No description available.

Electron impact excitation phenomena play an important role in atomic and molecular physics. The different energy levels of an atom or molecule interact differently with incoming electrons with different energies and that affects the excitation of the energy levels of the atoms and molecules. Studies involving electron impact excitation process are generally conducted with optical emission techniques or by the electron energy loss method. In the present study, for the first time, cavity ringdown spectroscopy (CRDS) has been used to investigate electron impact excitation phenomena of electronatom collision processes. The technique, i.e., electron impact excitation-cavity ringdown spectroscopy (EIE-CRDS), was employed for the purposes of fundamental study and of real-time applications. The fundamental study which was carried out in terms of determining electron impact excitation cross section (EIECS) has been demonstrated by measuring EIECS of a few excited levels of mercury (Hg) atom. For the application side, the EIE-CRDS technique has been employed for trace element detection. This dissertation first describes the fundamentals of electron impact excitationcavity ringdown spectroscopy (EIE-CRDS); afterwards its applications are demonstrated. A novel method of measuring excitation cross sections using this EIE-CRDS technique has been explained. In this method, first the excitation of atoms are achieved by electron impact excitation process, subsequently, CRDS measured absolute number density is utilized to determine the absolute EIECS values. Steps of the method are described in detail. Applicability of the method is demonstrated by measuring EIECS of three different energy levels of Hg, namely 6s6p 3P0, 6s6p 3P1, and 6s7s 3S1, and the obtained values are in agreement with those reported in the literature. Secondly, the EIE-CRDS technique was employed to investigate the absorption spectrum of mercury atom in the vicinity of 404.65 nm, corresponding to the transition 6s7s 3S1 -> 6s6p 3P0 levels of mercury. Elemental mercury was measured using a laser of wavelength 404.65 nm. The technological feasibility of developing a portable size instrument for mercury detection was explored. Subsequently, a portable size, dual-mode, plasma-CRDS based prototype instrument, capable of real-time trace element monitoring, was developed. The design, functioning, and specifications of the instrument are also explained.

No description available.

Measurements of the target thickness dependence of the target x-ray production yield for incident fast heavy ions are reported for thin solid Cu targets as a function of both incident projectile atomic number and energy. The incident ions were F, Al, Si, S, and CI. The charge state of the incident ions was varied in each case to study the target x-ray production for projectiles which had an initial charge state, q, of q = Z₁, q = Z₁ - 1, and q < Z₁ - 1 for F, Al, Si, and S ions and q = Z₁ - 1 and q < Z₁ - 1 for C1 ions. The target thicknesses ranged from 2 to 183 ug/cm². In each case the Cu K x-ray yield exhibits a complex exponential dependence on target thickness. A two-component model which includes contributions to the target x-ray production due to ions with 0 and 1 K vacancies and a three-component model which includes contributions due to ions with 0, 1, and 2 K vacancies are developed to describe the observed target K x-ray yields. The two-component model for the C1 data and the three-component model for the F, Al, Si, S, and C1 data are fit to the individual data for each projectile, and the cross sections for both the target and projectile are determined. The fits to the target x-ray data give a systematic representation of the processes involved in x-ray production for fast heavy ions incident on thin solid targets.

This thesis is concerned with the energy loss of light atomic projectiles in thin boron films. In Chapter I, a brief theoretical description of the stopping process is given. In Chapter II, the experimental procedure is described and a method is given by which electronic stopping cross-sections can be determined from the observed energy loss. In Chapter III, the thin boron films that are used in the experiments are described in detail. In Chapter IV, the experimental results are presented, and in Chapter V the results are compared with existing data and theory. / Thesis / Master of Science (MS)

<p> A new technique has be~n developed for the measurement of fast neutron total cross-sections in the range 0.8 -> 3.0 MeV using the fast neutrons from a reactor. The method was used to obtain the cross-section of six elements in the 2s-1d shell. Various analytical techniques have been applied to measure the behavior of the average cross-sections and delineate the properties of the observed resonances. From these results the resonances are confirmed as examples of "doorway" states, and conclusions are drawn about the distributions of the parameters of these states. The average properties are related to nuclear models of the origin and characteristics of states of high excitation in the target nuclides.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.

The present state of modeling radio-induced effects at the cellular level neglects to account for the microscopic inhomogeneity of the nucleus from the non-aqueous contents by approximating the entire cellular nucleus as a homogenous medium of water. Charged particle track-structure calculations utilizing this principle of superposition are thereby neglecting to account for approximately 30% of the molecular variation within the nucleus. To truly understand what happens when biological matter is irradiated, charged particle track-structure calculations need detailed knowledge of the secondary electron cascade, resulting from interactions with not only the primary biological component – water – but also the non-aqueous contents, down to very low energies. This paper presents developments for a novel approach, which to our knowledge has never been done before, to reducing the homogenous water approximation. The purpose of our work is to develop of a completely self-consistent computational method for predicting molecule-specific ionization, excitation, and scattering cross sections in the very low energy regime that can be applied in a condensed history Monte Carlo track-structure code. The present methodology begins with the calculation of a solution to the many-body Schrödinger equation and proceeds to use Monte Carlo methods to calculate the perturbations in the internal electron field to determine the aforementioned processes. Results are computed for molecular water in the form of linear energy loss, secondary electron energies, and ionization-to-excitation ratios and compared against the low energy predictions of the GEANT4-DNA physics package of the Geant4 simulation toolkit.

Thesis (Ph.D.); Submitted to the Catholic Univ. of America, Washington, DC (US); 1 May 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "JLAB-PHY-03-40" "DOE/ER/40150-2764" Brian Carnahan. 05/01/2003. Report is also available in paper and microfiche from NTIS.

In this thesis, the first-order and higher-order interferences on the total (Coulomb+nuclear), Coulomb and nuclear breakup cross sections in the 15C+208Pb, 11Be+208Pb breakup reactions are first studied at 68 MeV/u incident energy. It is shown that the first-order interference reduces by more than 60% the total breakup cross sections, by less than 3% the Coulomb breakup cross sections and by more than 85% the nuclear breakup cross sections, for both reactions. On the other hand, the high-order interference is found to reduce by less than 9% the total breakup cross section, less than 1% the Coulomb breakup cross section and less than 7% the nuclear breakup cross section for the 15C+208Pb reaction. For the 11Be+208Pb reaction however, the high-order interference reduces by less than 7% the total breakup cross section, by less than 1% the Coulomb breakup cross section and by less than 4% the nuclear breakup cross section. It is finally shown that even at first-order, the incoherent sum of the nuclear breakup cross sections is more important than the incoherent sum of the Coulomb breakup cross sections for the two reactions. The role of the diagonal and off-diagonal continuum-continuum couplings on total, Coulomb and nuclear breakup cross sections is also investigated for the 8B+58Ni, 8B+208Pb and 19C+208Pb at 29.3, 170.3 MeV and 1273 MeV incident energies respectively. Qualitatively, we found that, the diagonal continuum-continuum couplings are responsible for the large reduction of the differential total and nuclear breakup cross sections at backward angles. At forward angles, this reduction is due to the off-diagonal continuum-continuum couplings. In the absence of these couplings, the nuclear breakup is the more dominant process, while when they are included, the Coulomb breakup becomes dominant. This shows that, the nuclear breakup is more affected by the continuum-continuum couplings than its Coulomb counterpart. Quantitatively, we found that, the off-diagonal countinuum-countinuum couplings reduce by 13.39%, 12.71% and 11.11% the total breakup cross sections for the 8B+58Ni, 8B+208Pb and 19C+208Pb reactions, respectively. / Physics / D. Phil. (Physics)