1,163 research outputs found
The isovector effective charge and the staggering of the 2+ to 0+ transition probabilities in the Titanium isotopes
In an effort to understand the magical status of N=32 and N=34 at the very
neutron rich edge, experiments have been carried out in the Titanium isotopes
up to A=56. The measured staggering of the B(E2)'s is not reproduced by the
shell model calculations using the best effective interactions. We argue that
this may be related to the choice of the isovector effective charge and to the
value of the N=34 neutron gap.Comment: 2 pages, 2 figure
Lifetimes of 26Al and 34Cl in an astrophysical plasma
We study here the onset of thermal equilibrium affecting the lifetimes of
26Al and 34Cl nuclei within a hot astrophysical photon gas. The 26Al isotope is
of prime interest for gamma ray astronomy with the observation of its delayed
(t_1\2=0.74 My) 1.809MeV gamma-ray line. Its nucleosynthesis is complicated by
the presence of a short lived (t_1\2=6.34s) spin isomer. A similar
configuration is found in 34Cl where the decay of its isomer (34mCl, t_1\2=32m)
is followed by delayed gamma-ray emission with characteristic energies. The
lifetimes of such nuclei are reduced at high temperature by the thermal
population of shorter lived levels. However, thermal equilibrium within 26Al
and 34Cl levels is delayed by the presence of the isomer. We study here the
transition to thermal equilibrium where branching ratios for radiative
transitions are needed in order to calculate lifetimes. Since some of these
very small branching ratios are not known experimentally, we use results of
shell model calculations.Comment: 11 pages, 5 figures, Latex, accepted for publication in Phys. Rev.
Shape Coexistence in 78 Ni and the new Island of Inversion
Large Scale Shell Model calculations (SM-CI) predict that the region of
deformation which comprises the heaviest Chromium and Iron isotopes at and
beyond N=40 will merge with a new one at N=50 in an astonishing parallel to the
N=20 and N=28 case in the Neon and Magnesium isotopes. We propose a valence
space including the full pf-shell for the protons and the full sdg shell for
the neutrons; which represents a comeback of the the harmonic oscillator shells
in the very neutron rich regime. The onset of deformation is understood in the
framework of the algebraic SU3-like structures linked to quadrupole dominance.
Our calculations preserve the doubly magic nature of the ground state of 78 Ni,
which, however, exhibits a well deformed prolate band at low excitation energy,
providing a striking example of shape coexistence far from stability
A Shell Model Description of the Decay Out of the Super-Deformed Band of 36Ar
Large scale shell model calculations in two major oscillator shells (sd and
pf) describe simultaneously the super-deformed excited band of 36Ar and its
low-lying states of dominant sd character. In addition, several two particle
two hole states and a side band of negative parity are also well reproduced. We
explain the appearance of the super-deformed band at such low excitation energy
as a consequence of the very large correlation energy of the configurations
with many particles and many holes (np-nh) relative to the normal filling of
the spherical mean field orbits (0p-0h). We study the mechanism of mixing
between these different configurations, to understand why the super-deformed
band survives and how it finally decays into the low-lying sd-dominated states
via the indirect mixing of the 0p-0h and 4p-4h configurations.Comment: 4 pages 5 figures, revtex4, revised version, minor change
Nuclear shell evolution and in-medium NN interaction
We report on a quantitative study of the evolution of the nuclear shell structure, in particular, effective single-particle energies (ESPEs), based on the spin-tensor decomposition of an effective two-body shell-model interaction. While the global trend of the ESPEs is mainly due to the central term of the effective interaction, variations of shell gaps invoke various components of the in-medium NN force. From a detailed analysis of a well-fitted realistic interaction in the sdpf shell-model space, two most important contributions for the evolution of the N = 20 and N = 28 shell gaps are confirmed to be the central term and the tensor term. The role of the latter is dominant to explain the energy shift of spin-orbit partners. Spin-tensor analysis of microscopic effective interactions in sd, pf, and gds shell-model spaces, contrasted with that of the phenomenologically adjusted ones, shows no evidence of amplification of the tensor component contribution; however, it points toward the neglect of three-body forces in the present microscopic interactions
The crystal structure of rathite-I
Die Kristallstruktur von Rathit-I wurde mittels dreidimensionaler Intensitätsdaten bestimmt. Vier Formeleinheiten (Pb,Tl)3As4(As,Ag)S10 sind in der Einheitszelle der Symmetrie Ρ 21/a mit a = 25,16 Å, 6 = 7,94 Å, c = 8,47 Å, β = 100° 28' enthalten. Die wahre Symmetrie von Rathit-I ist möglicherweise triklin. Die Lösung lieferten die Ähnlichkeit der Struktur mit derjenigen von Rathit-III und spezielle Verhältnisse der Röntgendiagramme. Von drei unabhängigen Pb(Tl)-Atomen sind zwei von neun S-Atomen umgeben, das andere von sieben. Die As-Atome weisen trigonal-pyramidale Koordination durch die S-Atome auf. Von einem As-Atom wird angenommen, daß es statistisch von zwei verschiedenen trigonal -pyramidalen S-Koordinationen umgeben wird. Ein anderes As-Atom ist teilweise durch Ag ersetzt. Die Struktur besteht aus zweierlei Schichten parallel zu (100). Die erste Art hat die Zusammensetzung (Pb,Tl)S3 und besteht aus den Koordinationspolyedern um die Pb(Tl)-Atome mit Neuner-Koordination. Die zweite Art ist aus Pb(Tl)-, As(Ag)- und S-Atomen zusammengesetzt, welche ein deformiertes PbS-Gitter bilden. Trigonale As-S3-Pyramiden sind zu Ketten endlicher Länge vereinig
Nilsson-SU3 selfconsistency in heavy N=Z nuclei
It is argued that there exist natural shell model spaces optimally adapted to
the operation of two variants of Elliott' SU3 symmetry that provide accurate
predictions of quadrupole moments of deformed states. A selfconsistent
Nilsson-like calculation describes the competition between the realistic
quadrupole force and the central field, indicating a {\em remarkable stability
of the quadruplole moments}---which remain close to their quasi and pseudo SU3
values---as the single particle splittings increase. A detailed study of the
even nuclei from Ni to Cd reveals that the region of
prolate deformation is bounded by a pair of transitional nuclei Kr and
Mo in which prolate ground state bands are predicted to dominate, though
coexisting with oblate ones,Comment: Replacement I) Title simplified. II) Major revision: structure of
paper kept but two thirds totally rewritten (same number of pages); 20
references adde
The Nuclear Shell Model Toward the Drip Lines
We describe the "islands of inversion" that occur when approaching the
neutron drip line around the magic numbers N=20, N=28 and N=40 in the framework
of the Interacting Shell Model in very large valence spaces. We explain these
configuration inversions (and the associated shape transitions) as the result
of the competition between the spherical mean field (monopole) which favors
magicity and the correlations (multipole) which favor deformed intruder states.
We also show that the N=20 and N=28 islands are in reallity a single one, which
for the Magnesium isotopes is limited by N=18 and N=32.Comment: Proceedings of the Nordic Conference in Nuclear Physics 2011,
Stockholm, to appear in Physica Script
Coexistence of spherical states with deformed and superdeformed bands in doubly magic 40-Ca; A shell model challenge
Large scale shell model calculations, with dimensions reaching 10**9, are
carried out to describe the recently observed deformed (ND) and superdeformed
(SD) bands based on the first and second excited 0+ states of 40-Ca at 3.35-MeV
and 5.21-MeV respectively. A valence space comprising two major oscillator
shells, sd and pf, can accommodate most of the relevant degrees of freedom of
this problem. The ND band is dominated by configurations with four particles
promoted to the pf-shell (4p-4h in short). The SD band by 8p-8h configurations.
The ground state of 40-Ca is strongly correlated, but the closed shell still
amounts to 65%. The energies of the bands are very well reproduced by the
calculations. The out-band transitions connecting the SD band with other states
are very small and depend on the details of the mixing among the different
np-nh configurations, in spite of that, the calculation describes them
reasonably. For the in-band transition probabilities along the SD band, we
predict a fairly constant transition quadrupole moment Q_0(t)~170 e fm**2 up to
J=10, that decreases toward the higher spins. We submit also that the J=8
states of the deformed and superdeformed band are maximally mixed.Comment: 12 pages, 9 figure
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