An experimental study of solid-gas interactions was made for sodium chloride particles, using a specially fabricated environmental chamber in a transmission electron microscope. It was found that under suitable conditions the hydration and dehydration of these particles could be directly observed and quantitatively measured. Measurements of growth were obtained with a time resolution of one-thirtieth of a second for particles having diameters ranging from 0.02 μm to 1.0 μm.

Thesis --(MSc. (Medical Physics)), 2009. / Introduction The primary role of breast cancer treatment with radiation is to deliver a sufficient radiation dose to the cancer cells without unduly causing biological damage to the healthy tissues. For over 50 years, electron beam therapy has been an important modality for providing an accurate dose of radiation to superficial cancers and disease and for limiting the dose to underlying normal tissues and structures in particular to boost the dose to the tumour bed and surgical scars after mastectomy. The Monte Carlo code MCNP5 was used to determine the effect of silicone gel breast prosthesis on the electron beam dose distribution. Materials and Method Percentage depth dose curves (PDD) for 6, 9, 12, and 15 MeV electron energies along the electron central axis depth dose distributions in a water phantom and with silicone prosthesis immersed in a water phantom were simulated using MCNP5. In order to establish the accuracy of the MCNP5 code, the depth dose curves obtained using MCNP5 were compared against the measured depth dose curves obtained from the Varian 2100C linear accelerator. The simulated depth dose curves with silicone prosthesis immersed in water were compared to the measured depth dose curves with the vi silicone prosthesis in water. The dose at the interface of the prosthesis with water was measured using thermoluminiscent dosimeters. Results The simulated and measured depth dose curve and the investigated dosimetric parameters are within 2%. Simulations in the presence of silicone showed a decrease in dose as the at the interface as the beam passes from the prosthesis to water for most energies however, for 15 MeV beam there is an increase in dose at the interface between the prosthesis and water and this was verified by physical measurements. Conclusion There were good correlations between the measured and MCNP simulated depth dose curve. Differences were in order of 2%. Small deviations occurred due to the fact that the simulations assumed a monoenergetic beam that exits the accelerator head, while in the measured results the beam exiting from the accelerator head includes scatted radiation from the collimators and the applicator. The presence of the prosthesis does not perturb the electron beam central axis depth dose curve however, the 15 MeV beam enhanced the dose in front of the interface between the prosthesis and water. Despite the limitations mentioned above MCNP5 results agree reasonably with the measured results. Hence, MCNP5 can be very useful in simulating electron percentage depth dose data.

Measurements of the electron-induced electron emission properties of insulators are important to many applications including spacecraft charging, scanning electron microscopy, electron sources, and particle detection technology. However, these measurements are difficult to make since insulators can charge either negatively or positively under charge particle bombardment that in turn alters insulator emissions. In addition, incident electron bombardment can modify the conductivity, internal charge distribution, surface potential, and material structure in ways that are not well understood. A primary goal of this dissertation work has been to make consistent and accurate measurements of the uncharged electron yields for insulator materials using innovative instrumentation and techniques. Furthermore, this dissertation reports on the experimental work undertaken by our group to explore insulator charging rates as a function of incident electron energy and fluence. Specifically, these charging studies include: (i) the study of the effectiveness of charge-neutralization techniques such as low-energy electron flooding and UV light irradiation to dissipate both positive and negative surface potentials induced by incident electron irradiation, (ii) the exploration of several noncontacting methods used to determine insulator surface potentials and the insulator first and second crossover energies that are important in determining both the polarity and magnitude of spacecraft material potentials, (iii) the dynamical evolution of electron emissions and sample displacement current as a function of incident charge fluence and energy with ties to evolving surface potentials as an insulator reaches its current steady state condition, and (iv) the slow evolution of electron yields with continuous incident electron bombardment. These charging data are explained in the context of available insulator charging models. Specific insulator materials tested included chromic acid anodized aluminum, RTVsilicone solar array adhesives, and KaptonTM on aluminum.

This thesis quantitatively investigates gaseous electron-ion recombination in an environmental scanning electron microscope (ESEM) at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector (GSED) images immediately after a region of enhanced secondary electron (SE) emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the GSED electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks is then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients, and (iii) electron drift velocities, as well as absolute values of the total time constant of the detection system, to be determined as a function of microscope operating parameters. Results reveal the exact dependence that the effects of SE-ion recombination on signal formation have on reduced electric field intensity and time in ESEM. Furthermore, the model implicitly demonstrates that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought. Following that the generation and detection of gaseous scintillation and electro- luminescence produced via electron-gas molecule excitation reactions in ESEM is investigated. Here a novel gaseous scintillation detection (GSD) system is developed to efficiently detect photons produced. Images acquired using GSD are compared to those obtained using conventional GSED detection, and demonstrate that images rich in SE contrast can be achieved using such systems. A theoretical model is developed that describes the generation of photon signals by cascading SEs, high energy backscattered electrons (BSEs) and primary beam electrons (PEs). Photon amplification, or the total number of photons produced per sample emissive electron, is then investigated, and compared to conventional electronic amplification, over a wide range of microscope operating parameters, imaging gases and photon collection geometries. The main findings of the investigation revealed that detected electroluminescent signals exhibit larger SE signal-to-background levels than that of conventional electronic signals detected via GSED. Also, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases photon collection efficiencies. The attainment of such an improvement being a direct consequence of increasing the `effective' solid angle for photon collection. Finally, in attempt to characterize the scintillating wavelengths arising from sample emissive SEs, PEs, BSEs, and their respective cascaded electrons, such that future photon filtering techniques can be employed to extract nominated GSD imaging signals, the emission spectra of commonly utilized electroluminescent gases in ESEM, such as argon (Ar) and nitrogen (N2), were collected and investigated. Spectra of Ar and N2 reveal several major emission lines that occur in the ultraviolet (UV) to near infrared (NIR) regions of the electromagnetic spectrum. The major photon emissions discovered in Ar are attributed to occur via atomic de-excitation transitions of neutral Ar (Ar I), whilst for N2, major emissions are attributed to be a consequence of second positive band vibrational de-excitation reactions. Major wavelength intensity versus gas pressure data, for both Ar and N2, illustrate that wavelength intensities increase with decreasing pressure. This phenomenon strongly suggesting that quenching effects and reductions in excitation mean free paths increase with imaging gas pressure.

Ab initio prediction of the electronic properties of solids is traditionally performed using groundstate Density Functional Theory. These methods are unreliable however, for a class of important problems involving weak attractive forces. These problems include (i) the energetics of hydrogen storage and metal interactions in graphene, (ii) cohesion properties of some polymer systems and (iii) possibly, the weak hydrophobic forces in biomolecules. For these cases a more powerful method than groundstate DFT are timedependent DFT (tdDFT) methods related to the Random-Phase Approximation (RPA). All of these methods proceed by looking at the dynamic density-density response function, whose long-ranged properties naturally lead to the weak forces referred to above. In this thesis we have tested these ideas by investigating electronic response and correlation on the predicted properties of spherical atoms. We have developed and tested a variety of approximations to the timedependent response function through approximations of the tdDFT class and a new method involving greater self-consistency in the screening equation, the inhomogenous STLS approach. Through the development of new methods and computer code, we have solved the response equation allowing us to test our approximations on atoms. Calculation of certain dynamic and static properties of a variety of atoms within our approximations generally agree well with known results. In this thesis we have calculated excitation energies of Helium, dipole polarisabilities and C6 van der Waals (vdW) coefficients of a variety of atoms, and groundstate correlation energies Ec of some atoms. The excitation spectra of Helium generated in our new PGG+c approximation are in good agreement with experiment. The dipole polarisabilities are generally in good agreement with known results, with the exception of Magnesium, Beryllium and Sodium. The C6 coefficients are a little poorer with the exception of Helium where they are nearly exact. Correlation energies are generally reasonable in the PGG+c approximation although they are considerably less accurate than the other properties we have calculated for all atoms other than He. The ISTLS correlation energy of Helium is within 5% suggesting that this method may perform well for larger atoms where our present numerical techniques require improvement. These generally positive results suggest that the approximations we have developed may be applied to more complicated systems such as those described above with good results.

This thesis describes an experimental study of elastic electron scattering from CF2 radicals, in the intermediate energy regime. Measurements of the absolute differential, integral and momentum transfer cross sections for CF2 are presented. These measurements were performed using a new crossed beam spectrometer, incorporating a supersonic gas source and normalised using a new technique, with both of these features being extensively developed as a major part of this study. The organisation of this thesis is as follows: A brief justification for this research is presented in Chapter 1, together with a review of the spectroscopy and electron collision cross sections which are currently available for the CF2 radical. The crossed beamed apparatus and experimental techniques used to perform the present cross section measurements are then described in detail in Chapter 2, and the theory behind the new normalisation technique is subsequently presented in Chapter 3. Results from the present study are given in Chapter 4. Firstly, differential cross sections measurements for stable molecules are presented, to validate the new normalisation method. Characterisation data for the dissociation dynamics of C2F4 into CF2 radicals are then presented and, finally, differential cross section measurements for the CF2 radical are explored. Where possible, the measured data for CF2 are compared against results from theoretical calculations and the implications of the present results are discussed. The major findings of this research are then summarised in Chapter 5, and directions for future research using the present apparatus are also discussed here. Finally, some additional findings from this research and calibration data for the current apparatus are given in the appendices.

Includes bibliographical references (8 p.) Examines some of the consequences of the acceleration and production, and propagation, of high energy electrons and positrons in the Galaxy. In particular, predictions are made of the diffuse photon signals arising from the interactions of electrons and positrons with gas, low energy photons, and the galactic magnetic field during their transport in the Galaxy.

Le travail de cette thèse a été mené au sein de la collaboration CMS (Compact Muon Solenoid), une des quatre experiences en cours d'installation au LHC (Large Hadron Collider) du CERN. La thèse se focalise sur le calorimètre électromagnétique de CMS: les performances du detecteur sont étudiées en détail en utilisant soit des données de tests réalisés sur faisceau d'électrons soit une simulation complète du détecteur. Une étude du bruit de fond électronique dans la chaîne de lecture a conduit à l'élaboration de deux méthodes de reconstruction de l'amplitude des signaux, l'une en cas de saturation de la chaîne et l'autre en cas de bruit de fond cohérent. La thèse presente aussi une étude des méthodes de reconstruction des électrons dans CMS, qui a conduit à la première définition dans CMS de la qualité de mesure d'un électron, à une procédure pour établir l'échelle d'énergie des électrons et à la combinaison des informations du calorimètre et du trajectomètre pour obtenir une estimation optimale de l'impulsion de l'électron au vertex. Ces resultats sont appliqués à l'analyse du potentiel de découverte de l'Higgs dans le canal de désintegration H->ZZ(*)->4e pour lequel une te! chnique standard de selections séquentielles ainsi qu'une technique optimale utilisant des reseaux de neurones sont proposées.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 218-225). Also available on the World Wide Web.

In this thesis, we have analyzed one-dimensional fermion-spin interactions in the dilute limit. The two cases we analyze represent different paradigms. For the first part, we look at the existence of spins for all sites as an effective model to describe the rearrangement of core electrons within the dynamic Hubbard model. Within this model, the behavior of electrons and holes will be compared in the presence of fermion-spin coupling and on-site repulsion. It will be shown that in this framework, electrons and holes behave differently and even though electrons experience increased repulsion, holes show attraction for a range of on-site repulsions. The characteristics of the interaction show effective nearest-neighbor attraction though no such term exists within the model. By the analysis of dynamic properties, two regions of interaction are identified. The gradual change from weak to strong coupling of fermions is presented. The effect of introducing on-site repulsion for both ranges of coupling is presented for both the dynamic Hubbard model and electron-hole symmetric version. For the second case involving fermion-spin interaction, we look at the interaction of a fermion with spins existing only for a small portion of the lattice, representing a coupled magnetic layer that an itinerant fermion interacts with through Heisenberg-like spin flip interaction. The interaction represents a spin-flip interaction of a spin current and magnetic layer. This interaction has been extensively studied for its relevance to computer hard drives both experimentally and theoretically. Most theoretical descriptions utilize the semi-classical Landau-Lifshitz-Gilbert (LLG) formalism. However, with recent improvements in experimental methods with very small magnetic layers and very fast real time measurements, quantum effects become more pronounced. We present quantum mechanical results that show considerable modification to spin-flip interaction. We identify a set of conditions that exhibits the existence of an emerging bound state for the spin current both numerically and analytically. The bound state is a quantum mechanical state and cannot be achieved with a classical picture. We present results in a one-dimensional lattice for a spin-1/2 system, and generalize our arguments to higher dimension and spins with S > 1/2.