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Abstract
Neutron spectrometry came to play as the discovery of the neutron by Chadwick in 1932. During 1930’s, photographic film, calorimeters, color dosimeters, ionization chambers, proportional counters, Geiger Mueller counter, and scintillation counters were all available for radiation detection. The needs to develop these detectors and others to detect neutrons became more and more important. Notably, nuclear technology, fission reactors, fusion, nuclear materials, medical applications, industrial applications, national security, and high-energy physics are areas that drive neutron spectrometer development.
One of the most common moderating detector type is the multi-spheres, or more commonly, the Bonner sphere spectrometer (BSS), introduced by Bramblett, Ewing and Bonner in 1960. It has the advantage of having high efficiency and ability to discriminate between neutrons and the accompanying gamma rays. It can also detect neutron energies up to GeV values compared with other available neutron detectors.
The BSS system has been built and used more than any other neutron spectrometer, mainly because it has an almost isotropic response and cover the energy range from thermal to GeV neutrons by using materials with high mass number such as lead to improved the response of BSS to high energy neutrons. For example, spheres with diameters ranging from 2 to 12 inches are used to measure spectra from thermal energies up to 15 MeV. In this case the total system weight is about 100 kg. Higher energy measurements involve heavier systems. For very high-energy range measurements, the weight might reach 750 kg. Besides, in conventional BSS system, neutron field measurement is carried out individually for each individual sphere once at a time.
A portable-single-measurement spectroscopy system can be used effectively in many applications such as detection of explosives and landmines, nuclear facilities, and other in-situ measurements. In this work, neutron spectroscopy is carried out using multi-layered rectangular geometry moderated with light water. The system is consisted of successive rectangular parallel layers of BF3 detectors (96% ‘°B) followed by water layers. In total, ten BF3 layers and nine II2O ones in periodic order were used. Water has high moderation power to be used as neutron moderator. Although, it contains only 6.7x1022 hydrogen atoms/cm3 which is 18% less than polyethylene, it is still excellent for neutron moderation.
Chapter 1 of this work introduces the neutron spectroscopy status and problems in any
fields of interest.
Chapter 2 is a brief overview of the neutron detection techniques with special
attention to the Bonner sphere spectroscopy technique.
Chapter 3 describes the idea of multilayered detection system with a study of water
thickness appropriate for such system.
Chapter 4 reports the energy response functions of the virus layers (response matrix),
which has been studied using MCNP 4C code.