Neutron Spectrometer for Radiation Protection Purposes

One of the major problems encountered in dose assessment for neutron radiation protection arises from the imperfect dose equivalent response of the monitoring devices. Usually the neutron dosimeters are calibrated in a specific neutron spectrum (fast or thermal) and the response for different neutron spectra can be affected by errors up to 50% or higher. To correct dosimeter readings for dose 'missed' or for the imperfect energy responses it is important to know the neutron spectrum. One of most widely applied methods for measuring low intensity neutron fields is based on the detection of moderated neutrons in polyethylene of various sizes, also called Bonner Sphere Spectroscopy. The spectroscopic method developed is based on the use of a high sensitivity thermal neutron detector, a BF3 filled cylindrical proportional counter, with cylindrical polyethylene shieldings of various thicknesses. The response matrix was calculated using the Monte Carlo code MNCP 4C and for spectrum unfolding the SAND II code was used. A realistic detector model was used for the spectrometer response matrix computation. Isotopic fast neutron sources are used for the spectrometer calibration. Experimental devices, spectrometer response matrix and some results (neutron flux and spectrum) of measurements performed at experimental facilities are presented in the paper. The use of the high sensitivity neutron detector allows performing neutron spectrum measurements in very low intensity neutron fields, from the natural radiation background level up to about a few hundreds n/cm2s or neutron dose equivalent of 50 μSv/h. (authors)