National Physical Laboratory

Radionuclide source-based fluence and dose standards

Shadow Cone
Shadow cone being used
to measure neutron
scatter during calibration
of a survey meter

A variety of radionuclide neutron sources are available to provide, at NPL, well-characterised neutron fields for calibrating neutron sensitive devices or for irradiating samples. Typical applications include the calibration and type testing of area survey instruments, personal dosemeters, neutron spectrometers and measurements of the response of neutron sensitive devices as a function of energy. NPL is one of the few national standards laboratories worldwide that can offer radiological type testing facilities to determine device response functions.

Calibration Quantities

Calibrations are performed in terms of neutron fluence, ambient dose equivalent, or personal dose equivalent.

The dose equivalent quantities used are normally according to the recommendations of ICRP Publication 60.

Spectrum averaged conversion factors used are those recommended in ISO 8529 Reference neutron standards - Part 3. Calibration of area and personal dosimeters and determination of their response as a function of neutron energy and angle of incidence. (1998).

Range of sources available:

  • 241Am-Be, 241Am-B, 241Am-F, 241Am-Li, 252Cf, and heavy water moderated 252Cf.
  • Mean energies of the fluence spectra are between 0.5 MeV and 4.1 MeV.
  • Total neutron emission rates vary for the different source types, ranging from 2 x 104 to 4 x 107 s-1, corresponding to fluence rates between 0.2 and 3 x 102 cm-2 s –1 at 1 m. See table for further details.

Neutron Fluence:

  • The neutron fluence rate at an irradiation position is calculated from the absolute neutron emission rate of the source, as measured in the NPL manganese bath. Dose equivalent rates can be derived using internationally agreed conversion factors.
  • When specifying the fluence, allowance is made for source emission anisotropy and, if necessary, for scattered neutrons.

ISO Recommendations:

The 241Am-Be, the 241Am-B, the ‘bare’ 252Cf and the heavy water moderated 252Cf sources are all ISO recommended calibration standards. (See International Standard ISO 8529 Reference neutron radiations – Part 1 Characteristics and methods of production (2001).)

Calibration Facilities

Calibrations of neutron sensitive devices can be performed in the NPL low scatter facility. The source is placed at the centre of the room thus keeping the scattered neutron component as small as possible. Techniques are available to correct for this component if necessary.

Personal dosemeters can be mounted on an ISO recommended water phantom, an ICRU recommended perspex phantom, or free-in-air. The normal irradiation distance for this type of device is 75 cm from the source, unless otherwise specified by the customer.

Heavy water moderated 252Cf irradiations are performed by placing the ‘bare’ 252Cf source capsule at the centre of a 30 cm sphere of heavy water. This results in a neutron spectrum, which contains a significant low energy neutron component.

Custom radionuclide source irradiations are available on request. The maximum ambient dose equivalent rate achievable with an 241Am-Be source is in the region of 3.7 mSv h-1 at ~ 30 cm. he maximum ambient dose equivalent rate achievable with a 252Cf source is in the region of 2.4 mSv h-1 at ~ 30 cm.

Uncertainties

  • Reported uncertainties are based on a standard uncertainty multiplied by a coverage factor k = 2, providing a level of confidence of approximately 95%.

Sources available, with typical fluence and dose equivalent rate

Neutron Fluence Sources
* Based on fluence to ambient dose equivalent conversion factors published in ICRP publication 74 and ICRU Report 57

Contact

Customer Services tel: +44 20 8943 8637
E-mail: neutron_enquiries@npl.co.uk

Last Updated: 25 Mar 2010
Created: 17 Apr 2007

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