Iterative unfolding for Bonner sphere spectrometers using the MSANDB code - Sensitivity analysis and dose calculation

摘要

Since many decades, Bonner sphere spectrometers (BSSs) are routinely used for assessment of neutron spectra over a wide energy range from some meV to GeV. Typically, a spectrometer consists of a neutron detector sensitive to thermal neutrons located inside moderating polyethylene (PE) spheres of various sizes. Based on the response functions of the detector/sphere systems that must be calculated by Monte Carlo codes, an unfolding procedure is applied to deduce the present neutron spectrum from the count rates of the detectors. To start the unfolding procedure, a guess spectrum is required that includes some prior information on the physics of the investigated neutron spectrum, and that is iteratively modified to match the measured count rates. For the present investigation, a BSS-system consisting of an He-3 proportional counter and seventeen spheres containing PE or a combination of PE and lead is used. The system is used for example to measure secondary neutrons from cosmic radiation at mountain altitudes, or from high-energy accelerators outside the radiation shielding. A systematic study was performed to quantify the influence of the chosen guess spectrum and the number of iteration steps on the unfolded neutron spectrum, and on integral quantities deduced such as total neutron fluence or ambient dose equivalent. It turned out that none of these changes resulted in dose quantities that were more than a few percent different to those deduced when the optimised start spectrum was used. Similarly, use of the two different response matrices available for our BSS system provided similar values for the ambient dose equivalent.