A systematic investigation is carried out of the multiplicity of potential energy minima for up to 13 atoms interacting under central two-body potentials of Lennard-Jones and Morse type. The sets of minima discovered by various growth algorithms are believed to be virtually exhaustive of all possible N-atom isomers for the potentials used, with N 14. These are classified according to the presence of crystallographic or non-crystallograpnic (5-fold) symmetry, and their energy distributions are statistically analyzed. It is shown that non-crystallographic configurations predominate in structures of both greatest and least binding energy. A striking result is the extreme sensitivity of the number of possible stable minima tovariations in the range and softness of the pair potential. Thus, of no fewer than 988 energetically distinct minima for 13 Lennard-Jones (6-12) atoms, only some 36 are supported by the Morse (a = 3) potential. A vibrational analysis is performed for each configuration and the resulting vibrational frequencies are used to construct an approximation to the multi-configuration partition function (MCPF). This partition function is compared with the corresponding single-configuration partition function derived from the most stable cluster (SCPF) . Tnermodynamic properties for monatomic systems of rare gases, based on a rigid rotor/harmonic oscillator (RRHO) approach, are obtained via both single- and multi-configuration partition functions. It is seen that the validity of the single-configuration approximation depends strongly upon the distribution of isomer energies and less strongly upon the number of these isomers. From the computed partition functions equilibrium constants and relative concentrations are estimated, asis the size of the critical nucleus at various pressures. The first five reduced virial coefficients are calculated and a correspondence between the equilibrium constants and Mayer's Cluster Integrals presented. The use of graph theory in the enumeration of geometrically distinct isomers in 3 and higher dimensional spaces is noted, and adjacency matrices for some specific isomers constructed. Inspection of these matrices enables one to determine how compact the corresponding structures are.

This thesis is concerned with the study of the excitation of Hydrogen atoms to the n = 3 states from the ground state by electrons with incident energies ranging from just above the ionization threshold to energies where the first Born approximation is expected to be valid. The major physical effects in this region are exchange, the distortion of the wave describing the external electron, and the distortion of the atomic system. A model which includes these effects - the Distorted Wave Polarized Orbital (DWPO) approximation - is generalized for any excitation and used to investigate the excitation process for n = 3 in particular. Total (integrated) and differential cross sections, not previously calculated using this model, are presented and compared, where possible, with other theoretical and experimental work. Other sensitive indicators of the effects of the model are considered. These include the polarization of Balmer-a (Ha) radiation and the parameters which describe the orientation and alignment of the atomic system after collision and the coincidence rate for the observation of emitted photons with the ejected electrons (Fano-Macek and Macek-Jaecks parameters). Also studied is the asymmetry in the observed intensity of Ha radiation arising on sign reversal of an applied electric field along the interaction direction. There is a serious disparity between the results in this model or the Born approximation and the experimental observations. A number of reasons for this are discussed. The work here indicates a need for further theoretical and experimental study but that high levels of sensitivity are required in any experimental work particularly with regard to the polarization and asymmetry measurements. Addition any, this work illustrates a very serious failure in the DWPO model caused by the use of the adiabatic polarization potential rather than an energy dependent potential especially at higher energies in the 3d excitation where we found that for the total cross section, the results obtained by including full allowance for polarizationlie a factor of about 2.5 below the Bom result at 200 eV and do not approach the Born cross section even for impact energies measuredin keV. The most useful line of future research is expected to be the allowance for coupling to adjacent states by the unitarizaticn method and some preliminary work for this is included.

This thesis is concerned with a study of inelastic electron-atom collisions with the incident energy ranging from just above the first ionization threshold to some energy where the First Born approximation becomes valid. The main physical effects which need to be included in the theoretical treatment of such collisions are electron exchange and distortion of both the atomic system and the wave describing the external electron. A method which takes account of these effects, to be referred to as the Distorted Wave Polarized Orbital (DWPO) approximation, is described. Three models based on this approximation are developed and applied to electron collisions with the light atoms hydrogen and helium. In particular the models are applied to the following collision processes: e + H(ls) → e + H(2s, 2p, 2s + 2p); e + He(11S) → e+ He(n1,3L), n = 2,3,4,5, L= S,P. Results are presented for the total (integral)and differential cross sections and also, where appropriate, for the parameters describing the orientation and alignment properties of the excited atom. The results are compared with those of other theoretical methods and with experimental measurements.

<p> Atomic resonances present a difficult chapter in the study of atomic structure. The calculation and measurement of these resonant states have provided a challenge for both theorists and experimentalists. This work focuses on the numerical calculation of the resonant states. Some years ago a method to calculate the resonant states of three-body atomic interactions was developed. This method involves solving the Faddeev equations using a Merkuriev cut and a Coulomb-Sturmian basis, and will be the method used for the calculations in this work. This method was used as an alternative to the more conventional methods of calculating atomic resonant states. At the time of its derivation, the method was used to calculate the narrow-width resonances of the electron-Positronium (e &ndash; Ps) system, which showed accurate results with respect to the calculations done by other groups using alternative methods. Additionally, the method saw an emergence of resonances (broad-width) which line-up to the system thresholds. We have come to call these broad-width resonances the threshold resonances. However, at the time, these threshold resonances proved too computationally intensive to make confident results. Now, with the assistance of better computational resources and an improved code, this problem is again addressed. </p><p> New calculations of the narrow-width and threshold resonances were completed which support the appearance of the threshold resonances in the e &ndash; Ps system. The threshold resonances were observed lining up at the first, second, and third two-body thresholds, a trend that is assumed to continue at even higher energies. Calculations were carried out for both the 1S and 3S states. After successfully making calculations of the e &ndash; Ps system resonant states, calculations were also carried out for the electron-Hydrogen (e&ndash;H) 1S and 3S resonances. The calculations for the e &ndash; H system were carried out for the threshold resonances emerging from the 1st threshold. Additionally, we propose an explanation for the emergence of the threshold resonances.</p>

<p>Measurements of atomic and ionic data has been an active research area since the beginning of spectroscopic studies. The experimental data deduced from thesestudies are of great interest for our understanding of fundamental as well as applied physics. The metastability of certain energy levels has been used toexplain phenomena in widely different fields ranging from the principles of the laser to observations of forbidden spectral lines originating fromastrophysical objects. Eventhough measurements of radiative lifetimes have been carried out for many decades new interesting results are still found in theexplanation of phenomena observed in these studies. The technological development in the field of atomic physics has provided researchers with several toolswhich have opened up completely new possibilities in recent years. Atoms and ions can now be stored for long times which allows studies of extremely weakprocesses. In this thesis results from studies of weak radiative decays of metastable levels are presented. The measurements were carried out utilizing a laserprobing technique together with ions stored in the ion storage ring CRYRING at the Manne Siegbahn laboratory in Stockholm, Sweden. The longest radiativelifetime measured so far in a storage ring, 89 s in singly ionized barium, is presented along with a completely new method for determining extremelylong radiative lifetimes. The thesis also includes new interesting results from lifetime measurements in argon which revealed the unexpected importance of an E3 decay channel. This is to the best of our knowledge the first ever observation of an E3 transition rate of this magnitude in a singlycharged ion. All lifetime measurements are presented with complementary calculations by our collaborators at Université de Mons-Hainaut and Université de Liège, Belgium. The studies of BaII are also part of a collaboration with the Institute of solid state physics of the Bulgarian Academy of Sciences.</p>

Transmutation of plutonium and minor actinides in accelerator-driven systems (ADS) is being envisaged for the purpose of reducing the long-term radiotoxic inventory of spent nuclear reactor fuel. For this reason, the physics of sub-critical systems are being studied in several different experimental programs across the world. Three of these experiments have been studied within the scope of the present thesis; the MUSE experiments in France, the Yalina experiments in Belarus and the SAD experiments in Russia. The investigations of the MUSE experiments have focused on three different neutronic parameters; the neutron energy spectrum, the external neutron source efficiency and the dynamic neutron source response. It has been shown that the choice of external neutron source has negligible effect on the neutron energy spectrum in the core. Therefore, from this point of view, the MUSE experiments can be considered representative of an ADS. From the analyses of different reactivity determination methods in the Yalina experiments, it can be concluded that the slope fit method gives results in good agreement with the results obtained by the Monte Carlo method MCNP. Moreover, it was found that the Sjöstrand method underestimates keff slightly, in comparison with MCNP and the other investigated methods. In the radiation shielding studies of the SAD experiments, it was shown that the entire part of the effective dose detected at the top of the biological shielding originates from the proton-induced spallation reactions in the target. Thus, it can be concluded that the effective dose is directly proportional to the proton beam power, but independent of the reactivity of the sub-critical core. In order to study the energy gain of an ADS, i.e., the core power divided by the proton beam power, the proton source efficiency, ψ*, has been studied for various ADS models. ψ* is defined in analogy with the neutron source efficiency, φ*, but relates the core power directly to the source protons instead of to the source neutrons. φ* is commonly used in the physics of sub-critical systems, driven by any external neutron source (spallation source, (D,D), (D,T), 252Cf spontaneous fission etc.). On the contrary, ψ* has been defined only for ADS studies, where the system is driven by a proton-induced spallation source. The main advantages of using ψ* instead of φ* are that the way of defining the external source is unique and that ψ* is proportional to the energy gain. An important part of this thesis has been devoted to studies of ψ* as a function of different system parameters, thereby providing a basis for an ADS design with optimal properties for obtaining a high core power over beam power ratio. For instance, ψ* was found to decrease considerably with increasing spallation target radius. / QC 20101005

This thesis presents measurements of fundamental properties of singly charged ionic systems including spectral wavelengths, lifetimes of excited states and branching fractions with the main focus on time resolved laser spectroscopy of stored ions providing lifetime measurements of metastable states. The results of these measurements have been used to determine energy levels and transition probabilities of the studied systems. The included experimental data are compared with results from calculations which provides evaluations of different theoretical models. The presented results have been applied by others to the field of atomic astrophysics in order to deduce electron densities and elemental abundances in ejecta of the supermassive star eta Carinae and have also been followed by additional work of theoreticians interested in comparisons with laboratory data. The thesis is a result of several collaborations in which Stockholm University has been providing lifetime measurements of metastable states, Lund Observatory and the National Institute of Standards and Technology have been providing wavelength and branching fraction measurements, Lund Laser Centre has been providing lifetime measurements of short-lived states and calculations have been performed by theoretical physicists from Université de Mons-Hainaut, Université de Liège, Queen's University of Belfast and Laboratoire Aimé Cotton.

The charge distribution associated with an electron has surprising implications for a number of outstanding mysteries in physics. Why is the universe made out of matter versus anti-matter, instead of both equally? What new particles and interactions lie beyond the current reach of accelerators like the LHC? Models which propose answers to these questions, such as Supersymmetry, tend to predict a small, yet potentially measurable, asymmetric interaction between an electron and an electric field, characterized by an electric dipole moment (EDM). Despite over six decades of experimental searching, no EDM of any fundamental particle has ever been measured; however, these experiments continue to provide some of the most stringent limits on new physics. Here, we present the results of a new search for the electron EDM, \(d_e = (-2.1 \pm 3.7_{stat} \pm 2.5_{syst}) \times 10^{-29}\) e cm, which represents an order of magnitude improvement in sensitivity from the previous best limit. Since our measurement is consistent with zero, we present the upper limit of \(|d_e| < 8.7\times 10^{-29}\) e cm with 90 percent confidence. / Physics

In 1789 a German chemist, Martin Heinrich Klaproth, announced the discovery of a new element: uranium; few years later, the head of father of the modern chemistry, Antoine Lavoisier, was swept away by guillotine: a new era was destined to be opened, either where energy would have been produced in large scale by nuclear processes delivering hundreds of times the energy of chemical processes or where a mass of people, revolutionary or not, would have been melted down into a couple of seconds. After a quite long time, on the 2nd December 1942, the first nuclear reactor has been put into operation by Enrico Fermi in Chicago; few years later, came also the dark side utilization of fissile materials in Hiroshima and Nagasaki. Since those moments, three power plants generations succeeded, until the current one which is the generation IV of nuclear reactors. The latter has the goal of generating electricity in a safe manner, for the core is designed to provide an effective passive cooling of the decay heat. Amid generation IV of nuclear power plants, the Gas Turbine – Modular Helium Reactor, designed by General Atomics, is the only core with an energy conversion efficiency of 50%; the above consideration, coupled to construction and operation costs lower than ordinary Light Water Reactors, renders the Gas Turbine – Modular Helium reactor rather unequaled. In the present studies we investigated the possibility to operate the GT-MHR with two types of fuels: LWRs waste and thorium; since thorium is made of only fertile 232Th, we tried to mix it with pure 233U, 235U or 239Pu; ex post facto, only uranium isotopes allow the reactor operation, that induced us to examine the possibility to use a mixture of uranium, enriched 20% in 235U, and thorium. We performed all calculations by the MCNP and MCB codes, which allowed to model the reactor in a very detailed threedimensional geometry and to describe the nuclides transmutation in a continuous energy approach; finally, we completed our studies by verifying the influence of the major nuclear data libraries, JEFF, JENDL and ENDF/B, on the obtained results. / <p>QC 20100922</p>

A method is described for investigating the ionisation produced in helium by collisions between quasi-stationary atoms of helium - that is, atoms possessing only thermal velocities - and a beam of neutral helium atoms with kinetic energy less than one hundred electron volts. Ionisation is shown to begin when the kinetic energy of the impinging particle is twice as great as the minimum energy which an electron must possess in order to ionise helium. This value is lower than any which have been reported previously, and agrees with that predicted by a simple theory of collision, in which the impacting atoms are regarded as perfectly elastic spheres. Electrons resulting from the bombardment of metal surfaces by helium atoms were detected, and methods are described by means of which they were distinguished from those produced as a result of ionisation of the gas by atomic impact. Approximate values are obtained for the coefficient of liberation of electrons from nickel, by normally incident helium ions,and for the variation of the secondary electron emission from the same target, when the kinetic energy of an impacting beam of neutral helium atoms is increased from 42 to 70 electron volts. The theoretical implications of the results are discussed. The sensitivity of the apparatus used in this research iscompared with that of the arrangements employed by Varney* and by Rostagni** in similar investigations. Reasons are suggested for the complete failure of the former author to detect ionisation in helium,with impacting helium atoms of kinetic energy up to 400 electron volts, and for the high value reported by Rostagni for the critical energy for ionisation.