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This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions. By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection. The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced. Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements. Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.

In this work novel solid-state neutron detectors based on Gallium Nitride (GaN) have been produced and characterized. GaN is a radiation hard semiconductor which is commonly used in commercial optoelectronic devices. The important design consideration for producing GaN-based neutron detectors have been examined, and device simulations performed. Scintillators and p-i-n diode-type neutron detectors have been grown by metalorganic chemical vapor deposition (MOCVD) and characterized. GaN was found to be intrinsically neutron sensitive through the Nitrogen-14 (n, p) reaction. Neutron conversion layers which produce secondary ionizing radiation were also produced and evaluated. GaN scintillator response was found to scale highly linearly with nuclear reactor power, indicating that GaN-based detectors are suitable for use in the nuclear power industry. This work is the first demonstration of using GaN for neutron detection. This is a novel application for a mature semiconductor material. The results presented here provide a proof-of-concept for solid-state GaN-based neutron detectors which offer many potential advantages over the current state-of-the-art, including lower cost, lower power operation, and mechanical robustness. At present Helium-3 proportional counters are the preferred technology for neutron detection, however this isotope is extremely rare, and there is a global shortage. Meanwhile demand for neutron detectors from the nuclear power, particle physics, and homeland security sectors requires development of novel neutron detectors which are which are functional, cost-effective, and deployable.

There are a variety of uses for reference neutron fields including detector response and dosimeter studies. The Georgia Institute of Technology has a 252Cf spontaneous fission source and an AmBe (α, n) source available for use in its research programs. In addition, it has iron, lead, beryllium, tantalum, heavy water, and polyethylene spheres to modify the neutron energy distributions from these neutron sources. This research characterized the neutron leakage spectra from the source inside spherical shells using a Bonner sphere spectrometer. All the neutron fields measured were also computed with a Monte Carlo code to determine the neutron fluence rate and ambient dose equivalent rate. The comparison of experimental data and calculations are used to provide further insight into the neutron spectra as modified by the spheres. The characterization of these modified sources will provide data to assist in using the resulting neutron fields in other research activities. To measure each neutron field combination, one of the two sources was placed in the center of an attenuating sphere. The neutron field was first measured at a variety of source-to-detector distances with a Bonner Sphere System. The spectrometer measurements, specifically the count rates of the different Bonner spheres, as a function of distance from the source is fitted to obtain corrections for room-scatter and air-scatter of neutrons using the Eisenhauer, Schwartz, and Johnson method. Using these corrections, the count rates free of room return is obtained at 1 m from the source and unfolded using the BUMS software to obtain the reported fluence and dose equivalent rates. These results are compared to those generated by the Monte Carlo Neutral Particle (MCNP) code. Models were made in MCNP for each of the source and moderating sphere combinations. The neutron fluence and dose rates were tallied during the MCNP simulation. The unfolded experimental data and the MCNP calculations showed good agreement for most of source-attenuating sphere combinations, thereby reinforcing the experimental results.

This thesis presents an independent analysis of the data from 3-He-filled proportional counters from the third phase of the Sudbury Neutrino Observatory (SNO) data. These counters were deployed in SNO's heavy water to independently detect neutrons produced by the neutral current interaction of 8-B solar neutrinos with deuterium. Previously published results from this phase were based on a spectral analysis of the energy deposited in the proportional counters. The work in this thesis introduces a new observable based on the time-profile of the ionization in the counters. The inclusion of this observable in a maximum-likelihood fit increases the potential to distinguish neutrons from backgrounds which are primarily due to alpha-decays. The combination of this new observable with the energy deposited in the counters results in a more accurate determination of the number of neutrons. The analysis presented in this thesis was limited to one third of the data from the proportional counters, uniformly distributed in time. This limitation was imposed to reconcile different time-lines between the submission of this thesis, a thorough review of this work by the SNO Collaboration and results from an independent analysis that is still underway. Analysis of this reduced data set determined that 398 +/- 29 (stat.) +/- 9 (sys.) neutrons were detected in this reduced data-set. The number compares well to the previous analysis of the data, based only on a spectral analysis of the deposited energy, which determined that 410 +/- 44 (stat.) +/- 9 (sys.) were detected in the same time period. The analysis presented here has led to a substantial increase in the statistical accuracy. Assuming that the statistical accuracy will increase when the full data set is analyzed, the results from this thesis would bring the uncertainty in the 8-B solar neutrino flux to down 6.8% from 8.5% in the previously published results. The work from the thesis is intended to be included in a future analysis of the SNO data and will result in a more accurate measurement of the total flux of solar neutrinos from 8-B as well as reduce the uncertainty in the $\theta_{12}$ neutrino oscillation mixing angle. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-16 15:56:28.195