Projected Shell Model Calculations Research Articles

High-spin structure and multiphononγvibrations in very neutron-richRu114

High-spin levels of the neutron-rich $^{114}\mathrm{Ru}$ have been investigated by measuring the prompt $\ensuremath{\gamma}$ rays in the spontaneous fission of $^{252}\mathrm{Cf}$. The ground-state band and one-phonon $\ensuremath{\gamma}$-vibrational band have been extended up to 14${}^{+}$ and 9${}^{+}$, respectively. Two levels are proposed as the members of a two-phonon $\ensuremath{\gamma}$-vibrational band. A back bending (band crossing) has been observed in the ground-state band at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}$ 0.40 MeV. Using the triaxial deformation parameters, the cranked shell model calculations indicate that this back bending in $^{114}\mathrm{Ru}$ should originate from the alignment of a pair of ${h}_{11/2}$ neutrons. Triaxial projected shell model calculations for the $\ensuremath{\gamma}$-vibrational band structures of $^{114}\mathrm{Ru}$ are in good agreement with the experimental data. However, when using the oblate deformation parameters, both of the above-calculated results are not in agreement with the experimental data.

Competition in rotation-alignment between high-jneutrons and protons in transfermium nuclei

The study of rotation-alignment of quasiparticles probes sensitively the properties of high-$j$ intruder orbits. The distribution of very-high-$j$ orbits, which are consequences of the fundamental spin-orbit interaction, links with the important question of single-particle levels in superheavy nuclei. With the deformed single-particle states generated by the standard Nilsson potential, we perform Projected Shell Model calculations for transfermium nuclei where detailed spectroscopy experiments are currently possible. Specifically, we study the systematical behavior of rotation-alignment and associated band-crossing phenomenon in Cf, Fm, and No isotopes. Neutrons and protons from the high-$j$ orbits are found to compete strongly in rotation-alignment, which gives rise to testable effects. Observation of these effects will provide direct information on the single-particle states in the heaviest nuclear mass region.

Abrupt change of rotation axis inAg109

The electromagnetic transition rates for all the high spin levels of the yrast sequence of {sup 109}Ag have been measured. The observed behavior of the magnetic dipole transition rates as a function of angular momentum establishes that there is a sudden change in rotation axis associated with rotational alignment of two neutrons. The projected shell model calculations give a consistent picture of the observed phenomena in {sup 109}Ag.

Neutron–proton pairing competition in [formula omitted] nuclei: Metastable state decays in the proton dripline nuclei 8241Nb and 8643Tc

The low-lying structures of the self-conjugate (N=Z) nuclei 8241Nb41 and 8643Tc43 have been investigated using isomeric-decay spectroscopy following the projectile fragmentation of a 107Ag beam. These represent the heaviest odd–odd N=Z nuclei in which internal decays have been identified to date. The resulting level schemes shed light on the shape evolution along the N=Z line between the doubly-magic systems 5628Ni and 10050Sn and support a preference for T=1 states in Tz=0 odd–odd nuclei at low excitation energies associated with a T=1 neutron–proton pairing gap. Comparison with Projected Shell Model calculations suggests that the decay in 82Nb may be interpreted as an isospin-changing K isomer.

SHELL MODEL FOR HEAVY NUCLEI AND ITS APPLICATION IN NUCLEAR ASTROPHYSICS

Performing shell model calculations for heavy nuclei is a long-standing problem in nuclear physics. The shell model truncation in the configuration space is an unavoidable step. The Projected Shell Model (PSM) truncates the space under the guidance of the deformed mean-field solutions. This implies that the PSM uses a novel and efficient way to bridge the two conventional methods: the deformed mean-field approximations, which are widely applied to heavy nuclei but able to describe the physics only in the intrinsic frame, and the spherical shell model diagonalization method, which is most fundamental but feasible only for small systems. We discuss the basic philosophy in construction of the PSM (or generally PSM-like) approach. Several examples from the PSM calculations are presented. Astrophysical applications are emphasized.

Delayed alignments in theN=Znuclei84Moand88Ru

The yrast band of the $N=Z$ nucleus ${}^{84}\mathrm{Mo}$ has been extended up to the ${10}^{+}$ state. Its moment of inertia varies smoothly up to this state (rotational frequency 0.6 MeV). The new data confirm the systematic delay of the particle alignment frequency in $N=Z$ nuclei with respect to the neighboring $N>Z$ nuclei, which has been suggested as a signature of the neutron-proton pairing interaction. Projected shell model calculations performed for the heaviest $N=Z$ nuclei studied so far, ${}^{84}\mathrm{Mo}$ and ${}^{88}\mathrm{Ru},$ predict that the confirmation of a possible enhancement of the neutron-proton residual interaction requires the observation of still higher spins in these nuclei.

First observation of excited states in theTz=1/2nucleus85Mo

Excited states in the ${T}_{z}=\frac{1}{2}$ nucleus ${}^{85}\mathrm{Mo}$ have been observed for the first time with the reaction ${}^{58}\mathrm{Ni}{(}^{32}\mathrm{S},\ensuremath{\alpha}n\ensuremath{\gamma})$ at 105 MeV. $\ensuremath{\gamma}$-ray transitions in this nucleus have been assigned unambiguously by combining the information from the GASP $\ensuremath{\gamma}$-ray array, the ISIS silicon ball, and the n-Ring neutron detector. Two band structures have been observed in this nucleus; they continue the smooth evolution of the known bands from the lighter $N=43$ isotones and have been tentatively assigned spins and parity on this basis. After reaching a maximum of collectivity at ${}^{83}\mathrm{Zr},$ the trend with increasing mass is reversed, showing a smaller collectivity at ${}^{85}\mathrm{Mo}.$ The rotational behavior of the observed bands is discussed on the basis of the projected shell model calculations.

High-spin states in neutron-rich Dy isotopes populated in deep-inelastic reactions

High-spin states in neutron-rich Dy isotopes, populated in deep-inelastic processes produced by the interaction of 234 MeV 37Cl ions with a 160Gd target, have been studied using the highly sensitive EUROBALL IV gamma-ray detector array. The previously known level schemes for 159,160,161,162Dy have been extended to significantly higher spin ( ? 30?) and the i13/2 band crossing in 159Dy has been observed for the first time. The experimental results are discussed within the framework of cranked shell model and projected shell model calculations with particular reference to the observed delayed band crossing in 162Dy.

Does an independent quasiparticle picture hold in mass-190 superdeformed nuclei?

We investigate the independent quasiparticle (QP) picture for superdeformed nuclei of the mass-190 region, which is expected to lead to additivity in certain physical quantities. Obvious deviations from additivity are found from both projected shell model calculations and experimental data. The cause of the deviation can be decomposed and identified through this model. Our study suggests that the independent QP picture may not be fully appropriate for this mass region.

Lack of additivity in mass-190 superdeformed bands

We investigate the independent quasiparticle picture for superdeformed nuclei of the mass-190 region, which is expected to lead to additivity in certain physical quantities. Obvious deviations from additivity are found from both experimental data and projected shell model calculations. The cause of the deviation can be decomposed and identified through this model. Our study suggests that the independent quasiparticle picture may not be fully appropriate for this mass region. {copyright} {ital 1998} {ital The American Physical Society}

Rotational bands in170Yband171Yband the projected shell model

Rotational bands in ${}^{170}\mathrm{Yb}$ and ${}^{171}\mathrm{Yb}$ have been studied with the reactions ${}^{170}\mathrm{Er}(\ensuremath{\alpha}{,xn)}^{174\ensuremath{-}x}\mathrm{Yb}$ at 35 and 40 MeV. Considerable extensions have been made to the previously published level schemes, and new structures have been found in each nucleus. Angular correlation measurements and reduced transition probability $[B(M1:\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}1)/B(E2:\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}2)]$ ratios have been extracted. Detailed comparisons are made with the first major projected shell model calculations for excited bands in an odd-$N$ nucleus ${}^{171}\mathrm{Yb}$.

πg9/2structures in odd-odd130Prand alignment processes in superdeformed praseodymium nuclei

Two strongly coupled bands with large dynamic moment of inertia values have been observed for the first time in the neutron-deficient odd-odd nucleus ${}^{130}$Pr using Gammasphere and the Microball charged-particle detector array. These bands involve the important ${g}_{9/2}$[404]9/2 proton orbital. This new information on ${}^{130}$Pr coupled with Projected Shell Model calculations provide valuable insight on rotational alignment processes taking place in this mass region, the size of the $N$=72 deformed shell gap, and the position of the ${i}_{13/2}$[660]1/2 neutron intruder orbital at the boundary of the $A$=135 superdeformed region.

Rotational structures in 177Ta.

High-spin states in $^{177}\mathrm{Ta}$ were produced using the $^{170}\mathrm{Er}$${(}^{11}$B, 4n) reaction at 55 and 60 MeV. Considerable extensions have been made to the previously known level scheme, and new structures have been found. B(M1)/B(E2) ratios have been extracted for strongly coupled bands. The behavior of the different rotational cascades, in particular the anomalous crossing frequency observed in the [541]1/${2}^{\mathrm{\ensuremath{-}}}$ proton ${\mathit{h}}_{9/2}$ band and the occurrence of ``identical bands,'' is discussed. Comparisons are made with projected shell model calculations.