Further Characterization, Extension and Application of A Simplified Bonner Sphere Spectrometer

One disadvantage of a Bonner Sphere Spectrometer (BSS) is the impracticability due to the lengthy process of measurements as well as the weight issue. Hence, a simplified BSS was developed in the past but having limited spectrum accuracy at epithermal and high energy region. In addition, the use of cylindrical detector brings drawback of anisotropic angular response. The overall objective of this thesis was to improve the performance and features of the simplified BSS by first, utilizing a spherical 3He detector to obtain an almost flat angular response, and second, by adding two other spheres to refine the reconstructed spectrum at epithermal and high energy up to 250 MeV. As the final shape of the spectrum also depends on the use of unfolding methods (i.e., methods for spectrum reconstruction from BSS measurements), we aimed to study the unfolding performance of the new BSS using four unfolding methods. This work was divided into several tasks including characterization of energy and angular response of the current BSS, selection of the new sphere designs, and theoretical performance evaluation of the new BSS configurations. During the first two stages, MCNPX v2.7 was used as the primary tool for calculation. The energy response functions were calculated in 115 energy groups from meV up to 252 MeV while the angular responses were simulated for 13 angles from 0 to 180o. For the new sphere selection, there were 16 design options in total, consisting of each eight sphere designs for epithermal and high energy regions, respectively. The new spheres were selected by evaluating the effective rank of response matrices and by performing unfolding surveys over 45 different spectrums. The use of the unfolding codes (MAXED, GRAVEL, FRUIT, and Winbugs) was evaluated by simulating three types of fields, including fission, evaporation, and high energy neutron fields. All codes were fed with the same input, which was a realistic simulated counts data. The unfolded spectra and the integral quantities (e.g. neutron fluence, ambient dose equivalent) were then quantified to know the performance of the new configuration and to compare the performance of the codes. The simplified BSS using the spherical detector has been characterized concerning energy and angular response by means Monte Carlo calculation. The angular response was nearly isotropic, with minimum 78% efficiency. The new sphere design evaluation based on both effective rank, unfolding surveys obtained a similar results suggesting that sphere of diameter 4.5" with 1 mm Cd as well as 10" or 7" with 1 cm W were suitable for improving the spectrum accuracy at epithermal and high energy regions. Due to the weight consideration, the 4.5" 1 mm Cd and the 7" 1 cmWspheres were finally selected, with estimated mass of about 1 and 7.5 kg, respectively. After performing an extensive test on three kinds of spectrum, we conclude that the new BSS configuration, regardless of the use of the unfolding codes, showed its ability to improve the shape accuracy of the reconstructed spectrum. The expected unfolding performance may vary depending on the type of the fields, the unfolding technique, and the chosen unfolding parameters. The maximum deviations in terms of total fluence and ambient dose equivalent for the three kinds of neutron fields were expected to be within 10%, which is adequate for radiation protection. Though the discrepancy of the reconstructed spectrum among the different codes was observed, in general, they provide a conservative estimate of the integral quantities.