Energy levels of light nuclei A=8,9,10

Energy levels of A = 21−44 nuclei (V)

The mass of the nucleus, through its binding energy, continues to be of capital importance not only for various aspects of nuclear physics, but also for other branches of physics, notably weak-interaction studies and astrophysics. The authors first describe the modern experimental techniques dedicated to the particularly challenging task of measuring the mass of exotic nuclides and make detailed comparisons. Though tremendous progress in these and the associated production techniques has been made, allowing access to nuclides very far from stability, it is still not yet possible to produce many nuclides involved in stellar nucleosynthesis, especially the $r$ process, leaving no choice but to resort to theory. The review thus goes on to describe and critically compare the various modern mass formulas that may be used to extrapolate from the data towards the neutron drip line. Special attention is devoted to the crucial interplay between theory and experiment, showing how new measurements far from stability can considerably reduce the ambiguity in extrapolations to nuclides even further away.

Recent trends in the determination of nuclear masses

This article provides an overview of the basic principles of the physics of quantum halo systems, defined as bound states of clusters of particles with a radius extending well into classically forbidden regions. Exploiting the consequences of this definition, the authors derive the conditions for occurrence in terms of the number of clusters, binding energy, angular momentum, cluster charges, and excitation energy. All these quantities must be small. The article discusses the transitions between different cluster divisions and the importance of thresholds for cluster or particle decay, with particular attention to the Efimov effect and the related exotic states. The pertinent properties can be described by the use of dimensionless variables. Then universal and specific properties can be distinguished, as shown in a series of examples selected from nuclear, atomic, and molecular systems. The neutron dripline is especially interesting for nuclei and negative ions for atoms. For molecules, in which the cluster division comes naturally, a wider range of possibilities exists. Halos in two dimensions have very different properties, and their states are easily spatially extended, whereas Borromean systems are unlikely and spatially confined. The Efimov effect and the Thomas collapse occur only for dimensions between 2.3 and 3.8 and thus not for 2. High-energy reactions directly probe the halo structure. The authors discuss the reaction mechanisms for high-energy nuclear few-body halo breakup on light, intermediate, and heavy nuclear targets. For light targets, the strong interaction dominates, while for heavy targets, the Coulomb interaction dominates. For intermediate targets these processes are of comparable magnitude. As in atomic and molecular physics, a geometric impact-parameter picture is very appropriate. Finally, the authors briefly consider the complementary processes involving electroweak probes available through beta decay, electromagnetic transitions, and capture reactions.

Structure and reactions of quantum halos

Experimental studies on neutron halo nuclei are reviewed. The recent development of radioactive nuclear beams enabled us to study detailed structure of nuclei far from the stability line. The neutron halos have been observed in nuclei near the neutron dripline by reaction measurements with intermediate- and high-energy radioactive nuclear beams. These nuclei are of a special interest in relation to shell structures near the dripline and to new excitation modes associated with the excess neutron on the nuclear surface.

https://iopscience.iop.org/article/10.1088/0954-3899/22/2/004/pdf

Neutron halo nuclei

Pre-scission and post-scission neutron yields have been measured as a function of projectile mass, compound nucleus fissility, and fission mass split and total kinetic energy (TKE) for 27 fusion-fission and quasifission reactions induced by beams of $^{16,18}\mathrm{O}$, $^{40}\mathrm{Ar}$, and $^{64}\mathrm{Ni}$. A new method of interpretation of experimental pre-scission neutron multiplicities ${\ensuremath{\nu}}_{\mathrm{pre}}$ and mean kinetic energies ${\mathrm{\ensuremath{\varepsilon}}}_{\ensuremath{\nu}}$ allows the extraction of fission time scales with much less uncertainty than previously, all fusion-fission results being consistent with a dynamical time scale of (35\ifmmode\pm\else\textpm\fi{}15)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ s for symmetric fission. All reactions show that ${\ensuremath{\nu}}_{\mathrm{pre}}$ falls quite rapidly with increasing mass asymmetry; evidence is presented that for fusion-fission reactions this is partly due to a reduction of the dynamical fission time scale with mass asymmetry. For quasifission, the data indicate that the pre-scission multiplicity and mean neutron kinetic energy are very sensitive to the final mass asymmetry, but that the time scale is virtually independent of mass asymmetry. It is concluded that for fusion-fission there is no dependence of ${\ensuremath{\nu}}_{\mathrm{pre}}$ on TKE, while for $^{64}\mathrm{induced}$ quasifission reactions, a strong increase of ${\ensuremath{\nu}}_{\mathrm{pre}}$ with decreasing TKE is observed. This is probably largely caused by neutron emission during the acceleration time of the fission fragments in these fast reactions. Interpretation of post-scission multiplicities in terms of fragment excitation energies leads to deduced time scales consistent with those determined from the pre-scission data.

Neutron emission as a probe of fusion-fission and quasifission dynamics.

Nuclear rainbow structures in the elastic scattering of 16O on 16O at EL=350 Me

The nuclear density overlap, which occurs during refractive heavy-ion scattering, opens an alternative approach to study the equation of state (EOS) for cold nuclear matter. For this purpose elastic [sup 16]O+[sup 16]O scattering at incident enegies of 145, 250, 350, and 480 MeV has been measured very accurately, up to large angles. A systematic folding analysis of these data has been performed using an effective density dependent interaction based on the [ital G]-matrix elements of the Paris nucleon-nucleon potential. We find, with the observed refractive scattering patterns, that a soft EOS (with the nuclear incompressibility [ital K] around 200 MeV) is the most realistic one.

Equation of state for cold nuclear matter from refractive 16O+16O elastic scattering.

The time scale of fission at initial nuclear temperatures of about 5 MeV is deduced from the number of neutrons evaporated prior to and after scission in the reactions /sup 144/Sm and /sup 154/Sm+(838 MeV) /sup 32/S. The prescission lifetime for fission with symmetric mass splits is longer than for asymmetric mass splits. For symmetric mass splits the excitation energy at scission is only about 60 MeV and independent of the initial excitation energy which is consistent with a prescission lifetime in the order of 10/sup -20/ s.

M. Wilpert

Papers

Neutron emission as a probe of fusion-fission and quasifission dynamics.

Nuclear rainbow structures in the elastic scattering of 16O on 16O at EL=350 Me

Equation of state for cold nuclear matter from refractive 16O+16O elastic scattering.

Observation of cold scission of highly excited fissioning nuclei.

Spectroscopy of 10He