131 research outputs found
Population of isomers in decay of the giant dipole resonance
The value of an isomeric ratio (IR) in N=81 isotones (Ba, Ce,
Nd and Sm) is studied by means of the ( reaction.
This quantity measures a probability to populate the isomeric state in respect
to the ground state population. In ( reactions, the giant dipole
resonance (GDR) is excited and after its decay by a neutron emission, the
nucleus has an excitation energy of a few MeV. The forthcoming decay
by direct or cascade transitions deexcites the nucleus into an isomeric or
ground state. It has been observed experimentally that the IR for Ba
and Ce equals about 0.13 while in two heavier isotones it is even less
than half the size. To explain this effect, the structure of the excited states
in the energy region up to 6.5 MeV has been calculated within the Quasiparticle
Phonon Model. Many states are found connected to the ground and isomeric states
by , and transitions. The single-particle component of the wave
function is responsible for the large values of the transitions. The calculated
value of the isomeric ratio is in very good agreement with the experimental
data for all isotones. A slightly different value of maximum energy with which
the nuclei rest after neutron decay of the GDR is responsible for the reported
effect of the A-dependence of the IR.Comment: 16 pages, 4 Fig
Quadrupole Moments of Neutron-Deficient Na
The electric-quadrupole coupling constant of the ground states of the proton
drip line nucleus Na( = 2, = 447.9 ms) and the
neutron-deficient nucleus Na( = 3/2, = 22.49 s)
in a hexagonal ZnO single crystal were precisely measured to be kHz and 939 14 kHz, respectively, using the multi-frequency
-ray detecting nuclear magnetic resonance technique under presence of an
electric-quadrupole interaction. A electric-quadrupole coupling constant of
Na in the ZnO crystal was also measured to be
kHz. The electric-quadrupole moments were extracted as Na) = 10.3
0.8 fm and Na) = 14.0 1.1 fm, using
the electric-coupling constant of Na and the known quadrupole moment of
this nucleus as references. The present results are well explained by
shell-model calculations in the full -shell model space.Comment: Accepted for publication in Physics Letters
Isotope shift calculations for atoms with one valence electron
This work presents a method for the ab initio calculation of isotope shift in
atoms and ions with one valence electron above closed shells. As a zero
approximation we use relativistic Hartree-Fock and then calculate correlation
corrections. The main motivation for developing the method comes from the need
to analyse whether different isotope abundances in early universe can
contribute to the observed anomalies in quasar absorption spectra. The current
best explanation for these anomalies is the assumption that the fine structure
constant, alpha, was smaller at early epoch. We test the isotope shift method
by comparing the calculated and experimental isotope shift for the alkali and
alkali-like atoms Na, MgII, K, CaII and BaII. The agreement is found to be
good. We then calculate the isotope shift for some astronomically relevant
transitions in SiII and SiIV, MgII, ZnII and GeII.Comment: 11 page
Ionization balance of Ti in the photospheres of the Sun and four late-type stars
In this paper we investigate statistical equilibrium of Ti in the atmospheres
of late-type stars. The Ti I/Ti II level populations are computed with
available experimental atomic data, except for photoionization and collision
induced transition rates, for which we have to rely on theoretical
approximations. For the Sun, the NLTE line formation with adjusted H I
inelastic collision rates and MAFAGS-OS model atmosphere solve the
long-standing discrepancy between Ti I and Ti II lines. The NLTE abundances
determined from both ionization stages agree within dex with each other
and with the Ti abundance in C I meteorites. The Ti NLTE model does not perform
similarly well for the metal-poor stars, overestimating NLTE effects in the
atmospheres of dwarfs, but underestimating overionization for giants.
Investigating different sources of errors, we find that only [Ti/Fe] ratios
based on Ti II and Fe II lines can be safely used in studies of Galactic
chemical evolution. To avoid spurious abundance trends with metallicity and
dwarf/giant discrepancies, it is strongly recommended to disregard Ti I lines
in abundance analyses, as well as in determination of surface gravities.Comment: 16 pages, accepted for publication in MNRA
Atomic transition frequencies, isotope shifts, and sensitivity to variation of the fine structure constant for studies of quasar absorption spectra
Theories unifying gravity with other interactions suggest spatial and
temporal variation of fundamental "constants" in the Universe. A change in the
fine structure constant, alpha, could be detected via shifts in the frequencies
of atomic transitions in quasar absorption systems. Recent studies using 140
absorption systems from the Keck telescope and 153 from the Very Large
Telescope, suggest that alpha varies spatially. That is, in one direction on
the sky alpha seems to have been smaller at the time of absorption, while in
the opposite direction it seems to have been larger.
To continue this study we need accurate laboratory measurements of atomic
transition frequencies. The aim of this paper is to provide a compilation of
transitions of importance to the search for alpha variation. They are E1
transitions to the ground state in several different atoms and ions, with
wavelengths ranging from around 900 - 6000 A, and require an accuracy of better
than 10^{-4} A. We discuss isotope shift measurements that are needed in order
to resolve systematic effects in the study. The coefficients of sensitivity to
alpha-variation (q) are also presented.Comment: Includes updated version of the "alpha line" lis
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