A Bonner sphere set being used
to measure a neutron spectrum.

NPL has a suite of neutron spectrometers that are used for characterising workplace fields in the nuclear industry or anywhere neutrons are a hazard. The devices are also used to derive knowledge about the spectra of standard neutron fields used for calibration and type testing.

Two types of measurements are performed for radiation protection purposes. Survey instruments are used to measure radiation levels in areas to determine the occupancy allowable for workers. If the levels warrant it, the doses received by workers in these environments are measured and recorded using personal dosemeters worn by the individuals. In both cases, the devices are intended to measure a quantity, called the dose equivalent, which is a measure of the radiation risk.

To date, it has not been possible to devise survey instruments or dosemeters with the right dose equivalent response at all energies. It is thus important that spectrometric information is available for the fields in which individuals are exposed to neutrons, to explore the extent to which these instruments misread because of their poor dose equivalent response functions.

Neutron spectrometry in workplace fields requires instruments with particular properties. They must, for example, be robust, transportable, and not overly sensitive to influence quantities, such as noise, encountered in workplaces. At NPL a set of such spectrometers is available allowing low resolution measurements to be made over the entire neutron energy range of interest, and good resolution measurements in the 50 keV to 20 MeV region.

Work presently underway to improve these spectrometers includes:

• Measurements and calculations to improve the response functions for a Bonner sphere spectrometer (used for measuring stray neutron fields around medical accelerators).
• Development of a photon spectrometry capability for mixed n/γ fields. The increasing use of devices with both a neutron and photon sensitivity, but no n/γ discrimination capabilities, means that it is important to characterise the photon component in neutron calibration fields.
• High resolution spectrometry is at present restricted to the energy region above about 50 keV. Instruments, and techniques such as digital signal processing, are being investigated to extend the range to lower energies.

NPL spectrometry instruments being used to characterise the neutron spectrum at a simulated workplace field facility at Cadarache in France:

Bonner sphere spectrometry.
This provides a spectrum covering the whole energy range of interest, albeit with poor resolution.

An NE213 liquid scintillator
This spectrometer is used above 1.5 MeV, and exhibits n/γ discrimination properties. In the past the γ-ray information has been discarded, but in future the option will be available for further analysis.

A proton recoil counter.
One of a set which covers the energy range from about 50 keV to about 1.5 MeV with good energy resolution. Work is underway to extend the energy range to lower energies since this is an important region for radiation protection.