No abstract available

[en] A self-consistent theory for the generation of relativistic electron beams in foil-less diodes is developed. Steady-state situations are considered where the electron motion is collisionless. Two-dimensional rectangular and cylindrical geometries are considered with relativistic diode potentials phi/sub c/. A strong applied magnetic field of magnitude is assumed to exist parallel to the axis of the diode. In the ultrarelativistic limit, the electron velocity is equal to c almost everywhere. The problem of finding the self-consistent space-charge-limited flow of electrons is reduced to a singular linear integral equation. The equation can be solved analytically for the potential and space-charge density in two-dimensional rectangular geometry. For cylindrical geometry, a formalism is developed for numerical solution of the integral equation. A treatment of the diodes is then given for small but non-zero values of epsilon = k (MeC²/e phi/sub c/)/sup 1/2/, where k is of the order of 1. For finite epsilon, there is a nonrelativistic sheath at the cathode surface of a diode. The electrons are accelerated from v = 0 to v = c in the sheath region. The thickness x of this sheath is of the order of epsilon a. A Langmuir-Child dependence holds between the potential and the distance from the cathode only in the region of thickness much less than x. The existence of anode sheaths of thicknesses approx. epsilon a is identified. The methods used in analyzing two-dimensional foil-less diodes with epsilon = 0 (ultrarelativistic limit) and epsilon is not equal to 0 are applied to double diodes producing relativistic ions as well as electrons. It is found that the presence of relativistic ions makes the ion current and the electron current nearly equal and that the current varies as the square of the diode potential

[en] Long-term modulation of galactic cosmic rays in interplanetary magnetic field (IMF) turbulence is a fundamental subject for understanding the connection between cosmic ray transport and solar activity. The discovery of a new cosmic ray modulation phenomenon is reported. Analysis of neutron monitor data has revealed that the difference in amplitude of the cosmic ray diurnal anisotropy for IMF sectors directed toward and away from the Sun displays a solar cycle variation. Neutron monitor data recorded at Climax, Deep River, Hyancayo, Kiel, Mt. Washington, and Swarthmore/Newark over the period 1957 to 1988 show that the amplitude difference varied between approximately 0.1 to -0.1 percent, with peaks in 1960, 1972, and 1982. A theoretical expression for this difference was derived from a three-dimensional model. Analysis also showed that the latitudinal density gradient of cosmic rays changed between 1.6 and -1.6 percent/AU with a solar cycle variation, which may produce the variation of the amplitude difference of the cosmic ray diurnal anisotropy. In order to investigate the underlying physical mechanism which produces the observed effect, it was necessary to develop a new analytical technique for calculating energy spectra of the IMF turbulence. The cosmic ray scattering mean free path was calculated from the measured spectrum, solar wind speed, and mean magnetic field by using the quasilinear theory in the slab model. This transport parameter displays a solar cycle variation and is proportional to the amplitude difference of the cosmic-ray diurnal anisotropy

[en] In this thesis, two-nucleon transfer reactions are treated by the finite-range full-recoil Distorted-Wave-Born-Approximation (DWBA) method. The formulation of the one-step (simultaneous) and two-step (successive) transfer processes is derived. Monte Carlo and Gaussian quadrature methods are used to evaluate first order and second order transition amplitudes respectively. Competitions between simultaneous and successive processes are studied. The reaction ₂₀₈Pb_gs(₁₆O_gs, ₁₈O_gs)₂₀₆Pb_gs at E_1ab = 69, 73, and 86 MeV is analyzed. It is shown that the two-step transfer process dominates over the one-step transfer

[en] The tight-binding Hubbard model has been applied to alkali metal clusters with Hartree-Fock self-consistent methods and perturbation methods for the numerical calculations. The relaxation has been studied between the equilibrium structures and the range of the hopping matrix elements in the Hubbard Hamiltonian. The structures are not sensitive to the interaction range but are determined by the number of valence electrons each atom has. Inertia tensors are used to analyze the symmetries of the clusters. The principal axes of the clusters are determined to be the axes of rotational symmetries of clusters if the clusters have any. The eigenvalues of inertia tensors are compared between this model and the ellipsoidal jellium model. The agreement is good for large clusters. At a finite temperature, the thermal motion fluctuates the structures. A fluctuation function has been defined with the distance matrix of a cluster. The fluctuation has been studied with the Monte Carlo simulation method. Energy levels of clusters are calculated from the Hubbard model. Ionization potentials and magic numbers are also obtained from these energy levels. The results confirm that the Hubbard model is a good approximation for a small cluster. The excitation energy is presented by the difference between the original level and excited level, and the electron-hole interactions. The author also studied cooling of clusters between the nozzle and detector in molecular beam apparatus with three cooling mechanisms, adiabatic expansion, photon emission and cluster decay. The adiabatic expansion and the photon emission do not affect mass abundances of clusters. The studies show that cluster decay cooling produces a shift in the mass spectra. The shift is dependent on the temperature of the beam and increases as the temperature increases

No abstract available

[en] The d-orbitals dominate the electronic structure around the transition metal (T) sites in solids. In this dissertation we use a variety of X-ray absorption spectroscopy techniques to investigate the local structure about T-sites in a wide range of materials. We present transition metal L₂ and L₃ XANES measurements on 4d transition elements and their compounds. We will see that there is a direct correlation between the open-quotes White Lineclose quotes (WL) strength at these edges and the number of holes in the 4d band. This correlation is then used to extract the bonding induced variation of the 4d hole count for T_1-xX_x compounds with T = Ag, Pd and X = Al, Ga, In, Si, and Sn. To summarize our experimental results, we will discuss a general scale which characterizes the efficiency with which a given X element induces 4d charge transfer from a given T-element when compounded. We will see that our results agree very well with the results of similar 5d series studies. These results emphasize the ability of T-L_2,3 XANES in probing the d charge transfer effects. Studies on a number of binary Pd-B, ternary Rh-B and RT₂X₂ compounds with R = Gd or Ce; T = Ru, Rh, Pd and Ag; and X = Si, Ge, and Sn will evidence the formation of resolved open-quotes anti-bondingclose quotes states above the Fermi level due to T(4d)-B or X hybridization. Finally the Pt-ligand and Pt-Pt interactions in a series of square Pt-compounds are investigated by EXAFS measurements. The number, type and distances of neighboring atoms surrounding Pt atoms are obtained by careful room temperature as well as low temperature Pt L₃ EXAFS. We will see that EXAFS can reveal the structural difference for isomeric compounds with same stoichiometry

[en] The differential cross section for γ-deuteron Compton scattering from a tensor polarized deuteron is computed in an effective field theory. The first non-vanishing contributions to this differential cross section are the interference terms between the leading electric coupling diagrams and the subleading single potential pion exchange diagrams or the subleading magnetic moment coupling diagrams. At 90 deg. photon scattering angle, only the pion term contributes at this order to the tensor polarized differential cross section. This provides a clean way to study the photon pion dynamics in the two nucleon sector. The effect is measurable for photon energies between 40 and 80 MeV provided the uncertainty in the measured cross sections are ≤ 7%

[en] In the large N_c(number of colors) limit, quenched QCD and QCD are identical. This implies that, in the effective field theory framework, some of the low energy constants in (N_c = 3) quenched QCD and QCD are the same up to higher-order corrections in the 1/N_c expansion. Thus the calculation of the nonleptonic kaon decays relevant for the δI = ((1)/(2)) rule in the quenched approximation is expected to differ from the unquenched one by an O(1/N_c) correction. However, the calculation relevant to the CP-violation parameter ε'/ε would have a relatively big higher-order correction due to the large cancellation in the leading order. Some important weak matrix elements are poorly known that even constraints with 100% errors are interesting. In those cases, quenched calculations will be very useful

[en] Hydrothermal synthesis has proven to be a cost-effective, energy-efficient approach for the manufacture of lithium iron phosphate (LiFePO₄) and its related materials. However, hydrothermally prepared LiFePO₄ typically suffers from antisite defects, where some of the iron resides on lithium sites and restricts lithium-ion mobility. A post-heat-treatment temperature of around 700 C is generally used to eliminate cation disorder, but little is known about these antisite defects or their concentration as a function of the post-heat-treatment temperature. In this study, time-resolved, synchrotron X-ray diffraction reveals that antisite defects are completely eliminated above 500 C, suggesting that the electrochemical performance may be significantly enhanced by a milder postsynthesis heat treatment. The preliminary electrochemical results show a significant enhancement in the electrochemical capacity with the defect-free material, with the specific capacity increasing by approximately 60% at a C/20 rate.