Radiation protection dosimetry in mixed neutron/photon fields is still far less established than dosimetry for photon fields alone. Neutron dosimetry is complicated by the fact that neutron doses are strongly energy dependent and that neutron energies in typical workplace fields vary with at least 10 orders of magnitude. Although efforts are made by manufacturers of neutron dosemeters to reflect the energy dependency of the neutron dose in the energy response of the dosemeter, the approach is never perfect, which means that there is at present no neutron personal dosimeter available that measures with the same precision as photon dosemeters. Although neutron doses are in general much lower than the photon doses, neutron personal dosimetry is of importance for a specific number of workers in specific areas, eg. in the close proximity of reactor cores, around accelerators and spent fuel casks. To make reliable estimates of neutron doses, SCK•CEN is currently doing characterization measurements in terms of energy distribution of the neutron field in different workplace fields, such as nuclear power plants and hadron therapy centres, where neutron doses can be important.
Measuring the neutron energy spectrum is quite difficult, which has led to development of different methods in the past. All those different methods (proton-recoil, scintillators, activation foils, bubble detectors, …) have important drawbacks and cannot cover the whole energy range needed for radiation protection. There are very few commercial devices available, and all have a very limited resolution or energy range. Currently the method that is mostly used in radiation protection is Bonner Sphere Spectrometry. The basics of this technique consist of using a detector for thermal neutrons, and surrounding it by polyethylene spheres from different thicknesses. The more polyethylene that surrounds the thermal detector, the more the response shifts to higher energies. By doing several measurements using different moderator thicknesses, the neutron spectrum can be reconstructed, based on unfolding techniques using the different detector answers. In order to do this, the response curves from the different sphere thicknesses need to be very well characterised, which is mostly done with Monte Carlo simulations.
A simplified Bonner Sphere system has recently been made operational at SCK•CEN. It consists of a bare cylindrical He-3 proportional counter with a 3 inch and a 9 inch moderating polyethylene sphere. The system has been characterised in terms of its response functions for both the angular and the energy dependence by means of MCNPX simulations. The angular dependence and the sensitivity of the bare cylindrical He-3 proportional counter have also been validated experimentally with the thermal neutron beam at the Belgian Reactor 1 at SCK•CEN. The first measurements in the calibration lab at SCK•CEN with Cf-252 and Am-Be neutron sources, where thermal neutrons form a substantial contribution due to scattering, have demonstrated that even with this limited amount of spheres one can get a good idea about the neutron energy spectrum.