Environmental gamma-ray spectroscopy provides a powerful tool that can be used in environmental monitoring given that it offers a compromise between measurement time and accuracy allowing for large areas to be surveyed quickly and relatively inexpensively. Depending on monitoring objectives, spectral information can then be analysed in real-time or post survey to characterise contamination and identify potential anomalies. Smaller volume detectors are of particular worth to environmental surveys as they can be operated in the most demanding environments. However, difficulties are encountered in the selection of an appropriate detector that is robust enough for environmental surveying yet still provides a high quality signal. Furthermore, shortcomings remain with methods employed for robust spectral processing since a number of complexities need to be overcome including: the non-linearity in detector response with source burial depth, large counting uncertainties, accounting for the heterogeneity in the natural background and unreliable methods for detector calibration. This thesis aimed to investigate the application of machine learning algorithms to environmental gamma-ray spectroscopy data to identify changes in spectral shape within large Monte Carlo calibration libraries to estimate source characteristics for unseen field results. Additionally, a 71 × 71 mm lanthanum bromide detector was tested alongside a conventional 71 × 71 mm sodium iodide to assess whether its higher energy efficiency and resolution could make it more reliable in handheld surveys. The research presented in this thesis demonstrates that machine learning algorithms could be successfully applied to noisy spectra to produce valuable source estimates. Of note, were the novel characterisation estimates made on borehole and handheld detector measurements taken from land historically contaminated with 226Ra. Through a novel combination of noise suppression and neural networks the burial depth, activity and source extent of contamination was estimated and mapped. Furthermore, it was demonstrated that Machine Learning techniques could be operated in real-time to identify hazardous 226Ra containing hot particles with much greater confidence than current deterministic approaches such as the gross counting algorithm. It was concluded that remediation of 226Ra contaminated legacy sites could be greatly improved using the methods described in this thesis. Finally, Neural Networks were also applied to estimate the activity distribution of 137Cs, derived from the nuclear industry, in an estuarine environment. Findings demonstrated the method to be theoretically sound, but practically inconclusive, given that much of the contamination at the site was buried beyond the detection limits of the method. It was generally concluded that the noise posed by intrinsic counts in the 71 × 71 mm lanthanum bromide was too substantial to make any significant improvements over a comparable sodium iodide in contamination characterisation using 1 second counts.

Des calculs théoriques utilisant la méthode de champ moyen ont suggérée l'existence des formes du noyau avec la symétrie tétraédrique et/ou octraédrique dans la région des terres rares au voisinage des noyaux 156Gd et 160Yb. Dans les noyaux avec une symétrie tétraédrique pure, des transitions intra-bande E2 à bas spin dans des bandes de parité négative disparaissent ou sont très faibles. Ce travail est dédié à une recherche expérimentale de la symétrie tétraédrique dans le noyau 156Gd. Une expérience a été faite à Jyväskylä avec le multi-détecteur JUROGAM, sur la réaction de fusion - évaporation 154Sm(, 2n)156Gd. L'analyse des données, a établi les rapports d'embranchement de deux bandes de parité négative et a permis, par distribution angulaire, de connaître la nature d'une nouvelle transition. Les rapports d'embranchement obtenus sont comparables avec ceux des expériences précédentes et quelques limites supérieures ont été déterminées. L'absence de transitions à bas spin (I$^{pi}<9$) dans la bande de parité négative à spin impair a été confirmée. Ces résultats ont renforcé l'hypothèse d'une symétrie tétraédrique dans le noyau 156Gd. La spectroscopie gamma est l'outil majeure utilisé dans ce travail. Les principes ainsi qu'une étude de simulation réaliste sont détaillés dans ce manuscrit. La simulation avec des événements réalistes a été faite pour comparer la fonction de réponse de deux types de multidétecteurs EUROBALL et AGATA. Les résultats montrent que sous certain conditions la phase démonstrateur d'AGATA peut être utilisée pour la recherche d'événements rares. / Theoretical calculations using realistic mean-eld method suggest the existence of nuclear shapes with tetrahedral and/or octahedral symmetries in some rare earth nuclei around 156Gd and 160Yb. In nuclei with exact tetrahedral symmetry, the E2 intra-band transitions are predicted to vanish completely or to be very weak. This work is devoted to an experimental research of the tetrahedral symmetry in the nucleus 156Gd. An experiment has been carried out with the JUROGAM - rays detector at Jyväskylä by using the fusion - evaporation reaction 154Sm(, 2n)156Gd. In analysis, the branching ratios of two parity negative bands were determined, the angular distribution has been analyzed to determine the nature of one inter-band transition between these two bands. The transitions strength ratios determined are of the same order of magnitude as previous experiments, and some upper limits have been established for the lowest spin. The missing of the E2 transitions below the (I$^{pi}<9$)states in the odd spin parity negative band were conrmed. The results of this work reinforced the hypothesis of the tetrahedral symmetry in the nucleus 156Gd. -ray spectroscopy is the major tool used for this work. Detection principles and -ray simulations are detailed in the document. The simulation, based on realistic events, has been realised to compare the response function of two multidetectors EUROBALL and AGATA. The results show that under certain conditions the rst phase of AGATA (demonstrator) is also a tool to search for rare events

La structure nucléaire en couches évolue en allant vers des régions de plus en plus exotiques de la carte des noyaux, et par conséquent, les nombres magiques conventionnels (8, 20, 28, 50, 82, 126) peuvent disparaître loin de la stabilité, tandis que de nouveaux nombres magiques peuvent apparaître. Le noyau de ⁷⁸Ni, avec 28 protons et 50 neutrons, est un des noyaux supposés doublement magiques les plus exotiques et est donc d'un grand intérêt. L'évolution de la fermeture de couche à Z = 28 en allant vers N = 50 peut être étudiée en sondant le caractère de particule individuelle des niveaux dans la chaîne isotopique de cuivre, ayant un proton de plus que le nickel. Ce travail porte sur le ⁷⁹Cu, à N = 50. Afin d'effectuer la première spectroscopie gamma en ligne des noyaux autour du ⁷⁸Ni, une expérience a été réalisée à la Radioactive Ion Beam Factory du RIKEN, au Japon. Le noyau de ⁷⁹Cu était produit par la réaction de knockout (p,2p) à partir d'un faisceau de ⁸⁰Zn envoyé sur le dispositif MINOS, une cible d'hydrogène liquide couplée à une TPC servant à reconstruire la trajectoire des protons. L'émission de rayons gamma subséquente était détectée en vol par le scintillateur segmenté DALI2. Les spectromètres BigRIPS et ZeroDegree permettaient, respectivement, une identification sans ambiguïté des noyaux entrants et sortants.Une procédure d'analyse basée sur des coïncidences gamma-gamma a permis de construire le premier schéma de niveau du ⁷⁹Cu, avec des états jusqu'à 4.6 MeV, et les résultats ont été comparés à des calculs de modèle en couches Monte Carlo. Les conclusions montrent que le noyau de ⁷⁹Cu est bien décrit en termes d'un proton de valence en dehors d'un cœur fermé de ⁷⁸Ni, ce qui implique le caractère magique de ce dernier. / The nuclear shell structure is evolving when going into more and more exotic regions of the chart of isotopes and consequently, the conventional magic numbers (8, 20, 28, 50, 82, 126) may disappear far from stability, while some new magic numbers can appear. The ⁷⁸Ni nucleus, with its 28 protons and 50 neutrons, is one of the most exotic supposedly doubly-magic nuclei, making it of great interest. The evolution of the Z = 28 gap towards N = 50 can be studied by probing the single-particle character of the states in the copper isotopic chain, having one proton more than nickel. This work focuses on Cu, at N = 50.In the aim of performing the first in-beam gamma-ray spectroscopy of nuclei in the close vicinity of ⁷⁸Ni, an experiment was carried out at the Radioactive Isotope Beam Factory of RIKEN, in Japan. The ⁷⁹Cu nucleus was produced through the (p,2p) knockout reaction from a ⁸⁰Zn beam sent on the MINOS device, a liquid-hydrogen target coupled to a TPC used for proton tracking. The subsequent gamma-decay was detected in-beam with the DALI2 scintillator array. The BigRIPS and ZeroDegree spectrometers allowed an unambiguous identification of the incoming and outgoing nuclei, respectively.An analysis procedure based on gamma-gamma coincidences permitted to build the first level scheme of ⁷⁹Cu, with levels up to 4.6 MeV, and the results were compared to Monte-Carlo shell-model calculations for interpretation. The conclusions show that the ⁷⁹Cu nucleus is well described in terms of a valence proton outside a closed ⁷⁸Ni core, implying the magic character of the latter.

Um detetor HPGe foi calibrado na faixa de energia 4 a 10 MeV usando fótons provenientes do decaimento do produto da reação 27Al(p, g)28Si em linha, observando as ressonâncias com prótons de energias 992 e 1317 keV. Foram feitas calibrações de energia e eficiência de deteção de fótons e foi analisada a dependência da largura a meia altura dos picos com a meia vida do estado inicial e a energia da transição. Os efeitos do produto da reação estar em movimento, como o efeito Doppler no decaimento em vôo, perda de velocidade do núcleo ao recuar dentro do alvo e energia de recuo são importantes na calibração de energia e análise da largura do pico. A calibração da eficiência depende de outras calibrações: razão entre a área dos picos de absorção total de energia, escapes simples e duplo dos gamas de aniquilação; razão entre a área total do espectro pela área do pico. Os resultados experimentais foram comparados com simulações do espectro usando o código MCNP, sendo que as dimensões internas do detetor, inacessíveis para medição, foram escolhidas de modo a reproduzir parâmetros deduzidos de espectros gama experimentais. A simulação se mostrou capaz de descrever tanto a eficiência para absorção total de energia quanto a razão entre as áreas dos picos de absorção total de energia, escape simples e duplo do gama de aniquilação, mas não foi capaz de descrever a razão entre a área total do espectro e a do pico. / An HPGe detector was calibrated in the 4 to 10 MeV energy range using photons from the decay of the 992 and 1317 keV proton resonances in the reaction 27Al(p, g)28Si on-line. Energy and photon detection efficiencies were calibrated and the dependence of the peak full width at half maximum on the transition energy and initial level half-life was analyzed. The effects of the reaction product motion, like Doppler shift due to in-flight decay, slowing down in the target, and nuclear recoil energy from photon emission, are important for energy calibration and peak width analyses. The efficiency calibration required other calibrations: the full energy peak to single and double escape counting ratios, and total spectrum to peak areas ratio. The experimental results were compared to simulations with MCNP code, where internal detector dimensions, inaccessible to measurement, were chosen in order to fit the experimental gamma spectrum parameters. The simulation was able to explain the full energy peak to single escape and double escape counting ratios, and full-energy peak detection efficiency, but was unable to fit the total spectrum to peak areas ratio.

Um detetor HPGe foi calibrado na faixa de energia 4 a 10 MeV usando fótons provenientes do decaimento do produto da reação 27Al(p, g)28Si em linha, observando as ressonâncias com prótons de energias 992 e 1317 keV. Foram feitas calibrações de energia e eficiência de deteção de fótons e foi analisada a dependência da largura a meia altura dos picos com a meia vida do estado inicial e a energia da transição. Os efeitos do produto da reação estar em movimento, como o efeito Doppler no decaimento em vôo, perda de velocidade do núcleo ao recuar dentro do alvo e energia de recuo são importantes na calibração de energia e análise da largura do pico. A calibração da eficiência depende de outras calibrações: razão entre a área dos picos de absorção total de energia, escapes simples e duplo dos gamas de aniquilação; razão entre a área total do espectro pela área do pico. Os resultados experimentais foram comparados com simulações do espectro usando o código MCNP, sendo que as dimensões internas do detetor, inacessíveis para medição, foram escolhidas de modo a reproduzir parâmetros deduzidos de espectros gama experimentais. A simulação se mostrou capaz de descrever tanto a eficiência para absorção total de energia quanto a razão entre as áreas dos picos de absorção total de energia, escape simples e duplo do gama de aniquilação, mas não foi capaz de descrever a razão entre a área total do espectro e a do pico. / An HPGe detector was calibrated in the 4 to 10 MeV energy range using photons from the decay of the 992 and 1317 keV proton resonances in the reaction 27Al(p, g)28Si on-line. Energy and photon detection efficiencies were calibrated and the dependence of the peak full width at half maximum on the transition energy and initial level half-life was analyzed. The effects of the reaction product motion, like Doppler shift due to in-flight decay, slowing down in the target, and nuclear recoil energy from photon emission, are important for energy calibration and peak width analyses. The efficiency calibration required other calibrations: the full energy peak to single and double escape counting ratios, and total spectrum to peak areas ratio. The experimental results were compared to simulations with MCNP code, where internal detector dimensions, inaccessible to measurement, were chosen in order to fit the experimental gamma spectrum parameters. The simulation was able to explain the full energy peak to single escape and double escape counting ratios, and full-energy peak detection efficiency, but was unable to fit the total spectrum to peak areas ratio.

<p>Presently there is a clear trend of increasing demands on in-pile performance of nuclear fuel. Higher target burnups, part length rods and various fuel additives are some examples of this trend. Together with an increasing demand from the public for even safer nuclear power utilisation, this implies an increased focus on various experimental, preferably non-destructive, methods to characterise the fuel.</p><p>This thesis focuses on the development and experimental evaluation of such methods. In its first part, the thesis presents a method based on gamma-ray spectroscopy with germanium detectors that have been used at various power reactors in Europe. The aim with these measurements is to provide information about the thermal power distribution within fuel assemblies in order to validate core physics production codes. The early closure of the Barsebäck 1 BWR offered a unique opportunity to perform such validations before complete depletion of burnable absorbers in Gd-rods had taken place. To facilitate the measurements, a completely submersible measuring system, LOKET, was developed allowing for convenient in-pool measurements to be performed.</p><p>In its second part, the thesis describes methods that utilise in-pile measurements. These methods have been used in the Halden test-reactor for determination of fission gas release, pellet-cladding interaction studies and fuel development studies.</p><p>Apart from the power measurements, the LOKET device has been used for fission gas release (FGR) measurements on single fuel rods. The significant reduction in fission gas release in the modern fuel designs, in comparison with older designs, has been demonstrated in a series of experiments. A FGR database covering a wide range of burnup, power histories and fuel designs has been compiled and used for fuel performance analysis. The fission gas release has been measured on fuel rods with average burnups well above 60 MWd/kgU. The comparison between core physics calculations (PHOENIX-4/POLCA-7) and the in-pool measurements of thermal power indicates that the nodal power can generally be predicted with an accuracy within 4% and the bundle power with an accuracy better than 2%, expressed as rms errors.</p><p>In-pile experiments have successfully simulated the conditions that occur in a fuel rod following a primary debris failure, being secondary fuel degradation. It was concluded that massive hydrogen pick-up takes place during the first few days following the primary failure and that a pre-oxidized layer does not function as a barrier towards hydriding in an environment with a very high partial pressure of hydrogen. Another series of in-pile experiments clearly indicate that increased UO₂ grain size is an effective way of suppressing fission gas release in LWR fuel up to the burnup level covered (55 MWd/kgUO₂).</p>

This study is aimed at investigating the feasibility of using the radiometric technique as an alternative to traditional methods for determining soil physico-chemical parameters which are important for terroir characterization. In-situ and ex-situ radiometric analyses of soil from three vineyard blocks of Simonsig Wine Estate in the Stellenbosch district (Western Cape, South Africa) were studied. A mobile MEDUSA gamma-ray detection system comprising a CsI(Na) crystal (length 15 cm, diameter 7 cm) and associated electronics mounted on a portable trolley were used for partial terroir characterisation. Thereafter activity concentrations of 40K, 232Th series and 238U series in soil (top ~30 cm) from the measured MEDUSA spectra (0 – 3 MeV) were extracted by means of the full-spectrum analysis (FSA) method. A lead-shielded HPGe detector was used for analyzing collected soil samples while soil physico-chemical parameters were analysed using standard methods at research and commercial laboratories.

Presently there is a clear trend of increasing demands on in-pile performance of nuclear fuel. Higher target burnups, part length rods and various fuel additives are some examples of this trend. Together with an increasing demand from the public for even safer nuclear power utilisation, this implies an increased focus on various experimental, preferably non-destructive, methods to characterise the fuel. This thesis focuses on the development and experimental evaluation of such methods. In its first part, the thesis presents a method based on gamma-ray spectroscopy with germanium detectors that have been used at various power reactors in Europe. The aim with these measurements is to provide information about the thermal power distribution within fuel assemblies in order to validate core physics production codes. The early closure of the Barsebäck 1 BWR offered a unique opportunity to perform such validations before complete depletion of burnable absorbers in Gd-rods had taken place. To facilitate the measurements, a completely submersible measuring system, LOKET, was developed allowing for convenient in-pool measurements to be performed. In its second part, the thesis describes methods that utilise in-pile measurements. These methods have been used in the Halden test-reactor for determination of fission gas release, pellet-cladding interaction studies and fuel development studies. Apart from the power measurements, the LOKET device has been used for fission gas release (FGR) measurements on single fuel rods. The significant reduction in fission gas release in the modern fuel designs, in comparison with older designs, has been demonstrated in a series of experiments. A FGR database covering a wide range of burnup, power histories and fuel designs has been compiled and used for fuel performance analysis. The fission gas release has been measured on fuel rods with average burnups well above 60 MWd/kgU. The comparison between core physics calculations (PHOENIX-4/POLCA-7) and the in-pool measurements of thermal power indicates that the nodal power can generally be predicted with an accuracy within 4% and the bundle power with an accuracy better than 2%, expressed as rms errors. In-pile experiments have successfully simulated the conditions that occur in a fuel rod following a primary debris failure, being secondary fuel degradation. It was concluded that massive hydrogen pick-up takes place during the first few days following the primary failure and that a pre-oxidized layer does not function as a barrier towards hydriding in an environment with a very high partial pressure of hydrogen. Another series of in-pile experiments clearly indicate that increased UO2 grain size is an effective way of suppressing fission gas release in LWR fuel up to the burnup level covered (55 MWd/kgUO2).

The nucleus is a mesoscopic system that retains features from both the quantum and macroscopic worlds. A basic property of a macroscopic body is its shape. Nuclear shapes can be deduced from experimental data as they influence the excitation mode of the nucleus and hence the energies and lifetimes of its excited levels. Various short-lived nuclei were created in fusion-evaporation experiments performed at international heavy-ion accelerator facilities. The emitted γ rays and, in some experiments, also the charged particles and neutrons emitted in the reactions were detected. The studied neutron-deficient isotopes were either selected by the type and number of particles emitted in the reactions, or by using their characteristic decays. The excited states of the different isotopes were extracted from the γ-ray analyses. Spectroscopic properties, such as the lifetimes of the excited states or the angular distribution of the emitted γ rays were measured when possible. The experimentally obtained level schemes together with the other spectroscopic information were used to deduce the excitation modes and the shapes of the studied nuclei. The detector systems are described in the first chapter and in the second chapter some techniques used to extract information from the experimental data are explained. Finally, a brief theoretical overview on the nuclear models which were used to interpret the experimental results is given. / QC 20100621

This study is aimed at investigating the feasibility of using the radiometric technique as an alternative to traditional methods for determining soil physico-chemical parameters which are important for terroir characterization. In-situ and ex-situ radiometric analyses of soil from three vineyard blocks of Simonsig Wine Estate in the Stellenbosch district (Western Cape, South Africa) were studied. A mobile MEDUSA gamma-ray detection system comprising a CsI(Na) crystal (length 15 cm, diameter 7 cm) and associated electronics mounted on a portable trolley were used for partial terroir characterisation. Thereafter activity concentrations of 40K, 232Th series and 238U series in soil (top ~30 cm) from the measured MEDUSA spectra (0 – 3 MeV) were extracted by means of the full-spectrum analysis (FSA) method. A lead-shielded HPGe detector was used for analyzing collected soil samples while soil physico-chemical parameters were analysed using standard methods at research and commercial laboratories.