A scintillation counting system has been constructed with the use of BC-400 and EJ-212 series plastic scintillators along with a subminiature photomultiplier tube to investigate the effect of increasing plastic scintillator thickness on system-integrated counts. Measurements have been carried out using four different gamma sources with different energies ranging from 6keV to 1.332MeV and a Ni-63 beta source of maximum energy of 66keV. A simulation was also carried out in MCNP4a to verify the number of H-3 beta particles with max energy 18.6keV that would reach the plastic scintillator in a vacuum setting as well as in an air medium. Scintillator thicknesses ranged from 10μm to 2500μm. The response of the system was determined by measuring the integrated counts as a function of scintillator thickness. The results of these measurements showed the expected positive linear correlation between scintillator thicknesses and integrated counts for all the gamma sources while the slopes of the correlations of each gamma source was a function of the source energy. The beta particle response showed an initial increase of counts with scintillator thickness followed by a slight decrease. The MCNP simulation confirmed an analytical calculation of the fraction of H-3 beta particles for a given air concentration that would reach the scintillator. These results in conjunction with the experimental findings were used to assess the potential of a plastic scintillator system forming the basis of a tritium monitor for the detection of tritium in high-energy gamma backgrounds for Canadian nuclear power workers. / UOIT

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.

This thesis investigates the ability of transparent surface coatings to reduce xenon diffusion into plastic scintillators. The motivation for the work is improved radioxenon monitoring equipment, used with in the framework of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty. A large part of the equipment used in this context incorporates plastic scintillators which are in direct contact with the radioactive gas to be detected. One problem with such setup is that radioxenon diffuses into the plastic scintillator material during the measurement, resulting in an unwanted memory effect consisting of residual activity left in the detector. In this work coatings of Al2O3 and SiO2, with thicknesses between 20 and 400 nm have been deposited onto flat plastic scintillator samples, and tested with respect to their Xe diffusion barrier capabilities. All tested coatings were found to reduce the memory effect, and 425 nm of Al2O3 showed the most promise. This coating was deposited onto a complete detector. Compared to uncoated detectors, the coated one presented a memory effect reduction of a factor of 1000. Simulations and measurements of the expected light collection efficiency of a coated detector were also performed, since it is important that this property is not degraded by the coating. It was shown that a smooth coating, with a similar refractive index as the one of the plastic, should not significantly affect the light collection and resolution. The resolution of the complete coated detector was also measured, showing a resolution comparable to uncoated detectors. The work conducted in this thesis proved that this coating approach is a viable solution to the memory effect problem, given that the results are reproducible, and that the quality of the coating is maintained over time.

In this report the neutron-gamma discrimination capabilities of the new type of solid organic scintillator, EJ-299-33, was investigated using several pulse-shape discrimination (PSD) techniques. Among others, the analog zero-crossing method andthe digital charge-comparison and integrated-rise-time method were tested. The parameters of the digital PSD methods were optimised individually and the figure-of-merit was measured for each method and compared in different energy windows. The photoelectron yield of the setup was measured using two different photomultiplier tubes (PMT), a 3 inch diameter ET 9821 and a 5 inch diameter ET 9390KB. The highest photoelectron yield was measured with the ET 9390KB, which was the PMT used for the neutron-gamma discrimination capability measurements. In this work, four decay constants were found for the scintillator decay times. These were found by fitting average neutron and gamma-ray waveforms with the convolution of severeal exponential functions, that describes the light emission intensity of the scintillator, with an approximation of the PMT response function. Thebest agreement was found for the assumption that the scintillator light emission intensity is governed by four decay constants. The intensity of the two slowest components contain information about the incident particle.

The neutrino is, by its nature, an elusive particle that requires massive detectors with small backgrounds to capture a handful of events. Nevertheless, neutrino experiments stand at the heart of the current mysteries of particle physics and astrophysics. These include the origin and size of neutrino mass, the existence of additional types of neutrinos, CP violation and the matter--antimatter asymmetry, the amount of metals in the Sun's core, and the existence of non-nuclear energy sources in the Sun. This dissertation concerns the the use of a novel detector technology, the Raghavan Optical Lattice (ROL), in the Low-Energy Neutrino Spectrometer (LENS) and Neutrino Lattice (NuLat) experiments. LENS will measure the solar neutrino luminosity and the Sun's core metallicity using a ROL with indium-loaded liquid scintillator. NuLat will probe the existence of light sterile neutrinos with masses of $ \sim 1\,\mathrm{eV} $ using a ROL made from $ ^{6}\mathrm{Li} $-loaded plastic scintillator. For LENS we present an overview of the experiment and the present the ROL construction results from the LENS R\andD program. In particular we will present results from the micro- and mini-LENS prototypes. For both LENS and NuLat we present the development of an event reconstruction algorithm for ROLs and we apply these to the expected signals for these experiments. For NuLat we present an overview of the experiment including its theory of operation and its sensitivity to sterile neutrino oscillations. Finally, we present work toward the full-sized NuLat detector through bench-top tests and construction of the NuLat demonstrator. / Ph. D.

Over the past 20 years, the detection of neutrino oscillation has reported a lot of important results. The oscillation phenomenon itself has been well proved by various experiments. Some oscillation parameters has been measured and now in the area of precise determination. On the other hand, some new questions like the possibility of the existence of light sterile neutrinos and unexpected 5 MeV bump were raised during the measurement. The Neutrino Lattice Experiment (NuLat) is a detector based on the Raghavan Optical Lattice (ROL). It should be able to offer a compact design of an effective detector with good mobility. It can be extremely useful in the short baseline reactor neutrino oscillation detection community to resolve several confusing issues. In this thesis, we present the calibration results we got from the first active NuLat detector and show what kind of improvements we need for the next version of the NuLat detector based on these results. / Ph. D. / During the last century, physicists have developed a nice framework to describe the physics world we live. The model which we called Standard Model has been constructed to describe the behavior of elementary particles and nicely explain the phenomenon we found from various experiments. However there are still a lot mysteries which cannot be explained by this model and more precise measurements on different fields of particle physics are need to help us improve our understanding about this. Neutrino oscillation is one of the most important field related to this kind of concern. The Neutrino Lattice Experiment (NuLat) is a new type of neutrino detector. It has a good geometry reconstruction ability based on the the Raghavan Optical Lattice (ROL). Since we cannot directly see the elementary particles, we always rely on the signals generated by the reaction between particles and our detector. How to interpret the signals becomes crucial at this point to have high quality experimental data. NuLat is such kind of neutrino detector which offer good ability for us to interpret the signal right. It has a compact design compared to most of other detectors in this field. This is really useful because it can be implemented with limited space where other detectors might have difficulties. Simultaneously the ROL design can help offer nice background rejection ability and high energy resolution. In this thesis, we discuss the progress about develop and assembly of the first active NuLat detector with the preliminary calibration data which give us basic understanding about the performance of this first version.

Astrophysical observations give convincing evidence for a vast non-baryonic component, the so-called dark matter, accounting for over 20% of the overall content of our Universe. Direct dark matter search experiments explore the possibility of interactions of these dark matter particles with ordinary baryonic matter via elastic scattering resulting in single nuclear recoils. The ZEPLIN-III detector operated on the basis of a dualphase (liquid/gas) xenon target, recording events in two separate response channels { scintillation and ionisation. These allow discrimination between electron recoils (from background radiation) and the signal expected from Weakly Interacting Massive Particle (WIMP) elastic scatters. Following a productive first exposure, the detector was upgraded with a new array of ultra-low background photomultiplier tubes, reducing the electron recoil background by over an order of magnitude. A second major upgrade to the detector was the incorporation of a tonne-scale active veto detector system, surrounding the WIMP target. Calibration and science data taken in coincidence with ZEPLIN-III showed rejection of up to 30% of the dominant electron recoil background and over 60% of neutron induced nuclear recoils. Data taking for the second science run finished in May 2011 with a total accrued raw fiducial exposure of 1,344 kg days. With this extensive data set, from over 300 days of run time, a limit on the spin-independent WIMP-nucleon cross-section of 4.8 10-8 pb near 50 GeV/c2 WIMP mass with 90% confidence was set. This result combined with the first science run of ZEPLIN-III excludes the scalar cross-section above 3.9 10-8 pb. Studying the background data taken by the veto detector allowed a calculation of the neutron yield induced by high energy cosmic-ray muons in lead of (5.8 0.2) 10-3 neutrons/muon/(g/cm2) for a mean muon energy of 260 GeV. Measurements of this kind are of great importance for large scale direct dark matter search experiments and future rare event searches in general. Finally, this work includes a comprehensive measurement of the energy dependent quenching factor for low energy nuclear recoils in a plastic scintillator, such as from the ZEPLIN-III veto detector, increasing accuracy for future simulation packages featuring large scale plastic scintillator detector systems.

This thesis investigates surface coatings as xenon diffusion barriers on plastic scintillators. The motivation for the work is improved radioxenon detection systems, used within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). One type of radioxenon detection systems used in this context is the Swedish SAUNA system. This system uses a cylindrical plastic scintillator cell to measure the beta decay from radioxenon isotopes. The detector cell also acts as a container for the xenon sample during the measurement. One problem with this setup is that part of the xenon sample diffuses into the plastic scintillator material during the measurement, resulting in residual activity left in the detector during subsequent measurements. This residual activity is here referred to as the memory effect. It is here proposed, and demonstrated, that it is possible to coat the plastic scintillator material with a transparent oxide coating, working as a xenon diffusion barrier. It is found that a 425 nm Al2O3 coating, deposited with Atomic Layer Deposition, reduces the memory effect by a factor of 1000, compared an uncoated detector. Furthermore, simulations show that the coating might also improve the light collection in the detector. Finally, the energy resolution of a coated detector is studied, and no degradation is observed. The focus of the thesis is measurements of the diffusion barrier properties of Al2O3 films of different thicknesses deposited on plastic scintillators, as well as an evaluation of the expected effect of a coating on the energy resolution of the detector. The latter is studied through light transport simulations. As a final step, a complete coated plastic scintillator cell is evaluated in terms of memory effect, efficiency and energy resolution. In addition, the xenon diffusion process in the plastic material is studied, and molecular dynamics simulations of the Xe-Al2O3 system are performed in order to investigate the reason for the need for a rather thick coating to significantly reduce the memory effect.

Two measurements have been made, addressing gaps in knowledge for 235U(n,f) and 238U(n,f). The energy distribution for prompt fission neutrons is not well-understood below 1 MeV in 235U(n,f). To measure the 235U(n,f) prompt fission neutron distribution, a pulsed neutron beam at the WNR facility in Los Alamos National Laboratory was directed onto a 235U target with neutron detectors placed 1 m from the target. These neutron detectors were designed specifically for this experiment and employed a unique geometry of scintillating plastic material that was designed to reject backgrounds. Fission fragments were detected using an avalanche counter. Coincidences between fission fragment production and neutron detector events were analyzed, using a double time-of-flight technique to determine the energy of the prompt fission neutrons. A separate measurement was made, investigating the neutron-induced fission cross section for 238U(n,f). This measurement also used the pulsed neutron beam at the WNR facility. The neutron flux was normalized to the well-known hydrogen standard and the fission rate was observed for beam neutrons in the energy range of 130-300 MeV. Using an extrapolation technique, the energy dependence of the cross section was determined. These new data filled a sparsely populated energy region that was not well-studied and were measured relative to the hydrogen standard, unlike the majority of available data. These data can be used to constrain the fission cross section, which is considered a nuclear reaction standard.

Cette thèse étudie la détection de matière nucléaire avec la technique de la particule associée pour l’inspection de bagages abandonnés ou de conteneurs maritimes dans le domaine de la sécurité. Le principe consiste à mesurer, avec des scintillateurs plastique, les coïncidences entre particules de fissions induites par des neutrons de 14 MeV produits par un générateur basé sur la réaction 2H(3H,n)4He et équipé d’un détecteur alpha à localisation pour déterminer le temps d’émission et la direction du neutron opposé. La détection d’au moins trois particules de fission en coïncidence avec la particule qui permet de discriminer les matières nucléaires des matériaux bénins. Le système d’acquisition et les outils de simulation ont été qualifiés en passif avec des sources radioactives puis en actif avec le générateur et diverses cibles, validant les estimations de performances de systèmesd’inspection de bagages abandonnés ou de conteneurs maritimes réalisées par simulation numérique avec le code MCNP-PoliMi. Il est ainsi possible de détecter en quelques minutes, quelques kg d’uranium au centre d’un container rempli d’une matrice fer mêmesi l’échantillon est masqué par du plomb, à l’aide du signal des neutrons prompts de fission. La détection est plus difficile dans les matrices organiques en raison de la diffusion des neutrons interrogateurs et de fission sur les noyaux d’hydrogène. Par ailleurs, l’utilisation de scintillateurs plastiques à la place des compteurs gazeux à 3He a été évaluée pour caractériser le plutonium dans les colis de déchets radioactifs par mesure passive des coïncidences. La détection des neutrons de fission est beaucoup plus rapide,ce qui permet de minimiser le bruit accidentel dû aux réactions (,n). Les scintillateurs sont cependant plus sensibles aux rayonnements gamma et à la diaphonie entre détecteurs voisins, ce qui nécessite d’exploiter les coïncidences de multiplicité 3 avec un traitement des données spécifique pour limiter la diaphonie. / This thesis investigates the detection of Special Nuclear Materials (SNM) by neutroninterrogation with the Associated Particle Technique (APT). 14 MeV neutrons areproduced from the 3H(2H,n)α fusion reaction in a sealed tube neutron generatorembedding a position-sensitive alpha detector. The alpha detector determines thedirection of the nearly opposite neutron and its time of flight. The detection of at leastthree prompt fission particles in coincidence with the tagged neutron signs the presenceof SNM. The acquisition system and simulation tools have been qualified in passive modewith radioactive sources and active mode with the generator and various targets,validating the simulation of inspection systems with MCNP-PoliMi. Calculations showthat the detection of a few kilograms of shielded SNM with the ATP is possible in ironcargo container, with the prompt fission neutrons signal. Detection is more difficult inorganic matrices due to tagged- and prompt fission neutrons scattering on hydrogennuclei. Furthermore, the use of plastic scintillators instead of 3He counters was studied tocharacterize the plutonium in the radioactive waste by passive coincidences measurement.Measurements at fast time scales of fast-neutrons instead of the long time scales ofthermal-neutrons reduce random coincidences that can occur with high (,n) reactionrate. The scintillators are however sensitive to gamma rays and cross-talk betweenadjacent detectors. Therefore, we used data-analysis algorithms to minimize cross-talkcontribution to measured three-fold coincidences.