Shell structure of the near-dripline nucleus 23O

Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope 23O. The 22O fragments produced in one-nucleon removal from 23O at 938 MeV/nucleon in a carbon target were detected in coincidence with deexciting gamma rays. The widths of the longitudinal momentum distributions of the 22O fragments and the one-neutron removal cross sections were interpreted in the framework of a simple theoretical model which favors the assignment of Ipi = 1/2+ to the 23O ground state.     

Rearrangements in neutron shell closures far from stability

A survey of experimental results obtained at GANIL (Caen, Prance) on the study of the properties of very neutron-rich nuclei (Z = 6–20, A = 20–60) near the neutron drip line and resulting in an appearance of further evidence for the new magic number N = 16 is presented. Very recent data on mass measurements of neutron-rich nuclei at GANIL and some characteristics of binding energies in this region are discussed. Nuclear binding energies are very sensitive to the existence of nuclear shells and together with the measurements of instability of doubly magic nuclei ¹⁰He and ²⁸0 they provide information on changes in neutron shell closures of very neutron-rich isotopes. The behaviour of the two-neutron separation energies S_2n derived from mass measurements gives a very clear evidence for the existence of the new shell closure N = 16 for Z = 9 and 10 appearing between 2s_1/2 and ld_3/2 orbitals. This fact, strongly supported by the instability of C, N and O isotopes with N > 16, confirms the magic character of N = 16 for the region from carbon up to neon while the shell closure at N = 20 tends to disappear for Z ≤ 13. Decay studies of these hardly accessible short-lived neutron-rich nuclei from oxygen to silicon using the in-beam γ-ray spectroscopy are also reported.     

14O核中子闭壳效应的探讨

在引入了BCS理论的相对论平均场模型框架内,通过系统研究氧同位素偶偶核的单粒子能级间隔、单粒子能级占有概率比、对作用效应和粒子数涨落,比较了¹⁴O,²²O和²⁴O三个核的中子闭壳效应,最后从理论上预言丰质子核¹⁴O有着比丰中子核²²O和²⁴O更强的中子闭壳效应. 关键词： 相对论平均场模型 能级间隔 占有概率 粒子数涨落     

Experimental evidence of core modification in the near drip-line nucleus 23O

The longitudinal momentum (P(axially)) distributions of one- and two-neutron removal fragments ((21,22)O) of 23O from the reaction with a C target at 72 A MeV have been measured for the first time using a new direct time-of-flight method with nearly full acceptance for the breakup fragments. The unexpectedly narrow width of 21O ( 115 +/- 34 MeV/c in FWHM) is consistent with two neutrons occupying the 2s(1/2) orbital in 23O. This indicates modification of core (22O) structure for neutron halo-like sd shell nuclei near the drip line. This also suggests the lowering of the s orbital providing a justification for the N = 16 magic number.     

Pre-equilibrium neutron emission in the nucleon exchange transport model

We extend the nucleon exchange transport model to include the emission of pre-equilibrium neutrons as a result of nucleon transfer to unbound states. The treatment takes account of the heating of the two nuclei caused by the energy dissipation and the cooling due to the pre-equilibrium emission. Furthermore, the transferred nucleons may induce cascading in the receptor nucleus by means of direct two-body scattering with the resident nucleons, based on a reduced, Pauli-blocked, energy-dependent free NN cross section. Qualitative agreement with experimental data is achieved, but the absolute multiplicities are systematically underestimated by about a factor of two. The dependence of multiplicity on bombarding energy is reproduced. The energy spectra and their variation with angle are generally reproduced, although the yield of the highest energy particles is sometimes underestimated.     

Excited states populated via nucleon transfer in the reaction32S +208Pb

The population strengths of excited states in nuclei produced via transfer reactions in the 185 MeV³²S +²⁰⁸Pb reaction have been investigated by heavy-ion- coincidence techniques. The cross sections extracted from the spectra, have been analyzed in the framework of the Complex WKB approximation theory.     

Strange quarks in the nucleon and neutron electric dipole moment

Arguments in favour of a large value of the matrix element \(\left\langle {N\left| {\bar ss} \right|N} \right\rangle \) based on semiphenomenological data and low-energy theorems are given. As an application of these considerations, we study the contribution of strange quarks to the neutron electric dipole moment.     

Dynamics of the nucleon pairing and the alpha-correlation in the valence shell

In conventional theories, the origin of the pairing force and the formation of the superfluid state are treated separately. In real nuclei, however, the nucleon responsible for the attractive interaction is identical with the nucleon that stabilizes the superfluid state. Hence, the superfluid state modifies the attractive force, and this attractive force determines the superfluid state, leading to a feedback effect. In this paper, as a preliminary attempt, we incorporate the effective interaction and formation of the superfluid state into a common self-consistency scheme. Furthermore, the alpha-correlation is included into this scheme as well. Applying the Ginzburg-Landau formalism to this scheme, a dynamics of the nucleon pairing and alpha-correlation is studied.     

Selective population and neutron decay of an excited state of 23O

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of 23O at 2.8(1) MeV excitation energy which decays into the ground state of 22O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2+ hole state. This assignment and the recently observed 3/2+ particle state advance the understanding of 23O.     

Effect of nucleon correlations on pion condensation in neutron stars

It is pointed out that nucleon-nucleon correlations, play an important role in the pion condensation problem. Their effect is estimated by a simple analytical method. It is seen that they inhibit condensation by reducing the attraction a pion might otherwise feel in nuclear matter. Further the effect of correlations increases with nucleon density ?, offsetting the corresponding increase in the attraction felt by pions in free neutron matter.     

Quadrupole moment of the doubly closed shell +1 nucleon nucleus41Sc(Iπ=7/2−)

The nuclear electric quadrupole moment of⁴¹Sc(I^π=7/2⁻) was experimentally determined by use of the NMR detection in which the asymmetric β-ray distribution from spin polarized nuclei was monitored. The magnetic interaction of the state with high external magnetic field and the nuclear quadrupole interaction with the electric field gradient obtained in TiO₂ crystal were studied. The field gradient seen to the implanted⁴¹Sc was measured independently by the high field NMR detection on the stable isotope⁴⁵Sc located in the equivalent⁴¹Sc site with. |Q(⁴¹Sc;I^π=7/2⁻)|=(0.120±0.006) b was determined.     

N = 14 shell closure in 22O viewed through a neutron sensitive probe

To investigate the behavior of the N = 14 neutron gap far from stability with a neutron-sensitive probe, proton elastic and 2(1)+ inelastic scattering angular distributions for the neutron-rich nucleus 22O were measured using the MUr à STrip detector array at the Grand Accélérateur National d'Ions Lourds facility. A deformation parameter beta(p,p') = 0.26 +/- 0.04 is obtained for the 2(1)+ state, much lower than in 20O, showing a weak neutron contribution to this state. A microscopic analysis was performed using matter and transition densities generated by continuum Skyrme-Hartree-Fock-Bogoliubov and quasiparticle random phase approximation calculations, respectively. The ratio of neutron to proton contributions to the 2(1)+ state is found close to the N/Z ratio, demonstrating a strong N = 14 shell closure in the vicinity of the neutron drip line.     

The nucleon pair approximation(NPA) of the shell model

In this paper we discuss the nucleon pair approximation of the shell model,based on our recent results which include a brief introduction to its framework and validity.We exemplify this model by studies of low-lying states for odd-odd nuclei in the mass number A-210 region.     

Test of sum rules in nucleon transfer reactions

The quantitative consistency of nucleon transfer reactions as a probe of the occupancy of valence orbits in nuclei is tested. Neutron-adding, neutron-removal, and proton-adding transfer reactions were measured on the four stable even Ni isotopes, with particular attention to the cross section determinations. The data were analyzed consistently in terms of the distorted wave Born approximation to yield spectroscopic factors. Valence-orbit occupancies were extracted, utilizing the Macfarlane-French sum rules. The deduced occupancies are consistent with the changing number of valence neutrons, as are the vacancies for protons, both at the level of <5%. While there has been some debate regarding the true "observability" of spectroscopic factors, the present results indicate that empirically they yield self-consistent results.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N=14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N=16 shell closure in these isotopes, while indicating a disappearance of N=14 shell closure in Be and C isotopes.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N =14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N =16 shell closure in these isotopes, while indicating a disappearance of N =14 shell closure in Be and C isotopes.     

Charge transfer in the shell model

the mechanism of charge transfer is incorporated into the shell model in an effort to better describe the lattice dynamics of crystals of the zincblende structure. The long-wavelength aspects of the resulting new model are treated in detail. A preliminary application is made to GaAs.