A Watt

A shell-model investigation of the binding energies of some exotic isotopes of sodium and magnesium

Standard shell-model calculations of the binding energies of the neutron-rich isotopes of sodium and magnesium are in strong disagreement with the experimental values near N=20. The authors show that the discrepancy can be explained by allowing neutron excitations from the d3/2 shell into the f7/2 shell.

Crossing of single-particle energy levels resulting from neutron excess in the sd shell

It has been shown that the large discrepancies between calculation and experiment in the binding energies of neutron-rich isotopes of sodium and magnesium can be removed by extending the model space to include the f7/2 shell. It is shown that the physical explanation of this result lies in the severe distortion of the underlying single-particle spectrum by the large neutron excess.

Quark-model contributions to effective nuclear forces are repulsive

Abstract The contribution of direct interactions between quarks is found to produce a moderate repulsion in the effective nucleon-nucleon force at short distances. Previously published results which apparently contradict this conclusion are shown to be erroneous. Nuclear shell-model techniques are used in the calculation and channel coupling to hidden-colour states is found to be much more significant than coupling to Δ-isobar states.

Electromagnetic properties of the 01+, 21+, 41+ states in 20Ne, 24Mg, 28Si, 32S☆

Abstract The 0 + , 2 + , 4 + sequence in the 4 N nuclei in the 1s-0d shell has been examined by comparing the results of a large-basis shell-model calculation with electron scattering data. While the ground state is well described by the shell-model wave function there is evidence that extra s-d space configurations play an important role in defining the radial shapes of inelastic transition densities. For 0 + → 4 + transitions, mixing of the Og shell is important. The Chung-Wildenthal interaction shows erroneous behaviour in the description of some of the electromagnetic properties of these states.

Shell-model calculations in the sd shell. X. Termination of rotational bands

For pt.IX see Phys. Lett., vol.63B, p.385 (1976). It is shown that the shell-model states belonging to a given rotational band have very similar subshell occupancies which are interpreted as indicating similar intrinsic structures. Subshell occupancies can be used to help decide whether or not a state belongs to a given band and hence to determine the upper limits of rotational bands. In the sd shell, the prediction that no rotational bands extend beyond J=10 is made, and in many cases when one might expect them, rotational features are absent.

Elastic electron scattering from 23Na, 25Mg and 27Al and a shell-model interpretation

Elastic electron scattering cross sections for odd-A nuclei in the 1s-0d shell have been analysed in terms of large-basis shell-model wavefunctions. Even though these wavefunctions are expected to describe the configuration mixing adequately, the magnetic elastic scattering can only be explained if the valence orbitals are given a size which is 6-12% smaller than that required by the root-mean-square radius. No evidence is found to support the hypothesis that neutron and proton orbitals have different sizes. The M3 and M5 moments have been determined from the data.

Angular momentum projection from non-axial intrinsic states: A calculation for 20Ne

Abstract Projected Hartree-Fock calculations are reported for 20 Ne. States of definite angular momentum are projected from non-axial intrinsic wave functions and their energies are then minimized. The results are compared with exact shell-model calculations and good agreement is obtained.

Shell-model study of giant dipole resonances in open-shell nuclei using the Lancgos method

Abstract A microscopic investigation of giant-resonance states in open-shell nuclei is proposed using for the representation of the wave functions the shell-model basis in a large model space. Instead of an exact diagonalization of the hamiltonian, which is essentially impossible for the large model spaces considered, an iterative procedure is used, which is based on the Lanczos algorithm for matrix diagonalization. The choice for the initial state in this iteration ensures that the complete transition strength to the resonance of interest is taken into account, and the iteration allows an increasingly accurate estimate of the spreading of this transition strength to more complicate configurations. An application of this method to the giant dipole resonance in 20 Ne yields stable results after a few iteration steps and demonstrates the efficiency of this method.

Quark-antiquark pair creation effects in the study of S = 0 and S = −1 baryons

Abstract Improved wavefunctions of non-strange and strange baryons are obtained by incorporating quark-antiquark excitations inherent in the quark-gluon interaction into the quark model. The working framework is the shell model where the antiquark is assigned an intrinsic parity quantum number to distinguish it from the quark. The parameters of the interaction are chosen to be consistent with the experimental data. It is found that the amplitudes of the 3 q ( q q ) and 3 q ( q 2 q 2 ) components are not negligible and should be included in the calculation of the properties of the baryons. It is also found that the amplitude of the 3 q ( q q ) component for strange baryons is less than for nonstrange ones, indicating that the hyperon-nucleon(-hyperon) interaction is less attractive than the NN one.

Clustering in many-quark systems: the two-nucleon problem

The Pauli principle acting between quarks in different nucleons is expected to give rise to nuclear structure effects which have not yet been studied in detail. A general method of investigating the evolution of a many-quark system into colourless clusters with the quantum numbers of the nucleon is developed, and some results on the two-nucleon problem are presented.