Structure of unstable nuclei near proton drip line

Gamov-Teller (GT) states in nuclei near the proton drip line are studied by using a microscopic Hartree-Fock (H-F) + Tamm-Dancoff approximation (TDA) (or random phase approximation (RPA)). The calculations predict that giant Gamow-Teller (GT) β-decays are possible for N=Z nuclei heavier than ₂₈⁵⁶Ni₂₈, carrying most of the sum rule strength. The amplitude of isospin T=1 admixed to the T=0 ground state in N=Z nuclei is also discussed in relation with Fermi β-decay sum rule. Finally, the shapes of unstable nuclei near the proton drip line are studied by using the finite-range droplet model (FRDM).     

Laser spectroscopy and the properties of light nuclei near the neutron drip line

The interesting nuclear structure phenomena observed in some light nuclei at the neutron drip line suggest the measurement of basic ground state properties such as spins, magnetic moments and electric quadrupole moments. Here it will be discussed what experiments are presently feasable using laser spectroscopy. For the outstanding example of a halo nucleus,¹¹Li, the development of a technique combining -asymmetry detected optical pumping in a fast beam with NMR spectroscopy has yielded pertinent results, the most recent of which is an experimental value of the nuclear quadrupole moment.     

Specific features of the nuclear drip line in the region of light nuclei

The problem of the drip line in the region of the even isotopes ^4–12He, ^14–44O, and ^38–80Ca with respect to the emission of one or two neutrons is studied in the Hartree-Fock approximation by using the SLy4 and Ska effective Skyrme forces. The “stability peninsula” in the vicinity of ⁴⁰O is found.     

Study of drip line nuclei through two-step fragmentation

We have studied the structure of light neutron-rich nuclei around N = 16 by employing the in-beam -ray spectroscopy technique using the fragmentation of secondary beams of ^25,26Ne, ^27,28Na and ^29,30Mg isotopes. This secondary-beam cocktail was obtained by the fragmentation of a ³⁶S beam at 77.5 MeV by the SISSI/GANIL facility. By a second-step fragmentation, we have measured -ray-residue coincidences in ^17-20C and ^23,24O and described the obtained levels in the framework of the shell model.Received: 31 October 2002, Published online: 24 February 2004PACS: 23.20.Lv transitions and level energies - 21.60.Cs Shell model - 27.20. + n - 27.30. + t      

Studies with one- and two-proton drip line nuclei

In recent experiments at GANIL, we studied nuclei at the one- and two-proton drip line. The production rates allowed to search for direct 2p emission in the drip line nuclei ⁴²Cr, ⁴⁵Fe, and ⁴⁹Ni. No evidence for this decay mode was found in ⁴²Cr and ⁴⁹Ni, whereas the situation stays unclear for ⁴⁵Fe due to the limited statistics. In the medium-mass region ( A = 50-70), the half-life was measured for all proton-rich nuclei in the range T _z = 0 to T _z = - 1 between Z = 27 and Z = 36. First half-lives were determined for ⁶⁰Ga, ⁶²Ge, ⁶⁴As and ⁶⁶Se. Finally, β-decay studies of the 0⁺ → 0⁺ decay of ⁶²Ga have been performed at IGISOL in Jyv?skyl?. Non-analog transitions have been observed and are compared to theoretical predictions. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: blank@cenbg.in2p3.fr     

Recoil Beta Tagging: Application to the study of odd-odd near proton drip line nuclei, 74Rb and 78Y

We present a study of low-spin states in ⁷⁴Rb and ⁷⁸Y using the new technique of Recoil Beta Tagging. This yielded new information on ⁷⁴Rb and has provided the first evidence for non-isomeric T= 1 states in ⁷⁸Y.     

β-delayed proton decays near the proton drip line

We briefly reviewed the experimental study on β-delayed proton decays near the proton drip line published by our group during the period of 1996–2004, namely the first observation of the β-delayed proton decays of 9 new nuclides in the rare-earth region and the new measurements of β-delayed proton decays of 5 nuclides in the mass ∼90 region near theN=Z line with the aid of the “p-y” coincidence in combination with a He-jet tape transport system. In the meantime some important experimental technique details were supplemented. The experimental results, including the half-lives, spins, parities, deformations and production reaction cross sections for the 14 nuclei were summarized and compared with the current nuclear-model predictions, and then the following points were represented. (1) The experimental half-lives for⁸⁵Mo and⁹²Rh as well as the predicted “waiting point” nuclei⁸⁹Ru and⁹³Pd are 5–10 times longer than the theoretical predictions given by M?ller et al. using a macroscopic-microscopic model. It considerably influences the predictions of the abundances of the nuclides produced in the rp-process. (2) The current-model predictions are not consistent with the experimental assignments of the spins and parities for the proton drip-line nuclei¹⁴²Ho and¹²⁸Pm. However, the nuclear potential energy surface (PES) calculated by using a Woods-Saxon-Strutinsky method reproduced the experimental results. (3) The Alice code overestimated the production reaction cross sections of the studied 9 rare-earth nuclei by one order of magnitude or two, while HIVAP code overestimated them by one order of magnitude approximately.     

Nuclear structure near the drip line studied by RNB

The use of radioactive nuclear beams (RNB) has provided an opportunity to study nuclei far from the stability line. Reaction studies using intermediate and high-energy (30–1000 AMeV) radioactive beams have revealed new structures of nuclei that are not seen in nuclei near to the stability line [1]. One such new structure is the neutron halo, a long low-density tail of the neutron distribution [2]. It has been observed in neutron drip-line nuclei, such as⁶He,¹¹Li and¹¹Be. The neutron halo has put light on many new structure problems. Among those, recent studies of a softE1 mode, a low-frequency oscillation of halo neutrons against the core, are discussed in the following section in connection with a recent (p, p′) reaction measurement [3]. Also, the effect of changes in single-particle orbitals on halo formation is discussed [4]. Another new finding is the formation of neutron skins in neutron-rich unstable nuclei [5]. The relation between the equation-of-state (EOS) of asymmetric nuclear matter and the neutron-skin thickness as well as the density distribution of nuclei far from the stability line is discussed in the last section.     

Quadrupole strength function and core polarization in drip line nuclei

Using the self-consistent Hartree-Fock calculation plus RPA with Skyrme interactions, the RPA quadrupole strength function is estimated in the coordinate space, including simulataneously both the isoscalar and the isovector correlation. We discuss the result of the isoscalar, the isovector and the electric quadrupole polarization of the Ca-isotopes from the proton drip line towards the neutron drip line. We study also the comparison of the polarizations in the A = 48 mirror nuclei, ₂₈⁴⁸Ni₂₀ and ₂₀⁴⁸Ca₂₈, and the dependence of the polarizations of ₈²⁸O₂₀ on various Skyrme interactions.     

Hartree-Fock-Bogolyubov description of nuclei near the neutron-drip line

We consider the Hartree-Fock-Bogolyubov theory of nuclei in the coordinate representation and derive and solve the HFB equation for the Skyrme effective interaction. Ground-state wave functions and energies of the tin isotopes with 100 ? A ? 176 have been determined and the results have been compared with the predictions of the HF+BCS and macroscopic-microscopic models. The lightest tin isotope which is unstable with respect to a neutron emission is predicted by the HFB method to be ¹⁵³Sn. In the region of nuclei where experimental data are not available the macroscopic-microscopic and self-consistent approximations give substantially different results.     

Light nuclei near neutron and proton drip lines in relativistic mean-field theory

We have made a detailed study of the ground-state properties of nuclei in the light-mass region with atomic numbers Z = 10–22 in the framework of the relativistic mean-field (RMF) theory. The nonlinear σω model with scalar self-interaction has been employed. The RMF calculations have been performed in an axially deformed configuration using the force NL-SH. We have considered nuclei about the stability line as well as those close to proton and neutron drip lines. It is shown that the RMF results provide good agreement with the available empirical data. The RMF predictions also show reasonably good agreement with those of the mass models. It is observed that nuclei in this mass region are found to possess strong deformations and exhibit shape changes all along the isotopic chains. The phenomenon of shape coexistence is found to persist near the stability line as well as near the drip lines. It is shown that the magic number N = 28 is quenched strongly, thus enabling the corresponding nuclei to assume strong deformations. Nuclei near the neutron and proton drip lines in this region are also shown to be strongly deformed.     

Experimental studies of nuclei beyond the proton drip line by tracking technique

A novel experimental technique for measurements of in-flight decays of proton-unbound nuclei with pico-second lifetimes is described on the examples of the recent discovery of ¹⁹Mg and its two-proton (2p) radioactivity and the study of the reference 2p decay of the known isotope ¹⁶Ne . The method of measurements of 2p decays in flight by tracking all fragments with micro-strip detectors has also proven to be a potent tool for obtaining valuable spectroscopic information on exotic isotopes like ¹⁹Mg or ¹⁶Ne . Systematic studies of other 2p emitters predicted theoretically are foreseen with this powerful technique whose sensitivity is larger by factor of 20-30 in comparison with a conventional invariant-mass method. Information about the respective one-proton unbound subsystems can be obtained at the same time by evaluating proton-heavy-fragment correlations, which is illustrated on the example of the spectroscopy of ¹⁵F . This finding opens a way for systematic studies of exotic nuclei beyond the proton drip line, e.g., ⁶⁹Br . The properties of such nuclei may be important for the understanding of the element abundance in the Universe and may be used as input data for modeling the rp-process in various astrophysical sites.     

Deformation of nuclei close to the two-neutron drip line in Mg region

We present Hartree-Fock-Bogoliubov (HFB) calculations of the ground states of even Mg isotopes. A Skyrme force is used in the mean-field channel and a density-dependent zero-range force in the pairing channel. Our study shows that the ground states of^36,38,40Mg are strongly deformed with significantly different deformations for the neutrons and protons. Our study supports the disappearance of theN=28 shell gap in the Mg isotopes.     

Neutron-deficient and neutron-rich Fe and Ni isotopes near the drip line

Positions of the proton and neutron drip lines and characteristics of the neutron-deficient and neutron-rich Fe and Ni isotopes have been studied by the Hartree-Fock method with the Skyrme forces (Ska, SkM*, Sly4) with allowance for deformation. Pairing was taken into account on the basis of the BCS approximation with the pairing constant G = (19.5/2)[1 ± 0.51(N ? Z)/A]. Manifestation of magic numbers for the nickel isotopes ⁴⁸Ni, ⁵⁶Ni, and ⁷⁸Ni and for the nucleon-stable isotope ¹¹⁰Ni which is beyond the drip line is discussed.     

Charged pion electroproduction on light nuclei near the threshold

The experimental data for charged pion electroproduction on ¹²C and ¹⁶O measured at Mainz and Sendai are analyzed in a modified equivalent photon approximation. For the Sendai experiment we use distorted waves for the pion, while for the Mainz experiment we adopt a closure sum with a finite Fermi-gas model. We can roughly reproduce both sets of experimental data.     

Properties of Zr isotopes near the neutron drip line and beyond it

The properties of neutron-rich Zr isotopes up to the neutron drip line and beyond it have been investigated on the basis of the Hartree-Fock method with the Skyrme forces (Ska, Sly4), taking into account the deformation. By the example of chains of Zr isotopes, good agreement is shown for the two-neutron separation energies and mean-square radii with the known results of Hartree-Fock-Bogolyubov calculations with the Sly4 forces. For the extremely neutron-rich Zr isotopes, states with very large deformation parameters (β ≈ 0.4?0.45) of neutron and proton density distributions can be realized. Beyond the neutron drip line with respect to emission of two neutrons, the existence of ^150,152Zr isotopes, which are stable with respect to one-neutron emission, is predicted.     

β-delayed neutron decay of drip line nuclei 11Li and 14Be

Experimental studies of β-decay measurements of the neutron drip line nuclei ¹¹Li and ¹⁴Be are presented. β- decay schemes of these nuclei are determined by measuring β-rays, delayed neutrons and γ-rays in triple coincidence. The decay schemes of both ¹¹Li and ¹⁴Be associated with single neutron emission are unambiguously determined. New levels in their daughter nuclei, ¹¹Be and ¹⁴B, are found. In addition, the deduced level scheme of 14B indicates the lowering of 2s _1/2 single neutron orbital with respect to 1p _1/2 orbital in the N=9 isotones. Such behavior is known to exist in N=7.