Um dos grandes desafios em medidas de espectrometria gama utilizando fontes que não podem ser consideradas pontuais é a determinação da eficiência de detecção do sistema, uma vez que a geometria da fonte impõe complexidades na tarefa como o tratamento do ângulo sólido e da auto-absorção. Alguns métodos para o cálculo dessa eficiência já são bem estabelecidos e podem ser encontrados na literatura. Neste trabalho é proposto um método com características distintas aos já existentes cujas principais vantagens são, não depender de conhecimento prévio da geometria de detecção e da composição química da amostra a ser medida, e permitir o cálculo da eficiência de uma fonte extensa a partir de medidas com fontes padrão em formato pontual. O método proposto foi testado comparando os valores das eficiências obtidas por ele com os valores obtidos usando uma fonte padrão extensa. Para testar o método proposto em diferentes condições foi utilizado solo como matriz para a amostra em duas diferentes espessuras, 1cm e 3cm. Testou-se também o comportamento do método quando a matriz tem número atômico efetivo mais alto (aqui utilizou-se solo misturado à limalha de ferro) e mais baixo (adição de pó de grafite ao solo). Os resultados indicaram que o método é adequado para amostras de solo com espessura mais fina; já para fontes mais espessas, dois dos três resultados indicaram que a eficiência foi subestimada pelo método proposto, sugerindo que uma geometria mais adequada deva ser escolhida para a amostragem das medidas / One of the greatest challenges in gamma spectroscopy measurements using extense sources is the determination of the system\'s detection efficiency, since the source\'s geometry leads to added complexity to the treatment of parameters as geometry and self-absorption. Some well-established methods for the efficiency determination can be found in the literature. In the present work a method with distinct characteristics is proposed with the main advantages that it doesn\'t require previous knowledge of the detection geometry or of the sample composition; moreover, it allows the determination of the detection efficiency for a large sample based only on measurements using a point-like standard source. The proposed method was tested comparing the efficiency values obtained using the method with the values obtained using a real large source with the same composition and geometry. In order to verify the performance of the method under different conditions, soil matrixes of two different thicknesses (1cm and 3cm) were used; also, to asses the method under different sample compositions, 3cm-thick soil matrixes with increased effective atomic number (where iron filings was added) and with reduced effective atomic mass (where graphite powder was added) were used. The results indicate that the proposed method is suitable for thinner soil samples; as for thicker samples, two of the three results showed a subestimation of the detection efficiency value determined, suggesting that a more suitable sampling geometry should be analyzed.

Um dos grandes desafios em medidas de espectrometria gama utilizando fontes que não podem ser consideradas pontuais é a determinação da eficiência de detecção do sistema, uma vez que a geometria da fonte impõe complexidades na tarefa como o tratamento do ângulo sólido e da auto-absorção. Alguns métodos para o cálculo dessa eficiência já são bem estabelecidos e podem ser encontrados na literatura. Neste trabalho é proposto um método com características distintas aos já existentes cujas principais vantagens são, não depender de conhecimento prévio da geometria de detecção e da composição química da amostra a ser medida, e permitir o cálculo da eficiência de uma fonte extensa a partir de medidas com fontes padrão em formato pontual. O método proposto foi testado comparando os valores das eficiências obtidas por ele com os valores obtidos usando uma fonte padrão extensa. Para testar o método proposto em diferentes condições foi utilizado solo como matriz para a amostra em duas diferentes espessuras, 1cm e 3cm. Testou-se também o comportamento do método quando a matriz tem número atômico efetivo mais alto (aqui utilizou-se solo misturado à limalha de ferro) e mais baixo (adição de pó de grafite ao solo). Os resultados indicaram que o método é adequado para amostras de solo com espessura mais fina; já para fontes mais espessas, dois dos três resultados indicaram que a eficiência foi subestimada pelo método proposto, sugerindo que uma geometria mais adequada deva ser escolhida para a amostragem das medidas / One of the greatest challenges in gamma spectroscopy measurements using extense sources is the determination of the system\'s detection efficiency, since the source\'s geometry leads to added complexity to the treatment of parameters as geometry and self-absorption. Some well-established methods for the efficiency determination can be found in the literature. In the present work a method with distinct characteristics is proposed with the main advantages that it doesn\'t require previous knowledge of the detection geometry or of the sample composition; moreover, it allows the determination of the detection efficiency for a large sample based only on measurements using a point-like standard source. The proposed method was tested comparing the efficiency values obtained using the method with the values obtained using a real large source with the same composition and geometry. In order to verify the performance of the method under different conditions, soil matrixes of two different thicknesses (1cm and 3cm) were used; also, to asses the method under different sample compositions, 3cm-thick soil matrixes with increased effective atomic number (where iron filings was added) and with reduced effective atomic mass (where graphite powder was added) were used. The results indicate that the proposed method is suitable for thinner soil samples; as for thicker samples, two of the three results showed a subestimation of the detection efficiency value determined, suggesting that a more suitable sampling geometry should be analyzed.

<p>Dissertation in Environmental Physics to be publicly examined in Häggsalen (Ångström Laboratory), Uppsala University, on Friday, November 8, 2002 at 10:00 am for the degree of doctor of philosophy in Physics. The examination will be conducted in English. </p><p>Gamma spectrometry is one of the tools commonly used for the measurement of various environmental radionuclides. Simultaneous determination of the absolute activity of gamma emitting radiotracers in a wide range of environmental matrices and fractions necessitates proper and accurate evaluation of the sample-to-detector efficiency. Several radiotracers require, in addition, the use of sub-routines for self-absorption corrections. </p><p>Gamma spectrometry is an important and elegant tool for assessing environmental changes. Optimisation of ultra low-level gamma spectrometry for reliable assessment of such changes requires harmonisation of laboratory needs with sampling and site conditions.</p><p>Different aspects of the calculation of sample-to-detector efficiencies using empirical and Monte Carlo approaches are discussed here, including the uncertainties related to the simulation of the performance of different HPGe detectors and the effects of the incomplete collection of charges in Ge-crystals. Various simulation codes for the computation of peak efficiencies in planar and well Ge-detectors have been developed from scratch. The results of the simulations have been tested against experimental data and compared to other simulation results obtained with the Monte Carlo N-Particle code (MCNP). The construction of calibration sources with improved absorption and collimation characteristics have been, also, described in this work. These sources have been especially designed for the efficiency calibration of Ge-detectors at energies below 100 keV. </p><p>Flexible, fully tested and prototype approaches for the evaluation of self-absorption corrections, based on Monte Carlo simulations, are described. Special consideration is given to the problems related to the sample's variability in size, density and composition. Several examples of the absolute and simultaneous determination of environmental multitracers which benefited from self-absorption corrections and the optimised efficiency calibration algorithms are, also, presented and discussed. These examples include, among other things, a comprehensive analysis of the gamma spectrometry of ²³⁴Th in a wide range of matrices and the speciation of several radionuclides in sediments from a hard-water lake.</p>

Gamma-ray spectrometry measurements performed on site, or “in situ”, is a widely used and powerful method that can be employed both to identify and quantify ground deposited radionuclides. The purpose of this thesis is to improve the calibration of high purity germanium (HPGe) detectors for in situ measurements, and calculate the combined uncertainty and potential systematic effects. An improved semi-empirical calibration method is presented, based on a novel expression for the intrinsic detector efficiency that includes both the energy and angular response of the detector. A three-layer model for the description of the depth distribution of the radionuclide and the soil density is proposed. The combined uncertainty of intrinsic detector efficiency calibrations and in situ measurements according to the proposed method was estimated. The uncertainty in the intrinsic detector efficiency was found to be 5.1 and 8.1% (coverage factor k=1, i.e. for a confidence interval of about 68%), for the two detectors calibrated. These numbers were, however, at a later stage reduced to 3.7 and 4.2%, using a revised expression for the intrinsic detector efficiency. For in situ measurements, the combined standard uncertainty was found to be 15-20% (k=1), based on the original expression for the intrinsic detector efficiency. Monte Carlo models of the two detectors were created and Monte Carlo calculated values for intrinsic detector efficiency were compared with experimental data. As a discrepancy was found, a thorough investigation of the detector response was performed. Scanning of the detector surface with a collimated 59.5 keV photon beam revealed the detector response to be highly irregular over the detector surface. It was concluded that the efficiency deficit of the detector could most likely be attributed to an increase in dead layer thickness compared with manufacturer supplied data. The thickness of the dead layer was estimated to be 1.5-1.9 mm, whereas the nominal value was 0.7 mm. Radiographs of the detectors were produced that provided valuable information about the physical dimensions of the germanium crystal, as well as its actual location within the detector housing. The Monte Carlo models were employed to calculate in situ measurement efficiencies for measurements of 137Cs deposition from the Chernobyl fallout. Results from the Monte Carlo simulations were compared both with the semi-empirical method and with soil sample data, and satisfactory agreement was confirmed. It was then proceeded to employ the Monte Carlo model to calculate the effect on in situ measurement results by two influencing parameters: ground curvature and activity in trees. Neither of these parameters was found to influence the result by more than about 25%. This deviation is comparable with the measurement uncertainty, and should not deter from measurements in such terrain.

A boron neutron capture enhanced fast neutron therapy assembly has been designed for the Fermilab Neutron Therapy Facility (NTF). This assembly uses a tungsten filter and collimator near the patient¡¯s head, with a graphite reflector surrounding the head to significantly increase the dose due to boron neutron capture reactions. The assembly was designed using Monte Carlo radiation transport code MCNP version 5 for a standard 20x20 cm2 treatment beam. The calculated boron dose enhancement at 5.7-cm depth in a water-filled head phantom in the assembly with a 5x5 cm2 collimation was 21.9% per 100-ppm B-10 for a 5.0-cm tungsten filter and 29.8% for an 8.5-cm tungsten filter. The corresponding dose rate for the 5.0-cm and 8.5-cm thick filters were 0.221 and 0.127 Gy/min, respectively. To validate the design calculations, a simplified BNCEFNT assembly was built using four lead bricks to form a 5x5 cm2 collimator. Five 1.0-cm thick 20x20 cm2 tungsten plates were used to obtain different filter thicknesses and graphite bricks/blocks were used to form a reflector. Measurements of the dose enhancement of the simplified assembly in a water-filled head phantom were performed using a pair of tissue-equivalent ion chambers. One of the ion chambers is loaded with 1000-ppm natural boron (184-ppm 10B) to measure dose due to boron neutron capture. The measured dose enhancement at 5.0-cm depth in the head phantom for the 5.0-cm thick tungsten filter is (16.6 ¡À 1.8)%, which agrees well with the MCNP simulation of the simplified BNCEFNT assembly, (16.4¡À 0.5)%. The error in the calculated dose enhancement only considers the statistical uncertainties. The total dose rate measured at 5.0-cm depth using the non-borated ion chamber is (0.765 ¡À 0.076) Gy/MU, about 61% of the fast neutron standard dose rate (1.255Gy/MU) at 5.0-cm depth for the standard 10x10 cm2 treatment beam. The increased doses to other organs due to the use of the BNCEFNT assembly were calculated using MCNP5 and a MIRD phantom.

Dissertation in Environmental Physics to be publicly examined in Häggsalen (Ångström Laboratory), Uppsala University, on Friday, November 8, 2002 at 10:00 am for the degree of doctor of philosophy in Physics. The examination will be conducted in English. Gamma spectrometry is one of the tools commonly used for the measurement of various environmental radionuclides. Simultaneous determination of the absolute activity of gamma emitting radiotracers in a wide range of environmental matrices and fractions necessitates proper and accurate evaluation of the sample-to-detector efficiency. Several radiotracers require, in addition, the use of sub-routines for self-absorption corrections. Gamma spectrometry is an important and elegant tool for assessing environmental changes. Optimisation of ultra low-level gamma spectrometry for reliable assessment of such changes requires harmonisation of laboratory needs with sampling and site conditions. Different aspects of the calculation of sample-to-detector efficiencies using empirical and Monte Carlo approaches are discussed here, including the uncertainties related to the simulation of the performance of different HPGe detectors and the effects of the incomplete collection of charges in Ge-crystals. Various simulation codes for the computation of peak efficiencies in planar and well Ge-detectors have been developed from scratch. The results of the simulations have been tested against experimental data and compared to other simulation results obtained with the Monte Carlo N-Particle code (MCNP). The construction of calibration sources with improved absorption and collimation characteristics have been, also, described in this work. These sources have been especially designed for the efficiency calibration of Ge-detectors at energies below 100 keV. Flexible, fully tested and prototype approaches for the evaluation of self-absorption corrections, based on Monte Carlo simulations, are described. Special consideration is given to the problems related to the sample's variability in size, density and composition. Several examples of the absolute and simultaneous determination of environmental multitracers which benefited from self-absorption corrections and the optimised efficiency calibration algorithms are, also, presented and discussed. These examples include, among other things, a comprehensive analysis of the gamma spectrometry of 234Th in a wide range of matrices and the speciation of several radionuclides in sediments from a hard-water lake.

Cette thèse s’inscrit dans le domaine de la métrologie des rayonnements ionisants. Plus particulièrement dans la mesure par spectrométrie gamma des actinides contenus dans les colis et fûts de déchets. Le travail mené consiste à modéliser le coefficient d’étalonnage de la scène de mesure, élément indispensable à la quantification de l’activité (ou à la masse de radionucléides recherchée) de l’objet mesuré. La thèse comporte deux parties. La première partie traite de la modélisation de la réponse numérique spatiale et énergétique équivalente à la réponse réelle du détecteur, étape indispensable pour remonter à l’activité de l’objet. La seconde partie traite de la quantification du coefficient d’étalonnage de la scène de mesure sans hypothèse de l’opérateur. Le premier travail de thèse est la mise au point d’une méthodologie quasi automatisée d’obtention d’une réponse numérique équivalente à la réponse réelle du détecteur à un critère de convergence fixé. La réponse numérique est obtenue, sans expert, en conditions de terrain avec un critère de convergence inférieur à 5%. Le second travail est une étude de faisabilité sur la quantification de l’activité pour des colis complexes sans hypothèse de l’opérateur grâce à l’utilisation de métamodèles. Les métamodèles permettent de générer rapidement un ensemble de configurations du coefficient d’étalonnage par rapport aux données d’entrée. Les configurations sont ensuite triées pour sélectionner le coefficient d'étalonnage correspondant à la scène de mesure. / This thesis deals with radiation measurement. More particularly it concerns gamma ray spectroscopy for low level wastes. It consists in modeling the full efficiency calibration coefficient of the measured scene. It is essential to quantify the activity/mass of the measured object. This thesis is split in two parts. The first part consists in HPGE detector characterization. The HPGe characterization is available in space and energy range. The second part consists in determining the full efficiency calibration coefficient of the measured scene without operator hypothesis. First work is the development of an automated methodology to obtain detector characterization. HPGe detector characterization has similar performance to the real detector with a control of the discrepancy between them. HPGe detector characterization is achieved without expert, on field condition with a convergence criterion lower than 5%. Second work is a feasibility study for activity quantification of complex waste package without operator hypothesis. It will be possible by using metamodeling. Metamodeling generate quickly a set of configurations of the calibration coefficient with regard to input data. Configurations are sorted out according some criterions.