National Physical Laboratory

Measurement of radionuclide neutron source emission rates and emission anisotropy

Standard radionuclide neutron sources
Standard radionuclide neutron sources

NPL is one of the few national standards laboratories worldwide that offers an absolute measurement of the total emission rate of a radionuclide neutron source into 4π steradians. Comparative measurements of the total emission rate can also be performed. A further service offered is a measurement of the variation in emission rate relative to the axis of the source (anisotropy). NPL also has available for hire a number of sources with accurately known neutron emission rates and anisotropy factors.

Calibration facilities:

  • Absolute emission rate measurements are performed using the manganese bath technique. The neutron source is placed at the centre of a spherical bath, 1 m in diameter, containing an aqueous solution of manganese sulphate. The amount of radioactive manganese produced is measured by pumping the solution past two scintillation detectors, the γ-counting efficiencies of which are determined by absolute counting techniques. The neutron emission rate is calculated from the ã-count rate at saturation, applying appropriate correction factors.
  • A moderating assembly incorporating BF3 proportional counters is used to make comparative emission rate measurements. The source emission rate is obtained by comparison with a source of the same type which has been measured in the manganese bath.
  • Anisotropy measurements are made in a low-scatter area using a long counter to measure the output in steps of 10°.

Measurement capabilities:

  • The calibration limits are determined by the emission rate of a source. Lower and upper limits are outlined in the following table:
Note: 2.4 x 106 s-1 is the nominal emission rate from a 37 GBq (1 Ci) 241Am-Be source or a 1 ìg 252Cf source
Note: 2.4 x 106 s-1 is the nominal emission rate from a 37 GBq (1 Ci) 241Am-Be source or a 1 ìg 252Cf source
  • Calibrations are UKAS accredited.
  • The measurement of a source is generally completed within two weeks of receipt, and the calibration certificate issued within six weeks.


  • Uncertainties are estimated following the approach recommended in the 'Guide to the Expression of Uncertainties in Measurements' (GUM) published by the International Organization for Standardization (ISO).
  • Our reported uncertainties are based on a standard uncertainty, multiplied by a coverage factor k = 2, providing a level of confidence of approximately 95%.
  • Uncertainties depend on source emission rate and type. Typical values are summarised in the following table:

Neutron Source Uncertainties

Recalibration frequency:

It is recommended that 252Cf sources are recalibrated at least every 2.5 years, and 241Am-based sources at least every 10 years.

Sources available for hire:

Neutron Sources for Hire

Shielded transport containers are also available for use with these sources.


Customer Services tel: +44 20 8943 8637

Last Updated: 25 Sep 2013
Created: 17 Apr 2007


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