Neutron spectroscopy

Information for neutron radiation protection 

The need for spectral data arises because neither area survey instruments nor personal dosemeters give the correct dose equivalent results at all neutron energies. It is important therefore to know the spectra of the fields in which these devices are used. The spectra can be used either to provide information on the reliability of the results from devices used to measure the dose equivalent, or as a direct and accurate measurement of the dose equivalent. This is particularly valuable where doses approach statutory limits.

NPL can undertake on-site spectrometry measurements which, in addition to providing information for radiation protection applications, can also be used for example to determine the neutron output from a range of devices  (e.g. cyclotrons or LinAcs) which produce neutrons either as the primary radiation or as an unwanted contaminant.

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.
• Two high resolution NE213 scintillator spectrometers which cover the energy range from above about 1 MeV.
• A set of SP2 high resolution spherical hydrogen recoil counters which cover the energy range from about 50 keV to above 1 MeV.

Measurement timescales

Timescales for making a measurement depend on the intensity (dose rate) of the neutron field, and on the number of measurement sites.

For the active instruments it can take several hours to set up the instrumentation, and for the passive Bonner sphere set the efficiency is rather low so long measurement times may be required. 

Although maximum and minimum dose rates depend on the spectrum being measured, the passive Bonner sphere set can measure from about 10 mSv h-1 upwards, while the active set can measure, in a reasonable timescale, down to several tens of nSv h-1. The NE213 scintillator requires a dose equivalent rate in the tens of mSv h-1 range, while the hydrogen recoil counters need dose equivalent rates in the region of a hundred mSv h-1 or higher.

Spectrometer characteristics
Spectrometer	Energy range	Typical minimum dose equivalent rate
Active Bonner sphere set	Thermal to 20 MeV	Tens of nSv h-1
Passive Bonner sphere set	Thermal to 20 MeV	Tens of µSv h-1
NE213 scintillators	1 to 20 MeV	Tens of µSv h-1
Hydrogen recoil counters	50 keV to 1.5 MeV	About 100 µSv h-1

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