3,122 research outputs found
Solution of two-center time-dependent Dirac equation in spherical coordinates: Application of the multipole expansion of the electron-nuclei interaction
A non-perturbative approach to the solution of the time-dependent, two-center
Dirac equation is presented with a special emphasis on the proper treatment of
the potential of the nuclei. In order to account for the full multipole
expansion of this potential, we express eigenfunctions of the two-center
Hamiltonian in terms of well-known solutions of the "monopole" problem that
employs solely the spherically-symmetric part of the interaction. When combined
with the coupled-channel method, such a wavefunction-expansion technique allows
for an accurate description of the electron dynamics in the field of moving
ions for a wide range of internuclear distances. To illustrate the
applicability of the proposed approach, the probabilities of the K- as well as
L- shell ionization of hydrogen-like ions in the course of nuclear alpha-decay
and slow ion-ion collisions have been calculated
Dynamical symmetry of isobaric analog 0+ states in medium mass nuclei
An algebraic sp(4) shell model is introduced to achieve a deeper
understanding and interpretation of the properties of pairing-governed 0+
states in medium mass atomic nuclei. The theory, which embodies the simplicity
of a dynamical symmetry approach to nuclear structure, is shown to reproduce
the excitation spectra and fine structure effects driven by proton-neutron
interactions and isovector pairing correlations across a broad range of nuclei.Comment: 7 pages, 5 figure
Measurement of the Hyperfine Structure and Isotope Shifts of the 3s23p2 3P2 to 3s3p3 3Do3 Transition in Silicon
The hyperfine structure and isotope shifts of the 3s23p2 3P2 to 3s3p3 3Do3
transition in silicon have been measured. The transition at 221.7 nm was
studied by laser induced fluorescence in an atomic Si beam. For 29Si, the
hyperfine A constant for the 3s23p2 3P2 level was determined to be -160.1+-1.3
MHz (1 sigma error), and the A constant for the 3s3p3 3Do3 level is -532.9+-0.6
MHz. This is the first time that these constants were measured. The isotope
shifts (relative to the abundant isotope 28Si) of the transition were
determined to be 1753.3+-1.1 MHz for 29Si and 3359.9+-0.6 MHz for 30Si. This is
an improvement by about two orders of magnitude over a previous measurement.
From these results we are able to predict the hyperfine structure and isotope
shift of the radioactive 31Si atom, which is of interest in building a scalable
quantum computer
Isospin symmetry breaking in an algebraic pairing Sp(4) model
An exactly solvable sp(4) algebraic approach extends beyond the traditional
isospin conserving nuclear interaction to bring forward effects of isospin
symmetry breaking and isospin mixing resulting from a two-body nuclear
interaction that includes proton-neutron (pn) and like-particle isovector
pairing correlations plus significant isoscalar pn interactions. The model
yields an estimate for the extent to which isobaric analog 0+ states in light
and medium mass nuclei may mix with one another and reveals possible, but still
extremely weak, non-analog beta-decay transitions.Comment: 8 pages, 2 figure
Coupling transport and biodegradation of VOCs in surface and subsurface soils.
Volatile organic chemicals present at Superfund sites preferentially partition into the soil gas and may be available for microbial degradation. A simple mass transfer model for biodegradation for volatile substrates has been developed for the aerobic decomposition of aromatic and aliphatic hydrocarbons. The mass transfer analysis calculates diffusive fluxes from soil gas through water and membrane films and into the cell. This model predicts an extreme sensitivity of potential biodegradation rates to the air-water partition coefficients of the compounds. Aromatic hydrocarbons are removed rapidly while the aliphatic hydrocarbons are much slower by orders of magnitude. Furthermore, oxygen transfer is likely to limit aromatic hydrocarbon degradation rates. The model presents results that cast doubt on the practicality of using methane or propane for the co-metabolic destruction of trichloroethylene in a gas phase bioreactor. Toluene as a primary substrate has better mass transfer characteristics to achieve more efficient trichloroethylene degradation. Hence, in sites where these contaminants coexist, bioremediation could be improved
A structural evaluation of the tungsten isotopes via thermal neutron capture
Total radiative thermal neutron-capture -ray cross sections for the
W isotopes were measured using guided neutron beams from
the Budapest Research Reactor to induce prompt and delayed rays from
elemental and isotopically-enriched tungsten targets. These cross sections were
determined from the sum of measured -ray cross sections feeding the
ground state from low-lying levels below a cutoff energy, E, where
the level scheme is completely known, and continuum rays from levels
above E, calculated using the Monte Carlo statistical-decay code
DICEBOX. The new cross sections determined in this work for the tungsten
nuclides are: b and
b;
b and b; b and
b; and,
b and b. These results are consistent with
earlier measurements in the literature. The W cross section was also
independently confirmed from an activation measurement, following the decay of
W, yielding values for that are consistent
with our prompt -ray measurement. The cross-section measurements were
found to be insensitive to choice of level density or photon strength model,
and only weakly dependent on E. Total radiative-capture widths
calculated with DICEBOX showed much greater model dependence, however, the
recommended values could be reproduced with selected model choices. The decay
schemes for all tungsten isotopes were improved in these analyses.Comment: 25 pages, 15 figures, 15 table
Heavy Superheated Droplet Detectors as a Probe of Spin-independent WIMP Dark Matter Existence
At present, application of Superheated Droplet Detectors (SDDs) in WIMP dark
matter searches has been limited to the spin-dependent sector, owing to the
general use of fluorinated refrigerants which have high spin sensitivity. Given
their recent demonstration of a significant constraint capability with
relatively small exposures and the relative economy of the technique, we
consider the potential impact of heavy versions of such devices on the
spin-independent sector. Limits obtainable from a -loaded SDD
are estimated on the basis of the radiopurity levels and backgrounds already
achieved by the SIMPLE and PICASSO experiments. With 34 kgd exposure,
equivalent to the current CDMS, such a device may already probe to below
10 pb in the spin-independent cross section.Comment: 9 pages, 4 figures, accepted Phys. Rev.
Phenomenological description of the states and in some even-even nuclei
A sixth-order quadrupole boson Hamiltonian is used to describe the states
and identified in several nuclei by various types of experiments.
Two alternative descriptions of energy levels are proposed. One corresponds to
a semi-classical approach of the model Hamiltonian while the other one provides
the exact eigenvalues. Both procedures yield close formulas for energies. The
first procedure involves four parameters, while the second involves a compact
formula with five parameters. In each case the parameters are fixed by a
least-square fit procedure. Applications are performed for eight even-even
nuclei.
Both methods yield results which are in a surprisingly good agreement with
the experimental data. We give also our predicted reduced transition
probabilities within the two approaches, although the corresponding
experimental data are not yet available.Comment: 27pages, 18 figure
Internal conversion coefficients for superheavy elements
The internal conversion coefficients (ICC) were calculated for all atomic
subshells of the elements with 104<=Z<=126, the E1...E4, M1...M4
multipolarities and the transition energies between 10 and 1000 keV. The atomic
screening was treated in the relativistic Hartree-Fock-Slater model. The Tables
comprising almost 90000 subshell and total ICC were recently deposited at LANL
preprint server.Comment: 6 pages including 3 figures, needs files myown.sty and epsfig.sty
(both included
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