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Nuclear metrology

Neutron spectrometry

Characterising workplace fields in the nuclear industry or other environments where neutrons are a hazard

The difficulty in measuring neutron fields arises partly from the complexity of their interaction with matter, and partly from the very large range of neutron energies that can be encountered. This range extends from a few meV (thermal neutrons) in nuclear power production, to hundreds of MeV in clinical accelerators and up to the GeV region for cosmic rays. This covers 12 orders of magnitude with the risk level varying across the energy range.

NPL has a suite of neutron spectrometers that are used to characterise and environment and also derive knowledge about the spectra of standard neutron fields used for calibration and type testing. Neutron spectrometry in workplace fields requires instruments with particular properties. They must be robust, transportable and not overly-sensitive to outside influences, such as noise.

The spectrometers available at NPL can be divided into two types: firstly, Bonner sphere sets which cover a very broad energy range, albeit with rather poor resolution; secondly, devices with high resolution but which only cover a limited energy range. The particular instruments available are listed below:

  • An 'active' Bonner sphere system, based on a sensitive 3He proportional counter. This set can be used for dose equivalent rates from several hundred mSv per hour down almost to natural background.
  • A 'passive' Bonner sphere system, based on gold activation foils as sensor elements. This system is less sensitive than the 3He-based set, but can be used in areas where the neutron field is pulsed, where there is an intense photon field, or where radioactive contamination is a potential problem.
  • A set of SP2 high resolution spherical hydrogen recoil counters, which cover the energy range from about 50 keV to above 1 MeV.

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