-scatter (measured with shadow cones), and out-scatter (calculated from the known cross-sections for oxygen and nitrogen).
• When dosimetric quantities are of interest, the gamma-ray contribution to the total dose can also be determined by use of energy compensated Geiger-Muller counters.
• Our reported uncertainties are based on a standard uncertainty and multiplied by a coverage factor k = 2, providing a level of confidence of approximately 95%.

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