Unstable Oxygen Isotopes Research Articles

Mean field and beyond in nuclei far from stability lines

We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14) in a Hartree-Fock basis. The resulting gap accounts for about half of the experimental gap. Going beyond the mean field in the particle-particle channel, the combined effect of the bare nucleon-nucleon potential and of the induced pairing interaction arising from the exchange of low-lying surface vibrations between nucleons moving in time reversal states close to the Fermi energy accounts for the experimental gap. Examples for light, halo nuclei are also reported. The more studied effects of the particle-vibration coupling in the particle-hole channel are discussed for the low-lying quadrupole vibration in 120Sn and the giant dipole resonance in the unstable oxygen isotopes and 132Sn.

Quadrupole resonances in unstable oxygen isotopes in time-dependent density-matrix formalism

The strength functions of quadrupole modes in the unstable oxygen isotopes 22O and 24O are calculated using an extended version of the time-dependent Hartree–Fock theory known as the time-dependent density-matrix theory (TDDM). It is found that TDDM gives the lowest quadrupole states which are energetically shifted upward and become significantly collective due to the coupling to two-body configurations. It is pointed out that these features of the lowest quadrupole states are similar to those obtained in the quasi-particle random phase approximation.

A Hartree-Fock Calculation with Yukawa Interaction

An effective interaction for the mean-field calculations is explored from a semimicroscopic standpoint. The M3Y interaction is modified so as to reproduce the saturation density, energy and the LS splittings. This semi-microscopic interaction describes the properties of the symmetric nuclear matter reasnoably well. This interaction is also applied to the Hartree-Fock calculations for the stable and unstable oxygen isotopes by using the recently proposed numerical method. Recent experimental data have disclosed the exotic structure of unstable nuclei. 1) We have halos or skins in several nuclei close to the neutron drip line. It has also been clarified that the well-known magic numbers in the β-stability can disappear and new magic numbers may appear in the neutron-rich region. These observed phenomena cast some quests on the nuclear mean-field and the single-particle orbits built on it. The existence of the halos indicates the importance of the wavefunction asymptotics, which is not easy to be handled in numerical calculations. The effective interactions for the mean-field calculations have been developed so as to reproduce the properties of the nuclei in and around the β-stability. However, depending on the parameters, these interactions behave differently from one another in the neutron-rich region. In order to investigate the effective interaction for the neutron-rich nuclei, microscopic interactions would provide us with a guidance. In this regard, the so-called M3Y interaction, 2) which is derived from the bare NN interaction and still have a tractable form, seems to give a suitable starting point. The M3Y interaction has been successfully applied to the low-energy nuclear reactions with slight modification, and is also known to be moderately good as a shell-model interaction. We have recently proposed a new method to implement the Hartree-Fock (HF) calculations. 3) In this method we can handle finite-range interactions even with the Yukawa form, and the wave-function asymptotics can be properly reproduced. In this work we discuss modification of the M3Y interaction from the mean-field viewpoints, and apply the modified interaction to stable and unstable oxygen isotopes.

Pigmy and Giant Dipole States in Stable and Unstable Oxygen Isotopes

We have studied dipole states of oxygen isotopes in large scale shell model calculations. The calculated photoreaction cross sections in 16O, 17O and 18O give reasonable agreement with experimental observations both in the low energy region below ħω=15 MeV and in the high energy giant resonance region (15 MeV < ħω ≤ 30 MeV). We found that the transition strength below dipole giant resonance (ħω ≤ 15 MeV) exhausts about 10% of the classical Thomas-Reiche-Kuhn sum rule and more than 40% of the cluster sum rule in heavier oxygen isotopes than 17O. The predicted Pigmy strengths in 20O and 22O are also confirmed by recent Coulomb excitation experiment at GSI.

Giant resonances in unstable oxygen isotopes

Electromagnetic and nuclear breakup of the neutron-rich Oxygen isotopes ranging from A = 17 to A = 22 is studied experimentally in reactions at energies around 600 MeV/u. The beams were produced in fragmentation reactions and separated by the GSI Fragment Separator FRS. By measuring the four-momenta of all decay products after inelastic scattering and neutron decay of the projectile, the excitation energy is determined. From the differential cross sections dσ dE ∗ for electromagnetic excitation, the E1-strength distributions can be deduced. For 18,20,22O, low-lying dipole strength is observed, exhausting about 5% of the Thomas Reiche Kuhn sumrule for energies up to 5 MeV above the continuum threshold.