Recent breakthroughs in effective interaction and effective operator techniques allow us to take a new look at this field that has seen limited progress in the past twenty years. A comparison of the old and new techniques will shed some new light on the use of effective interactions and effective operators in shell model calculations of light nuclei. Three different methods of calculating the effective interaction and effective operators are described and compared. A large model-space no-core shell-model calculation for ⁶Li is used as the basis for comparison. In the no-core calculation all nucleons are active in a model space involving all configurations with energies up to 8ħΩ. The second method is a perturbation expansion for the effective interaction and effective operators, using an inert ⁴He core and two valence particles. In particular, the electric quadrupole and magnetic dipole operators are studied to determine the effective charges to be used in connection with one-body operators in this shell-model space. The third method is a model-space truncation scheme, which maps operators in a large model space into operators in smaller, truncated model spaces. The effect of going to larger excitation spaces will be examined as well as the convergence trends regarding increases in the excitation space. The results from these three approaches are compared in order to gain new insight into the nature of effective interactions and operators in truncated model spaces. We find that by going to energies of 8ħΩ we can accurately reproduce the experimental values for the binding energy, excitation spectrum, electric quadrupole moment and magnetic dipole moment of ⁶Li and that there is a definite model-space dependence for these operators. To obtain results similar to the 8ħΩ ones in a truncated 2ħΩ model space we use effective operators and effective charges. Effective charges of approximately 1.1e for the effective proton charge and 0.3e for the effective neutron charge are obtained in the perturbation-expansion technique, while the model-space truncation calculations yield effective charges of 1.5e for the proton and.36e for the neutron. These values can be compared with empirically obtained values of eᵖ(eff) ≈ 1.5e and eⁿ(eff) ≈ 0.5e.

The no-core shell-model approach has proven to be extremely useful for the theoretical determination of the properties of light (A ≤ 12) nuclei. However, at present this method does not accommodate a three-nucleon interaction into the potential that it employs. The problem is introduced with a largely historical development of both the effective interaction formalism and three-nucleon interactions, placing the motivation in context. This work makes a first attempt to incorporate such a three-nucleon interaction into the no-core shell-model ansatz. To this end, a variant of the two-pion-exchange Tucson-Melbourne three-nucleon interaction has been chosen. A three-body translationally-invariant harmonic-oscillator basis is constructed, and matrix elements of the three-nucleon interaction in this basis are calculated. The majority of this is accomplished through standard angular-momentum algebraic techniques, with the most expensive component being the spatial one, as it requires a transformation of the basis set with computationally intensive transformation brackets. Given the ability to determine the matrix elements for the chosen Tucson-Melbourne force, the practicality of employing these in calculations is demonstrated, with calculations on the three-body nuclei ³H and ³He. These are simple calculations, where the Tucson-Melbourne matrix elements are added to those of the two-body effective potential (a slight inconsistency which future studies will aim to fix). The dependence of binding energies on the harmonic-oscillator parameter, hΩ, and the Tucson-Melbourne cutoff parameter, Λ are examined. The former is found to be small in the range of hΩ considered, while the latter is shown to be consistent with previous works that have explored this dependence using other methods. The convergence of the binding energy with increasing model space size is slow, but this is perhaps attributable to the unrenormalized nature of the three-body matrix elements. The ultimate aim of this research is to find a viable method for constructing a three-body effective interaction from a given "realistic" three nucleon interaction, for use in no-core shell-model calculations. The current work demonstrates that such a scheme is feasible, and should yield results more consistent with experiment. Such a three-body effective interaction should also achieve quicker convergence with model space size than shown here, as the three-nucleon matrix elements will be renormalized to account for the geometry of the model space. Thus, one will have constructed an ab initio method for calculations on light nuclei, that includes a three-nucleon interaction, and converges quickly in the determination of nuclear properties.

We interpret theoretically electron pair data observed in Pb(158 AGeV)-Au collisions at the CERN SPS by considering the system as an evolving fireball with parameters fit to experimental observables. Dilepton production in the QGP phase is found via standard finite temperature field theory techniques where annihilating quarks have thermally generated effective masses. After the phase transition, contributions from rho and omega meson decays are found via from experimentally determined forward scattering amplitudes which account for the effects of emission from a medium with finite temperature and density. All results are folded with a model which considers bias created by the CERES detector's acceptance. Our calculations agree well with existing data dilepton production at low and intermediate invariant masses. / Nous interprétons les données expérimentales sur les paires de leptons mesurées dans les collisions Pb-Au à 158 AGeV, au SPS du CERN. Nous traitons l'évolution du système hadronique en considérant une modélisation thermodynamique ajustée aux observables asymptotiques. La production de leptons dans la phase du plasma quark-gluon est obtenue avec les techniques reconnues de la théorie des champs à température finie, où les quarks ont des masses thermiques non-nulles. Après la transition de phase, les contributions des désintégrations des mésons rho et omega sont évaluées en partant des amplitudes de diffusion vers l'avant, ce qui tient compte des effets de milieu. Tous nos résultats sont filtrés par l'acceptance du détecteur CERES. Nos calculs sont en accord avec les données mesurées sur la production de dileptons de petite et moyenne masses invariantes.

This thesis presents the systematic analysis of azimuthal distributions of identified charged particles produced in Au+Au collisions at 11.5 A·GeV/c at the AGS measured with respect to the reaction plane. A Fourier expansion is used to describe the azimuthal anisotropy in particle production. Directed flow of protons, deuterons, pi+/-, K+/- and antiprotons is presented as a function of transverse momentum for different particle rapidities and collision centralities. Directed flow of protons and that of deuterons are well described by a simple sideward moving and radially expanding thermal source model; the observed behavior of the directed flow of pi+/- indicates the interplay between shadowing and expansion; direct flow of K + and K- is observed to be anti-correlated to that of protons at low pt, implying that the role of absorption and rescattering is significant for kaons in the dense nuclear medium; the first observation of a large "antiflow" signal for antiprotons suggests that a strong annihilation process occurs in heavy-ion collisions at the AGS. The experimental results are compared to the predictions of the event generator RQMD (version 2.3) in both the mean-field and cascade modes.

We apply the methods of statistical mechanics and field theory at finite temperature to understand phenomena in intermediate and high energy heavy-ion collisions. / In the intermediate energy scenario we concentrate on the pervasive phenomena of multifragmentation. We introduce various extensions of the recently proposed Recursive Statistical Multifragmentation Model (RSM model). In particular, we devise a novel Monte-Carlo technique to improve the treatment of the excluded volume in the model. We consider extensions to account for Coulomb effects and inclusion of isospin degrees of freedom. We then devise a consistent decay formalism to account for the change in isotope ratios due to decay of particle unstable clusters produced in such systems. We demonstrate how, with the above mentioned extensions, populations of various intermediate mass fragments observed in experiments may be explained. We then focus on the possible observation of critical phenomena in experiments, and provide a criticism of the parametrization techniques currently used by practitioners in the field. We demonstrate how such techniques may lead to misleading interpretations and identifications of critical phenomena. / In high energy heavy-ion collisions, we focus on the ongoing search for the Quark-Gluon-Plasma (QGP). The QGP is formed for a very short time and hence its presence is inferred through indirect signatures. In this thesis, we concentrate on the electromagnetic signatures of such a plasma. We demonstrate how the explicit breaking of charge conjugation invariance by the QGP may lead to the appearance of processes hitherto considered absent due to symmetry considerations. These processes allow for gluons to fuse to form lepton pairs and turn out to be comparable, in certain regions of parameter space, to the tree level rate for lepton pair formation from quark anti-quark annihilation. We then investigate the issue of collinear and infrared divergences in two-loop dilepton production rates. This is done by calculating the imaginary part of the retarded two-loop self-energy of a static vector boson in a plasma of quarks and gluons. We recombine the various cuts of the self-energy to generate physical processes. We demonstrate how cuts containing loops may be reinterpreted in terms of interference between O(alpha) tree diagrams and the Born term along with spectators from the medium. We apply our results to the rate of dilepton production in the limit of dilepton invariant mass M >> T. We find that all infrared and collinear singularities cancel in the final result obtained in this limit.

A new linear high electric field radiofrequency quadrupole (RFQ) ion trap, dubbed HVTrap, was developed and tested to study the phase space properties of confined particles and determine the suitability of such a device as an ion source/delivery system for high-resolution time-of-flight (TOF) mass spectrometry and to test the possibility of high field RFQ beam cooling in an ion guide. / A segmented quadrupole rod structure was built and shown to easily withstand over 4kV between adjacent electrodes placed 1mm apart in 1x10 -4 Torr of helium buffer gas. An innovative resonating circuit design using hollow air-cored induction coils was used to simultaneously deliver the necessary RF and DC trapping potentials to the linear RFQ system as well as deliver the extraction voltages used to eject test ions (Cs+, m/z = 133) produced by a surface ionization source. The resulting ion bunches were delivered to a TOF system for time profile analysis using a multichannel plate detector. Optimal trapping parameters were found to vary with applied RF potentials and extraction voltages. The presence of helium buffer gas at pressures of 10-4 Torr and long cooling times, in the range of 500ms, were found to improve the number of detected ions. / A thermodynamic model of the confined ions was used to simulate the extraction process and follow the phase space evolution of the ejected particles through the TOF region. Analysis of the experimental data showed that confined Cs + ions reached equilibrium temperatures as low as 0.45 eV after 500ms cooling periods in 1x10-4 Torr of helium buffer gas. These equilibrium temperatures were also found to be dependent on the number of detected ions and applied RF potentials. Harmonic frequencies in the resonant circuit are thought to play an important role in determining the thermal energies of the trapped ions. / This thesis has shown that high field RFQ confinement of ions in buffer gas is indeed feasible and that trapped particles can be manipulated using DC fields superimposed on the RF. In its present form, the HVTrap would be capable of mass resolutions of roughly 1000. Beam cooling using a high field RFQ would also be possible and could potentially accommodate beam currents of up to 100nA. If the ion temperature could be reduced to 0.05 eV, TOF mass resolutions of 30 000 would be possible.

A great portion of the experimental work in nuclear physics today has as its object the determination and study of energy levels in the nucleus. The energies of these levels can be determined by various types of experiments. In the work to be described in this thesis, two different types of experiments were conducted, both with the object of studying energy levels of nuclei. Energy levels in excited C14 and N14 were studied by measuring the energy of the gamma rays emitted by these excited states. The excited levels were obtained by bombarding C13 with deuterons. Energy levels in Be8 and Be9 were studied by observing resonances in the yield of product particles in the various reactions of Li6 and Li7 with deuterons. Since the two experiments are of such a different nature, this thesis will be written in two parts---one to describe each experiment.

Magnetic beta-ray spectrometers of several types have been used extensively to obtain information by the detection of low energy gamma-rays. The photoelectric effect, Compton scattering, and atomic internal conversion have yielded information regarding nuclei energy level spacings, transition schemes, relative and absolute intensities of gamma radiation, and the multipole character of nuclear radiation. However, these processes have low cross sections for high energy gamma rays, so that some other process is necessary for the study of the high energy region. Pair production, unlike the secondary processes mentioned, is increasingly probable at higher gamma ray energies. Useful information can be gained by the study of the pairs produced in the external, internal, and nuclear pair formation processes. Most generally, the total energy and number of the pairs detected are used to determine energy level spacings in nuclei, and to find what gamma ray transitions occur as well as the probability for their occurrence. A potentially fruitful field of study is opened by studying internal and nuclear pairs. Information concerning the multipole character of the radiation and consequently the angular momentum and parity changes in nuclear transitions can possibly be gained by the study of internal pair formation. Nuclear pairs immediately indicate the angular momenta of the excited levels from which they arise. This paper describes the development and use of a magnetic lens spectrometer for the detection of pairs. The spectrometer has been used as a medium resolution spectrometer for high energy gamma rays. It is capable of detecting either external, internal, or nuclear pairs. Measurements made using these three processes will be described.

Abstract Not Available.

This thesis will be devoted to describing two rather different problems of nuclear physics which have been studied with the Rice University 180° magnetic spectrometer. The first part of the thesis describes a series of experiments which has been performed in an effort to obtain a set of energy standards which could be employed in calibrating analyzing magnets associated with various experimental nuclear physics problems. The second part of the thesis describes a study of the decay scheme for three-body nuclear decay. A recent paper by Phillips, Griffy, and Biedenharn has treated the three-body decay process as a time sequence of two-body interactions; and the cross section predictions resulting from this treatment have been investigated.