National Physical Laboratory

Transfer standards for thermal neutron fields

Neutron Transfer Standards

The transfer standard for thermal neutron fluence is based on the activation of gold foils and the subsequent measurement of the induced activity. The activity is directly proportional to the fluence but also depends on the neutron spectrum.

  • The thermal neutron fluence rate associated with a nuclear reactor, a radioactive neutron source or any other neutron-producing equipment can be measured at the customer’s premises using gold foils as the transfer standard.
  • The measurements involve activating gold foils both bare and under 1 mm thick cadmium covers and measuring the induced 198Au activity. The fluence rate is determined from the thermal neutron cross section for gold.
  • The quantity measured is the conventional thermal neutron fluence rate nth vo, where nth is the neutron density below the cadmium cut-off for 1 mm thick cadmium covers, and vo is the neutron velocity, 2200 m s-1 (corresponding to an energy of 0.0253 eV), conventionally used when specifying thermal fluences.
  • The fluence rate is given by:

nthv0 = F(D - DCdFCd) / Ngσ

Where:

F is a correction factor for self-shielding and perturbation of the neutron field,
D, DCd are the saturation disintegration rates per gram of foil for the bare and cadmium-covered foils respectively,
FCd is a correction factor for attenuation of neutrons in the cadmium cover,
N is the number of gold atoms per gram,
is the effective value of the neutron capture cross section for gold for the thermal component of the spectrum where σ is the cross section at 2200 m s-1 and g is the Westcott factor for the departure of the cross section from the 1/v law.

  • The activated foils must be returned to NPL for measurement of the induced 198Au activity using 4πβ, counting, the counting efficiency of each foil having been determined previously following irradiation in the NPL thermal neutron field.
  • If the foils have been irradiated in a fairly intense thermal neutron fluence, such as that in a reactor, the activity may be determined more directly using the 4πβ-γ coincidence counting technique.
  • The gold foils are 1 cm2 in area with thicknesses ranging from 20 mg cm-2 to 100 mg cm-2.
  • Irradiation times necessary for an adequate activity vary from about 15 minutes, for a high fluence rate, up to a maximum useful period of about a week for a fluence rate of 100 cm-2 s-1.
  • Foils must be returned to NPL within two days of irradiation.

Uncertainties

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

Contact

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

Last Updated: 25 Mar 2010
Created: 17 Apr 2007

Registration

Please note that the information will not be divulged to third parties, or used without your permission

Login