Structure Of Unstable Nuclei Research Articles

Towards the drip lines of the nuclide landscape

In the paper we review the recent progress of studies in unstable nuclei, mainly affiliated with the facilities of radioactive ion beams in China, including both experimental and theoretical aspects of researches. Many experiments for reactions, decays and structures have been performed targeting better understandings of properties of unstable nuclei. Special experimental measurements related to nuclear astrophysics have been done to seek insights into the processes of syntheses of elements in the universe. Theoretical calculations have provided many useful predictions on the behaviors of unstable nuclei, with model developments. Studies covered many mass regions from light to superheavy nuclei, giving plenty of information about the structures of unstable nuclei, towards the limits of existence of atomic nuclei.

Inelastic Scattering of 32Mg at 190 MeV/Nucleon from a Thick Proton Target

The first excited 2+ state (885 keV) of 32Mg and its corresponding deformation parameter β2 are studied with proton inelastic scattering at 190 MeV/nucleon using in-beam γ technology with a thick target. Angular distributions of the inelastic scattering cross sections are analyzed with coupled-channel calculations. The deformation parameter β2 = 0.41(3) obtained in this work agrees with the earlier experimental results obtained at lower beam energies. Our work demonstrates that the structure of unstable nuclei can be studied with proton inelastic scattering at high incident energies in the vicinity of 200 MeV/nucleon using thick targets.

Structure of unstable nuclei in the sd-pf shell region by shell model with proper tensor force

Focusing on the importance of the tensor force in the effective interaction, we investigate the structure of unstable nuclei around N=28 with large-scale shell-model calculations. From the analysis of the spin-tensor decomposition for some interactions, the tensor force in the effective interaction should be close to the π+ ρ force, whereas it is much weaker in the Millener-Kurath (MK) interaction which is often used as the cross-shell interaction. The significance of the tensor force appears in the structure around 42Si: the proper tensor force predicts that it is deformed contrary to the result from MK.

Structure of unstable nuclei from nuclear moments and β decays

The structure of neutron-rich nuclei around the N = 20 magic number is studied through the measurements of static electromagnetic moments. Recent data on the magnetic dipole moments and electric quadrupole moments for neutron-rich Al isotopes are discussed to clarify whether the emergence of a strong deformation reported for Z = 10−12 nuclei in this region might happen also in these Z = 13 nuclei. Possibility to study the nuclear structure via the β transition matrix elements is also discussed.

Measurements with radioactive ion beams: progress and prospects

Radioactive nuclei play an important role in many astrophysical phenomena. Beams of radioactive ions allow the structure of unstable nuclei to be studied and cross sections for reactions of astrophysical interest to be directly measured. Some recent results obtained with radioactive ion beams that are of interest to nuclear astrophysics are surveyed. The capabilities of some radioactive ion beam facilities and prospects for future work are also briefly discussed.

Implication of proton-nucleus scattering for density distributions of unstable nuclei

The use of elastic proton scattering at intermediate and high energies to obtain information about the density distributions of unstable nuclei is investigated. A comparison between the relativistic impulse approximation (RIA) and Glauber model for proton scattering from 16O, 40Ca, 44Ca and 48Ca at medium energies is performed. We used density distributions derived from the relativistic mean-field theory, employing the recent relativistic force NL-RA1, as well as experimental and phenomenological densities. It is found that the eikonal approximation can describe the cross-section only at small scattering angles and is weakly sensitive to the density distributions, while the RIA nicely produced the experimental cross-sections, even at medium and larger angles, and was very sensitive to the nuclear densities. Furthermore, the RIA better describes the isospin dependence of the cross-section. We used the RIA to investigate the density distribution of 58Ca for proton scattering at different energies. It is found that the cross-section strongly depends on the parameters of the density distribution even at a small scattering angle. These results are important in extracting information about the structure of unstable nuclei. We also investigated the RIA and its sensitivity in describing halo nuclei such as 6He. We used for 6He a no-halo Gaussian density and a realistic-halo density that derived in the cluster orbital shell model approximation and contains the extended distribution of the valence nucleons. Comparison with the recent experimental data at GSI at 717 MeV/nucleon shows that the RIA successfully described the data at all considered range of the momentum transfer and on the other hand favor the halo structure of 6He.

Vanishing of the N = 20 Magic Number Studied by the Monte Carlo Shell Model

The structure of unstable nuclei around N = 20 is studied theoretically by the Monte Carlo shell model (MCSM). We focus mainly upon the vanishing of the N = 20 magic structure in this region. The binding energies, the energy levels, and the B(E2) values are well reproduced by the MCSM, and some of them are predicted successfully. We discuss the mechanism of the vanishing in terms of the effective single-particle energy, pointing out that the strong T = 0 monopole interaction between the 0d5/2 and 0d3/2 plays an important role on the vanishing of the N = 20 magic number and the appearance of the new N = 16 one. The location of F drip line is discussed along the lines.

Isospin Symmetry in the Structure Study of Unstable Nuclei

Assuming isospin T is a good quantum number, the analog states of the low-lying states in T z = ′ (T 0 +1) nuclei can be found as excited T 0 +1 states in T z = ′T 0 nuclei. Therefore, if To + 1 states are identified in theexcited region of T z = ′T 0 nuclei, the structure of unstable nuclei with T z = ′(T 0 + 1) can be deduced. We measured and compared the transition strengths to pairs of excited analog states in inelastic and charge-exchange reactions on the same target nucleus. The T values of these analog states were identified by using the fact that the transition strengths to them are different depending on T. The identification of T = ½ and 3/2 nature of analog M1 and GT states in the mirror nuclei pair 2 7 Al and 2 7 Si is reported by comparing the transition strengths in high-resolution 2 7 Al(p,p') and 2 7 Al( 3 He, t) 2 7 Si reactions at 0°. The identified T = 3/2 states are analogous to the low-lying states of 2 7 Mg, which plays an important role in rp-process of nucleosynthesis.

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.

Structure of Unstable Nuclei Studied with Number-Projected HFB Calculation

Structure of Unstable Nuclei Studied with Number-Projected HFB Calculation

Structure of unstable nuclei

The structure of unstable nuclei is discussed in terms of large-scale shell model calculations. First, the development and limitations of the conventional shell model calculations are mentioned. In order to overcome the limitations, the Quantum Monte Carlo Diagonalization (QMCD) method has been proposed. The basic formulation and features of the QMCD method are presented as well as its application to the nuclear shell model, as referred to as Monte Carlo Shell Model (MCSM). The MCSM provides us with a breakthrough in the shell model calculation: the structure of low-lying states can be studied with realistic interactions for a wide, nearly unlimited basically, variety of nuclei. Thus, the MCSM will contribute significantly to the physics to be developed by means of Radioactive Ion Beams. Applications to 56Ni, Ba isotopes and N ∼20 unstable nuclei far from the β stability line are mentioned. The mixing of normal, intruder and higher intruder states will be presented for N = 20 isotones, and 29F nucleus is shown to be bound due to such mixing.

Mean-field and shell-model approaches to deformation of unstable nuclei

The structure of unstable nuclei is discussed. First, the experimentally studied neutron skin of Na and Mg isotopes is compared to the Hartree-Fock calculations. The standard Skyrme Hartree-Fock calculation is shown to produce reasonable predictions as well as the relativistic mean field approaches. The deformation of 32Mg is revisited by comparing calculated results to the observed B(E2) value. It is suggested that this nucleus shows γ-unstable deformation to a certain extent, as a consequence of the N=20 closed shell breaking. We point out that the deformation is also an important aspect in reactions involving unstable nuclei. Finally, we present basic points and a recent result of the Quantum Monte Carlo Diagonalization method. The application is made for a large-scale shell model calculation for a proton-rich N=Z unstable nucleus 64Ge, suggesting its γ-unstable deformation.

Structure of unstable nuclei near proton drip line

Abstract 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 28 56 Ni 28 , 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).

Radiation-detected optical pumping in solids

Radiation-detected optical pumping in solids has been developed to investigate the structure of unstable nuclei. Appreciable nuclear polarization of implanted or doped unstable-nuclei in a host crystal is achieved with the optical pumping in solids. The nuclear polarization achieved is enhanced/reduced by applying a radio-frequency magnetic field together with the optical pumping, radiation-detected magnetic resonance being thus observed to get information on electromagnetic properties of unstable nuclei. Two schemes have been successfully developed for the optical pumping of unstable nuclei in solids. One is to directly pump the atoms by the excitation from the ground state to a broad absorption band in visible and UV regions which shows a large magnetic circular dichroism. This scheme is applicable to many rare-earth atoms in alkaline-earth fluoride host. The other scheme is to pump the electrons in the conduction band of direct-type semiconductor and thus indirectly polarize the nuclei in the host material via a hyperfine interaction between the nuclei and the polarized conduction electrons. This scheme can be especially applied to the III to VI families of atoms in direct-type semiconductors. Principle of the methods, on-line experimental system, and a few examples of the results obtained so far are presented and discussed.