Neutron Fermi Levels Research Articles

Mean-field analysis of ground-state and low-lying electric dipole strength inC22

Properties of neutron-rich $^{22}$C are studied using the mean-field approach with Skyrme energy density functionals. Its weak binding and large total reaction cross section, which are suggested by recent experiments, are simulated by modifying the central part of Skyrme potential. Calculating $E1$ strength distribution by using the random-phase approximation, we investigate developments of low-lying electric dipole ($E1$) strength and a contribution of core excitations of $^{20}$C. As the neutron Fermi level approaches the zero energy threshold ($\varepsilon_F >\sim -1$ MeV), we find that the low-lying $E1$ strength exceeds the energy-weighted cluster sum rule, which indicates an importance of the core excitations with the $1d_{5/2}$ orbit.

Low-energyE1strength in select nuclei: Possible constraints on neutron skin and symmetry energy

Correlations between low-lying electric dipole (E1) strength and neutron skin thickness are systematically investigated with the fully self-consistent random-phase approximation using the Skyrme energy functionals. The presence of strong correlation among these quantities is currently under dispute. We find that the strong correlation is present in properly selected nuclei, namely in spherical neutron-rich nuclei in the region where the neutron Fermi levels are located at orbits with low orbital angular momenta. The significant correlation between the fraction of the energy-weighted sum value and the slope of the symmetry energy is also observed. The deformation in the ground state seems to weaken the correlation.

Shell and Neutron-Skin Effects on Pygmy Dipole Resonances

We have performed a systematic calculation of low-lying dipole responses for even-even nuclei, up to zirconium isotopes including deformed nuclei within the fully self-consistent random phase approximation using the Skyrme interaction. We found the low-lying peaks in the dipole strength, so-called the pygmy dipole resonance (PDR), in every isotopic chain. The E1 strength of the PDR strongly depends on the position of the neutron Fermi level and shows a clear correlation with the occupation of the orbits with low orbital angular momenta (low-� ). We also found a strong correlation between the neutron skin thickness and the pygmy dipole strength in the region where the neutron Fermi levels are located at low-� orbits.

Multiparticle excitations in 194Pb

High-spin states of 194Pb have been populated by the reaction 184W( 16O, 6n) at 113 MeV beam energy and studied using the EUROGAM-1 array. The level scheme has been extended up to spin of 33 ℏ and excitation energy of 11.1 MeV. The high-spin part of the level scheme is dominated by bands of dipole transitions. New dipole bands have been observed and the configurations of their band heads are discussed. Besides the well-known coupling of the K π =11 − two-proton state to several νi 13/2 rotation-aligned neutron states, the K π =8 + two-proton state is also involved. Such a coupling is identified for the first time in a lead isotope. Moreover, the K π =11 − two-proton state is coupled to the rotation-aligned proton pair ( πh 11/2) 2. Thus many multiparticle configurations involving all the high- j subshells located around the proton and neutron Fermi levels, πh 11/2, πh 9/2, πi 13/2, and νi 13/2 have been identified in this nucleus.

IBFM2 description of odd Xe and Cs isotopes

Excitation energies, branching ratios and E2/M1 mixing ratios of the negative-parity states in 125,127Xe and 125,127,129Cs have been calculated by using the proton-neutron Interacting Boson Fermion Model (IBFM2). A good agreement with the experiment has been obtained. The sensitivity of the calculated observables to changes in model parameters is discussed and the results are compared with those obtained from the IBFM1. We examine a symmetry of the Hamiltonian with respect to the change of sign of the parameters χ π , χ ν and Г. The different behaviours of the odd Xe and Cs isotopes arise from the different position of the proton and neutron Fermi levels. In particular the exchange term of the Hamiltonian in 125,127,129Cs can be neglected.

A new type of band crossing at large deformation

A detailed study of the positive parity yrast and yrare rotational bands in 77Rb is presented. Using the reaction 40Ca( 40Ca, 3 p) and the EUROGAM I spectrometer, γγγ coincidences enabled us to follow both bands over a large spin range and to measure many E2 strengths. The moments of inertia and transition quadrupole moments indicate that a more deformed band ( ϵ 2 ≈ 0.38) is crossed by a less deformed one ( ϵ 2 ≈ 0.29). Since the yrare band starts at the extremely low spin value of I = 9 2 , the conventional band crossing mechanism of two aligning high- j quasiparticles is excluded. The frequency-dependent equilibrium shapes were calculated with Nilsson-Strutinsky type calculations using a diabatic tracing of configurations near the neutron Fermi level. This is the first observation of such a band crossing.

128,130,132,134La in the axially symmetric rotor model.

Recent experimental data on positive-parity yrast states, based on the {pi}{ital h}{sub 11/2}{direct_product}{nu}{ital h}{sub 11/2} configuration, of odd-odd {sup 128,130,132,134}La are compared in a systematic way with the energies and transition probabilities calculated within the framework of the two quasiparticle, axially symmetric core model. It is found that very good agreement is obtained in {sup 128,130}La assuming prolate deformation. Influence of the neutron Fermi level on the level properties in these two nuclei has also been studied. In the case of {sup 132,134}La, this model seems to indicate that this positive parity yrast band may have an oblate deformation.

Low-lying isovector monopole resonances

Considering the equality of the proton and neutron Fermi levels as an indication of a phenomenological interaction between pairs of protons and neutrons, low-energy isovector monopole resonances are proved to appear in the superfluid nuclei. The required phenomenological interaction is presented as an isospin symmetry-breaking mean field for the four-particle interaction.