Neutron dosimetry in mixed fields with monoblock neutron spectrometer

Full text: The multi-sphere method of neutron spectrometry or namely Bonner spheres neutron spectrometry is currently playing an increasing role in the mixed radiation field measurements. The growing popularity of this methodology is caused by its relative availability, simplicity of measurement in a wide energy range, high sensitivity and satisfactory gamma-ray suppression. These qualities allow the usage of multi-sphere neutron spectrometers for adequate characterization of neutron field, particularly reliable measurements of neutron dose rate. However, the main difficulties in the application of this kind of neutron detector are the perturbation of the neutron field, caused by the detector itself, and the complex procedure required for unfolding the neutron spectrum. Furthermore, it is necessary to perform a relatively high number of measurements, one for each spherical moderator (as a rule, 5-7 pieces). This in turn may require a dedicated source monitoring system, otherwise significant errors may occur. These requirements hamper the application of the multi-sphere spectrometry method to pulsed neutron sources, for example. Other difficulties occur in the characterization of reactor neutron beams, in case the beam diameter is smaller than those of the spherical moderators. In this situation it is necessary to carry out a beam scanning and integrate the acquired data. To improve the methodology of neutron field parameter measurement the Monoblock Neutron Spectrometer (MNS) has been developed recently. The basic idea of the novel detector is to determine the neutron energy spectrum by unfolding a set of count rates from thermal neutron detectors located at different depths in the common polyethylene moderator. The unfolding algorithms for neutron spectrum and neutron dose rates have been specifically improved for operation with MNS. The testing results with well-know neutron reference fields and reactor neutron beam are presented. The application of MNS for neutron dosimetry and the uncertainties in the neutron dose assessment are discussed. (author)