Speaker
Dr
Nigel Hawkes
(National Physical Laboratory)
Description
This paper describes the facilities at the UK’s National Physical Laboratory for generating monoenergetic and broad-spectrum fast neutron fields.
Monoenergetic neutrons within the energy range 50 keV - 17 MeV are produced, via a variety of nuclear reactions, by directing charged particle beams from the Neutron Metrology Group's 3.5 MV Van de Graaff accelerator on to appropriate neutron-producing 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 maximum useable beam current depends on the heat sensitivity of the target and varies from 3 - 20 microamps.
The 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. Experiments are set up at the desired angle to the beam and distance from the target by means of lightly constructed detector supports accessed by low mass walkways. This arrangement minimises corrections for room- and air-scattered neutrons. In addition, a multi-energy facility is available, which makes use of the variation of neutron energy with angle to allow several irradiations to be carried out simultaneously at different energies.
Neutron fluences are measured using a well characterised long counter. Fluence uncertainties depend on the particular arrangements but are typically in the range 3 to 5%, and the instrumentation is subject to periodic international comparisons.
NPL has a wide range of isotopic neutron sources, including 252Cf, 241Am - Be, , 241Am - B, 241Am - F, and 241Am - Li. These may be mounted in the low-scatter area in place of the Van de Graaff neutron target for irradiations with a broad energy spectrum. The neutron output of all these sources has been measured to better than 1% in the NPL Manganese Bath facility, and their anisotropy characterised using a long counter. An accelerator-based broad-spectrum source has been constructed to simulate the neutron spectra typical of workplaces at pressurised water reactors and UK gas-cooled reactors.
Summary
This paper describes the facilities at the UK’s National Physical Laboratory for generating monoenergetic and broad-spectrum fast neutron fields.
Author
Dr
Nigel Hawkes
(National Physical Laboratory)
Co-author
Dr
David Thomas
(National Physical Laboratory)
