This diploma thesis deals with electron microscopy. Examined equipment is environmental scanning electron microscope (ESEM), namely scintillation a detector of the microscope. There is solved the influence of the profile of holes in diaphragms to the resulting pressure and gas flow on the path of secondary electrons at the detector. Introductory part of thesis informs about microscopy in general, with concentration on electron microscopy; especially on scanning microscope, because research is taken just on the environmental scanning electron microscope. Another part informs about both general principles of dynamics of gases and with finite volume method. Another part deals with concrete used software and with setting of individual parameters for calculation. At the beginning of calculation are used five basic profiles of holes in diaphragms for pressure 1000 Pa in the chamber of the sample. For modelling individual shapes is used 3D parametric modeller SolidWorks. Analysis of circulation of secondary electrons through detector is made by using Cosmos FloWorks module. The most suitable type of diaphragms is chosen from measured models. Another part of diploma thesis deals with measuring of chosen types of diaphragms for more pressures in the chamber of the sample; the pressure is 200, 400, 600, 800 and 1000 Pa. The outcomes of this research are both models of pressure and speed of circulation inside the detector and graphically processed values by using different diaphragms, respectively one type of a diaphragm in different pressures. Production drawings of each diaphragm, together with calculated models, are enclosed.

This master´s thesis provides basic properties and measurement of optical beams. In the first chapter is shown division of light on ray, wave and beam optics. Atmospheric optics and properties associated with propagation of light through the earth's atmosphere is presented in the second chapter. In the third part are shown basic techniques for Gaussian beam shaping. The last chapter deals with measurement of optical beam after propagating through a turbulent medium. In this section is shown distribution of relative variance of optical intensity in Gaussian and Top-Hat beam. There is also measured spatial coherence of laser beam in the turbulent atmospheric transmission media. Finally effect of the beam wander is investigated.

The aim of this thesis is to study the issue of eniveromental scanning electron microscopy and pumping gas to create vacuum in the newly designed scintillation detector. Further, creating a model of recently proposed scintillation detector and simulating and analyzing pumping gas in differentially pumped chamber of detector. The theoretical part deals with electron microscopy, electron sources, electron optics and secondary electrons detectors. It is also presented which signals are generated by the electron beam on the surface of a solid. Further fluid flow issues and equations describing the flow in the solved chamber are dismantled. Furthermore, the impact of gaseous environment on the trajectory of primary electrons, because there are collisions of primary beam with atoms and molecules of gas. The following section discusses creating, quality and importance of the network in mathematical modelling. A method of a final volume used to calculate the differential equations describing the flow of gas at the premises of the detector is described . The practical part consists in creating a model of scintillation detector and analyzing the gas flow in drawing a vacuum in the newly designed scintillation detector. At the end the simulation results of gas flow are compared for different variants of apertures and various pressures on the neck of a scintillation detector designed for optimum performance of the detector. The outcome of this thesis is model of newly designed scintillation detector with optimized shapes of apertures according to functional requirements.

The aim of this thesis is to study the issue of eniveromental scanning electron microscopy and pumping gas to create a vacuum in the newly designed scintillation detector. Further, creating a model of recently proposed scintillation detector and simulating and analyzing pumping gas in differentially pumped chamber of detector and the results compare with the previous model. The theoretical part deals with electron microscopy, electron sources, electron optics and secondary electrons detectors. It is also presented which signals are generated by the electron beam on the surface of a solid. Further fluid flow issues and equations describing the flow in the solved chamber are dismantled. Furthermore, the impact of gaseous environment on the trajectory of primary electrons, because there are collisions of primary beam with atoms and molecules of gas. The following section discusses creating, quality and importance of the network in mathematical modelling. A method of a final volume used to calculate the differential equations describing the flow of gas at the premises of the microscope is described . The practical part consists in creating a model of scintillation detector and analyzing the gas flow in drawing a vacuum in the newly designed scintillation detector. Furthermore, the simulation results are compared with the results of simulations on the older type of scintillation detector. The output of this thesis is model of recently proposed scintillation detector with visualized simulation results.

The design of an optical communication link must plan for the random effects of atmospheric turbulence. This study analyses data from an experiment which transmitted from a laser located 8 meters above ground over a 13 Km range to coherent detection devices approximately 162 meters above ground. The effects of a fading and surging beam wave were considered in regards to code techniques for error correction, amplitude modulation and hardware architecture schemes. This study simulated the use of arrays and large apertures for the receiving devices, and compared the resultant scintillation index with the theoretical calculations.

An accurate PDF of irradiance for a FSO channel is important when designing a laser radar, active laser imaging, or a communications system to operate over the channel. Parameters such as detector threshold level, probability of detection, mean fade time, number of fades, BER, and SNR are derived from the PDF and determine the design constraints of the receiver, transmitter, and corresponding electronics. Current PDF models of irradiance, such as the Gamma-Gamma, do not fully capture the effect of aperture averaging; a reduction in scintillation as the diameter of the collecting optic is increased. The Gamma-Gamma PDF of irradiance is an attractive solution because the parameters of the distribution are derived strictly from atmospheric turbulence parameters; propagation path length, Cn2, l0, and L0. This dissertation describes a heuristic physics-based modeling technique to develop a new PDF of irradiance based upon the optical field. The goal of the new PDF is three-fold: capture the physics of the turbulent atmosphere, better describe aperture averaging effects, and relate parameters of the new model to measurable atmospheric parameters. The modeling decomposes the propagating electromagnetic field into a sum of independent random-amplitude spatial plane waves using an approximation to the Karhunen-Loeve expansion. The scattering effects of the turbulence along the propagation path define the random-amplitude of each component of the expansion. The resulting PDF of irradiance is a double finite sum containing a Bessel function. The newly developed PDF is a generalization of the Gamma-Gamma PDF, and reduces to such in the limit. An experiment was setup and performed to measure the PDF of irradiance for several receiver aperture sizes under moderate to strong turbulence conditions. The propagation path was instrumented with scintillometers and anemometers to characterize the turbulence conditions. The newly developed PDF model and the GG model were compared to histograms of the experimental data. The new PDF model was typically able to match the data as well or better than the GG model under conditions of moderate aperture averaging. The GG model fit the data better than the new PDF under conditions of significant aperture averaging. Due to a limiting scintillation index value of 3, the new PDF was not compared to the GG for point apertures under strong turbulence; a regime where the GG is known to fit data well.

La scintillation organique correspond au phénomène d’émission de lumière par un matériau moléculaire à la suite de l’excitation de celui-ci par un rayonnement externe d’énergie donnée. Lors de l’interaction, le dépôt d’énergie induit des transitions électroniques peuplant des états dont la plupart se désexcite de manière non radiative, à l’exception d’une, entre le premier état électronique singulet et l’état fondamental de la molécule. Lors de cette relaxation, un photon de fluorescence est émis. Cette émission a deux origines : i) l’excitation directe par le rayonnement primaire et les électrons secondaires ; elle donne lieu à une émission dite rapide ou prompte ; ii) l’ionisation par le rayonnement primaire et les électrons secondaires ; elle donne lieu à une émission dite lente ou différée. Ce travail de recherche fondamentale, à la fois théorique et expérimental, fait l’analyse de toutes les étapes du processus, de l’interaction primaire à l’émission de fluorescence, de manière à relier la dose déposée à la quantité de lumière émise, à des fins d’applications en dosimétrie médicale. Il repose sur la mesure des déclins de fluorescence de deux molécules modèles, l’anthracène et le paraterphényle, excitées par un flux continu de rayons X, et la séparation des contributions rapide et lente de la lumière émise, aux énergies médicales. Une modélisation analytique des processus physiques conduisant à l’émission de lumière, au regard de la dose déposée, a ensuite été effectuée, faisant apparaître de nombreux résultats originaux. Dans un premier temps, un dispositif expérimental original a été développé, basé sur la technique TCSPC (Time-Correlated Single Photon Counting), afin de pouvoir mesurer des déclins temporels de fluorescence en résolution nanoseconde et sous flux d’irradiation continu. Dans un second temps, nous avons développé une nouvelle approche mathématique permettant d’extraire finement les composantes rapides et lentes du signal. L’analyse des résultats a montré, pour la première fois, l’existence d’un rapport R constant et uniquement fonction du matériau, entre les rendements d’excitation et d’ionisation. Le caractère constant de ce rapport ne peut être attribué qu’à un mécanisme d’autoionisation moléculaire au sein d’un matériau se comportant intrinsèquement comme une chambre d’ionisation proportionnelle pour l’ionisation secondaire de basse énergie. Ceci est en accord total avec la linéarité observée entre l’intensité totale de lumière différée (ionisation) et la dose mesurée par une chambre d’ionisation proportionnelle. Une étude plus approfondie des mécanismes d’excitation, au regard du rapport R, a également permis de montrer, pour la première fois, une proportionnalité directe entre l’intensité totale de la lumière prompte et le dépôt d’une dose que nous avons baptisé dose d’excitation. Cette dose a été observée comme étant de 4 à 14 fois supérieure à celle mesurée par une chambre d’ionisation. Ce résultat original majeur devra impérativement conduire à des études futures afin de mieux comprendre les dégâts infligés à la matière organique et biologique par les excitations. / Organic scintillation is the emission of light by an organic scintillator when irradiated by an external source of radiation depositing enough energy to excite the molecule. Electronic states are populated by the electronic transitions generated by the deposited energy. The states de-excite through radiationless transitions, except for one, the transition between the first electronic state and the ground state where a photon of fluorescence is emitted. This light has two different origins: i) direct excitation caused by primary radiation or secondary electrons which leads to an emission knows as prompt; ii) ionization caused by primary radiation or secondary electrons generate what is known as the delayed component. This fundamental research was based on both theoretical and experimental work. We studied all the different processes in organic scintillation, from the interaction between the incident radiation and matter to the emission of light in order to find the relationship between fluorescence and the deposited dose, to the application to medical dosimetry. Two well known organic scintillators, anthracene and p-terphenyl, were excited using an X-ray source set at typical medical imaging parameters. The light emitted was acquired and an analytical model was used to describe the different processes that led to light emission revealing interesting new results.An experimental setup, based on the Time Correlated Single Photon Counting (TCSPC) technique, was developed to acquire fluorescence decay curves with nanosecond resolution using a continuous X-ray source. Afterwards, these curves were analyzed using an innovative mathematical approach in order to determine the prompt and delayed components.Results showed the ratio, defined as R, between the prompt and delayed components of fluorescence was constant and independent of the energy of the incident X-rays and that the response of the delayed component of fluorescence was linear to an ionization chamber. These observations were explained by considering that the only process taking place within the molecule after excitation was autoionization. Hence, the response of organic scintillator was the same as the one of an ionization chamber. Furthermore, due to the constant ration R, the response of prompt component of fluorescence was linear to the ionization chamber as well. This was the first time this behavior was observed and we referred to it as excitation dose. This dose was between 4 and 14 times bigger than the one measured with the ionization chamber. These original results suggested that energy is deposited mainly through excitation processes, suggesting the need for further studies to better understand the damage caused by excitation to the living.

The thesis deals with modifying and biulding of scintilation detector of secondary electrons for environmental scanning electron microscopy. It describes dilemma of environmental scanning electron microscopy, types of detectors and secondary electrons detection. The experimental part of this thesis focuses on the design and construction of new scintillation detector on the basis of simulations secondary electrons trajectories. Identifying the parameters, pressure dependencies and optimizations of electrode system of the detector realized.

Thesis deals with theoretical knowledge about scanning electron microscopy and environmental scanning electron microscopy. It describes principle of operation, signals generated by interaction between primary electron beam and specimen and means of detection of secondary electron signal in environmental conditions using scintillation detector. Furthermore, thesis focuses on optimization of detection od secondary electrons by adjusting electrode system of scintillation detector. Computer program Simion is used for modelling signal electron trajectories for proper adjustments. Simulation were starting-point for adjusting the design of the detector. Detection efficiency of adjusted detector was determined by evaluating signal magnitude from captured images, secondary electron detection capability from voltage contrast and quality of the captured images from signal/noise ratio.

The aim of the research study was to evaluate the levels of 14C and 3H radionuclides in Hartbeespoort Dam water and to determine if these radionuclides are within regulatory concerns. Water samples from Hartbeespoort Dam were prepared using the Sample Oxidiser Method and measurements of selected radionuclides were done using Liquid Scintillation Counter Quantulus 1220. The results evaluated suggest that water from Hartbeespoort Dam contains levels of 14C and 3H radionuclides that are within regulatory limits. The highest average concentration for 14C measured was 3.77E+01 (+/-2.47E-01) Bq/L, whereas the highest average concentration measured for 3H was 2.74E+01 (+/- 2.30E-01) Bq/L. The observations made regarding the impacts of climate on the 14C radionuclide were that, the concentration levels were higher during winter season when there was a rain than during rainy seasons. Tritium results showed that the climate conditions did not have any significant impacts on the concentration levels. When the concentrations of these radionuclides are above regulatory levels (14C is 100 Bq/L and 3H is10000 Bq/L), their impacts may cause harm to public`s health and the environment. Therefore, Necsa as a nuclear facility owner and National Nuclear Regulator (NNR) as a regulator are responsible for ensuring the public protection from radioactive effluents that contain not just 3H and 14C, but any radionuclide which may cause harm to public`s health. / Environmental Sciences / M. Sc. (Environmental Science)