192 research outputs found
A new analysis of 14O beta decay: branching ratios and CVC consistency
The ground-state Gamow-Teller transition in the decay of 14O is strongly
hindered and the electron spectrum deviates markedly from the allowed shape. A
reanalysis of the only available data on this spectrum changes the branching
ratio assigned to this transition by seven standard deviations: our new result
is (0.54 \pm 0.02)%. The Kurie plot data from two earlier publications are also
examined and a revision to their published branching ratios is recommended. The
required nuclear matrix elements are calculated with the shell model and, for
the first time, consistency is obtained between the M1 matrix element deduced
from the analog gamma transition in 14N and that deduced from the slope of the
shape-correction function in the beta transition, a requirement of the
conserved vector current hypothesis. This consistency is only obtained,
however, if renormalized rather than free-nucleon operators are used in the
shell-model calculations. In the mirror decay of 14C a similar situation
occurs. Consistency between the 14C lifetime, the slope of the shape-correction
function and the M1 matrix element from gamma decay can only be achieved with
renormalized operators in the shell-model calculation.Comment: 9 pages; revtex4; one figur
An improved calculation of the isospin-symmetry-breaking corrections to superallowed Fermi beta decay
We report new shell-model calculations of the isospin-symmetry-breaking
correction to superallowed nuclear beta decay. The most important improvement
is the inclusion of core orbitals, which are demonstrated to have a significant
impact on the mismatch in the radial wave functions of the parent and daughter
states. We determine which core orbitals are important to include from an
examination of measured spectroscopic factors in single-nucleon pick-up
reactions. We also examine the new radiative-correction calculation by Marciano
and Sirlin and, by a simple reorganization, show that it is possible to
preserve the conventional separation into a nucleus-independent inner radiative
term and a nucleus-dependent outer term. We tabulate new values for the three
theoretical corrections for twenty superallowed transitions, including the
thirteen well-studied cases. With these new correction terms the corrected Ft
values for the thirteen cases are statistically consistent with one another and
the anomalousness of the 46V result disappears. These new calculations lead to
a lower average Ft value and a higher value of Vud. The sum of squares of the
top-row elements of the CKM matrix now agrees exactly with unitarity.Comment: 15 pages, 2 postscript figures, revtex
Time-odd triaxial relativistic mean field approach for nuclear magnetic moments
The time-odd triaxial relativistic mean field approach is developed and
applied to the investigation of the ground-state properties of light odd-mass
nuclei near the double-closed shells. The nuclear magnetic moments including
the isoscalar and isovector ones are calculated and good agreement with Schmidt
values is obtained. Taking F as an example, the splitting of the single
particle levels (around MeV near the Fermi level), the nuclear current,
the core polarizations, and the nuclear magnetic potential, i.e., the spatial
part of the vector potential, due to the violation of the time reversal
invariance are investigated in detail.Comment: 26 pages, 8 figures. PHYSICAL REVIEW C (accepted
New Cl(p,)Ar reaction rate for astrophysical rp-process calculations
The Cl(p,)Ar reaction rate is of potential importance
in the rp-process powering type I X-ray bursts. Recently Clement et al.
\cite{CBB04} presented new data on excitation energies for low lying proton
unbound states in Ar obtained with a new method developed at the
National Superconducting Cyclotron Laboratory. We use their data, together with
a direct capture model and a USD shell model calculation to derive a new
reaction rate for use in astrophysical model calculations. In particular, we
take into account capture on the first excited state in Cl, and also
present a realistic estimate of the remaining uncertainties. We find that the
Cl(p,)Ar reaction rate is dominated entirely by capture
on the first excited state in Cl over the whole temperature range
relevant in X-ray bursts. In the temperature range from 0.2 to 1 GK the rate is
up to a factor of 70 larger than the previously recommended rate based on shell
model calculations only. The uncertainty is now reduced from up to a factor of
1000 to a factor of 3 at 0.3-0.7 GK and a factor of 6 at 1.5 GK.Comment: To be published in Phys. Rev.
Shell-model calculations for the energy levels of the N=50 isotones with A=80â87
The detailed features of the calculated energy-level schemes and of the single-particle, orbit-occupancy properties of the low-lying levels of the N=50 isotones 80Zn, 81Ga, 82Ge, 83As, 84Se, 85Br, 86Kr, and 87Rb are presented and discussed. These results are obtained with a new effective Hamiltonian operator obtained empirically from an iterative fit to experimental energies taken from all experimentally studied (A=82â96) N=50 nuclei. The model space for the calculations consists of active 0f5/2, 1p3/2, 1p1/2, and 0g9/2 proton orbits relative to a nominal 78Ni core. This space is truncated internally by restricting the number of particles excited from the negative-parity orbits into the g9/2 orbit to be no greater than four. The typical structures predicted for these lighter N=50 isotones are found to be dominated by well-mixed combinations of fp-orbit configurations, with the g9/2 orbit playing a minor role in all but a few special cases. The model energy-level spectra are compared with existing experimental information, as are calculated spectroscopic factors for single-proton stripping and pickup reactions
Gamow-Teller transitions from 24Mg and its impact on the electron capture rates in the O + Ne + Mg cores of stars
Electron captures on nuclei play an important role in the collapse of stellar
core in the stages leading to a type-II supernova. Recent observations of
subluminous Type II-P supernovae (e.g. 2005cs, 2003gd, 1999br) were able to
rekindle the interest in 8 - 10 which develop O+Ne+Mg cores. We used the
proton-neutron quasiparticle random phase approximation (pn-QRPA) theory to
calculate the B(GT) strength for 24Mg \rightarrow 24Na and its associated
electron capture rates for incorporation in simulation calculations. The
calculated rates, in this letter, have differences with the earlier reported
shell model and Fuller, Fowler, Newman (hereafter F2N) rates. We compared
Gamow-Teller strength distribution functions and found fairly good agreement
with experiment and shell model. However, the GT centroid and the total GT
strength, which are useful in the calculation of electron capture rates in the
core of massive pre-supernova stars, lead to the enhancement of our rate up to
a factor of four compared to the shell model rates at high temperatures and
densities.Comment: 13 pages, 3 figure
Shell-model study of the N=82 isotonic chain with a realistic effective hamiltonian
We have performed shell-model calculations for the even- and odd-mass N=82
isotones, focusing attention on low-energy states. The single-particle energies
and effective two-body interaction have been both determined within the
framework of the time-dependent degenerate linked-diagram perturbation theory,
starting from a low-momentum interaction derived from the CD-Bonn
nucleon-nucleon potential. In this way, no phenomenological input enters our
effective Hamiltonian, whose reliability is evidenced by the good agreement
between theory and experiment.Comment: 7 pages, 11 figures, 3 tables, to be published in Physical Review
On the Strength of Spin-Isospin Transitions in A=28 Nuclei
The relations between the strengths of spin-isospin transition operators
extracted from direct nuclear reactions, magnetic scattering of electrons and
processes of semi-leptonic weak interactions are discussed.Comment: LaTeX, 8 pages, 1Postscript with figur
Towards the solution of the anomaly in shell-model calculations of muon capture
Recently many authors have performed shell-model calculations of nuclear
matrix elements determining the rates of the ordinary muon capture in light
nuclei. These calculations have employed well-tested effective interactions in
large scale shell-model studies. For one of the nuclei of interest, namely
Si, there exists recent experimental data which can be used to deduce
the value of the ratio by using the calculated matrix elements.
Surprisingly enough, all the abovementioned shell-model results suggest a very
small value () for , quite far from the PCAC prediction
and recent data on muon capture in hydrogen. We show that this rather
disturbing anomaly is solved by employing effective transition operators. This
finding is also very important in studies of the scalar coupling of the weak
charged current of leptons and hadrons.Comment: Revtex, 6 pages, 2 figs include
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