No description available.

No description available.

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.

No description available.

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.

<p> We present a method to map the spectral and spatial distributions of radioactive sources using a small number of detectors. Locating and identifying radioactive materials is important for border monitoring, accounting for special nuclear material in processing facilities, and in clean-up operations. Most methods to analyze these problems make restrictive assumptions about the distribution of the source. In contrast, the source-mapping method presented here allows an arbitrary three-dimensional distribution in space and a flexible group and gamma peak distribution in energy. To apply the method, the system&rsquo;s geometry and materials must be known. A probabilistic Bayesian approach is used to solve the resulting inverse problem (<p style="font-variant: small-caps">IP</p>) since the system of equations is ill-posed. The probabilistic approach also provides estimates of the confidence in the final source map prediction. A set of adjoint flux, discrete ordinates solutions, obtained in this work by the Denovo code, are required to efficiently compute detector responses from a candidate source distribution. These adjoint fluxes are then used to form the linear model to map the state space to the response space. The test for the method is simultaneously locating a set of ¹³⁷Cs and ⁶⁰Co gamma sources in an empty room. This test problem is solved using synthetic measurements generated by a Monte Carlo (<p style="font-variant: small-caps">MCNP</p>) model and using experimental measurements that we collected for this purpose. With the synthetic data, the predicted source distributions identified the locations of the sources to within tens of centimeters, in a room with an approximately four-by-four meter floor plan. Most of the predicted source intensities were within a factor of ten of their true value. The chi-square value of the predicted source was within a factor of five from the expected value based on the number of measurements employed. With a favorable uniform initial guess, the predicted source map was nearly identical to the true distribution, and the source intensities agreed within the predicted uncertainty. Using experimental data, the mapping was more difficult due to laboratory limitations. However, by supplanting 14 flawed measurements (out of 69 total) with synthetic data, the proof-of-principle source mapping was nearly as accurate as the synthetic-only prediction. </p>

<p> This study focused on developing a better understanding of granular dynamics in pebble bed reactor cores through experimental work and computer simulations. The work completed includes analysis of pebble motion data from three scaled experiments based on the annular core of the Pebble Bed Fluoride Salt-Cooled High- Temperature Reactor (PB-FHR). The experiments are accompanied by the development of a new discrete element simulation code, GRECO, which is designed to offer a simple user interface and simplified two-dimensional system that can be used for iterative purposes in the preliminary phases of core design. The results of this study are focused on the PB-FHR, but can easily be extended for gas-cooled reactor designs.</p><p> Experimental results are presented for three Pebble Recirculation Experiments (PREX). PREX 2 and 3.0 are conventional gravity-dominated granular systems based on the annular PB-FHR core design for a 900 MWth commercial prototype plant and a 16 MWth test reactor, respectively. Detailed results are presented for the pebble velocity field, mixing at the radial zone interfaces, and pebble residence times. A new Monte Carlo algorithm was developed to study the residence time distributions of pebbles in different radial zones. These dry experiments demonstrated the basic viability of radial pebble zoning in cores with diverging geometry before pebbles reach the active core.</p><p> Results are also presented from PREX 3.1, a scaled facility that uses simulant materials to evaluate the impact of coupled fluid drag forces on the granular dynamics in the PB-FHR core. PREX 3.1 was used to collect first of a kind pebble motion data in a multidimensional porous media flow field. Pebble motion data were collected for a range of axial and cross fluid flow configurations where the drag forces range from half the buoyancy force up to ten times greater than the buoyancy force. Detailed analysis is presented for the pebble velocity field, mixing behavior, and residence time distributions for each fluid flow configuration.</p><p> The axial flow configurations in PREX 3.1 showed small changes in pebble motion compared to a reference case with no fluid flow and showed similar overall behavior to PREX 3.0. This suggests that dry experiments can be used for core designs with uniform one-dimensional coolant flow early in the design process at greatly reduced cost. Significant differences in pebble residence times were observed in the cross fluid flow configurations, but these were not accompanied by an overall horizontal diffusion bias. Radial zones showed only a small shift in position due to mixing in the diverging region and remained stable in the active core. The results from this study support the overall viability of the annular PB-FHR core by demonstrating consistent granular flow behavior in the presence of complex reflector geometries and multidimensional fluid flow fields.</p><p> GRECO simulations were performed for each of the experiments in this study in order to develop a preliminary validation basis and to understand for which applications the code can provide useful analysis. Overall, the GRECO simulation results showed excellent agreement with the gravity-dominated PREX experiments. Local velocity errors were found to be generally within 10-15% of the experimental data. Average radial zone interface positions were predicted within two pebble diameters. GRECO simulations over predicted the amount of mixing around the average radial zone interface position and therefore can be treated as a conservative upper bound when used in neutronics analysis. Residence time distributions from the GRECO velocity data based on the Monte Carlo algorithm closely matched those derived from the experiment velocity statistics. GRECO simulation results for PREX 3.1 with coupled drag forces showed larger errors compared to the experimental data, particularly in the cases with cross fluid flow. The large discrepancies suggest that GRECO results in systems with coupled fluid drag forces cannot be used with high confidence at this point and future development work on coupled pebble and fluid dynamics with multidimensional fluid flow fields is required.</p>

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.