Energy levels of A = 21−44 nuclei (V)

The experimental and theoretical evidence for intrinsic reflection-asymmetric shapes in nuclei is reviewed. The theoretical methods discussed cover a wide spectrum, from mean-field theory and its extensions to algebraic and cluster approaches. The experimental data for nuclear ground states and at low and high spin, cited as evidence for reflection asymmetry, are collected and categorized. The extensive data on electric dipole transition moments and their theoretical interpretation are surveyed, along with available data on electric octupole moments. The evidence for reflection-asymmetric molecular states in light nuclei is summarized. The application of reflection-asymmetric theories to descriptions of the fission barrier, bimodal fission, superdeformation, and hyperdeformations is reviewed, and some other perspectives in the wider context of nuclear physics are also given. [S0034-6861(96)00102-X]

Intrinsic reflection asymmetry in atomic nuclei

The concept of spontaneous symmetry breaking is applied to the rotating mean field of nuclei. The description is based on the tilted-axis cranking model, which takes into account that the rotational axis can take any orientation with respect to the deformed density distribution. The appearance of rotational bands in nuclei is analyzed, focusing on weakly deformed nuclei at high angular momentum. The quantization of the angular momentum of the valence nucleons leads to new phenomena. Magnetic rotation represents the quantized rotation of the anisotropic current distribution in a near spherical nucleus. The restricted amount of angular momentum of the valence particles causes band termination. The discrete symmetries of the mean-field Hamiltonian provide a classification scheme of rotational bands. New symmetries result from the combination of the spatial symmetries of the density distribution with the vector of the angular momentum. The author discusses in detail which symmetries appear for a reflection-symmetric density distribution and how they show up in the properties of the rotational bands. In particular, the consequences of rotation about a nonprincipal axis and of breaking the chiral symmetry are analyzed. Also discussed are which symmetries and band structures appear for non-reflection-symmetric mean fields. The consequences of breaking the symmetry with respect to gauge and isospin rotations are sketched. Some analogies outside nuclear physics are mentioned. The application of symmetry-restoring methods to states with large angular momentum is reviewed.

Spontaneous symmetry breaking in rotating nuclei

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model. An experimental study of certain short-lived isotopes of radon and radium has found clear octupole deformation in the nuclei of the latter — that is, these nuclei are pear-shaped; the results enable discrimination between differing theoretical approaches to octupole correlations. The atomic nucleus is a many-body quantum system with a shape determined by the number of nucleons that it contains and the interactions between them. Most of the several thousand known stable and radioactive atomic nuclei, with differing numbers of protons and neutrons, are spherical or rugby-ball shaped. But there is circumstantial evidence that some heavy, unstable nuclides are distorted into a pear shape through the phenomenon of octupole deformation. Samples of these rare atomic species can be accelerated to 8% of the speed of light in the REX-ISOLDE facility at CERN, and now Coulomb excitation experiments on beams of the short-lived isotopes radium-224 and radon-220 have demonstrated clear octupole deformation in the former. The results make it possible to discriminate between the various theoretical models of octupole-deformed nuclei, and are also relevant to the pursuit of physics beyond the Standard Model.

/pdf/studies-of-pear-shaped-nuclei-using-accelerated-radioactive-3u58wl8m3a.pdf

Studies of pear-shaped nuclei using accelerated radioactive beams

A new type of explanatory mechanism is proposed to account for the fact that many of the dimensionless numbers which characterize particle physics and cosmology take unnatural values. It is proposed that all final singularities 'bounce' or tunnel to initial singularities of new universes at which point the dimensionless parameters of the standard models of particle physics and cosmology undergo small random changes. This speculative hypothesis, plus the conventional physics of gravitational collapse, together comprise a mechanism for natural selection, in which those choices of parameters that lead to universes that produce the most black holes during their lifetime are selected for. If our Universe is a typical member of the ensemble that results from many generations of such reproducing universes then it follows that the parameters of our present Universe are near a local maximum of the number of black holes produced per universe. Thus, modifications of the parameters of particle physics and cosmology from their present values should tend to decrease the number of black holes in the universe. Three possible examples of this mechanism are described.

https://iopscience.iop.org/article/10.1088/0264-9381/9/1/016/pdf

Did the Universe Evolve

https://web-docs.gsi.de/~wolle/TELEKOLLEG/KERN/LECTURE/Wollersheim/PAPERS/NPA556.pdf

Coulomb excitation of 226Ra

High-spin states in 224,226,228Th and the systematics of octupole effects in even Th isotopes

We have measured the circular polarization of the deexcitation $\ensuremath{\gamma}$ rays following the bombardment of natural Ag targets with 284-MeV and 303-MeV $^{40}\mathrm{Ar}$ ions. A symmetric setup containing two polarimeters normal to the reaction plane was used. From the observed count-rate asymmetry of + 0.65%\ifmmode\pm\else\textpm\fi{}0.15% for deep-inelastic events we conclude that the excited product nuclei are polarized in the direction of the reaction normal ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{i}\ifmmode\times\else\texttimes\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{f}$ which provides evidence for a predominance of negative classical deflection angles.

Evidence for Negative Deflection Angles in Ar 40 + Ag Deep-Inelastic Reactions from γ -Ray Circular Polarization Measurements

Yrast levels of 220Ra populated in the 208Pb(14C, 2n) reaction

The intensities and angular correlations of yrast transitions measured in coincidence with light fragments from 96-MeV $^{16}\mathrm{O}$+$^{48}\mathrm{Ti}$ reactions are used to determine amount and alignment of the angular momentum transferred to the heavy fragment as a function of the reaction $Q$ value. It is found that fluctuations of the spin axis about the scattering normal are small at the most probable $Q$ value of - 32 MeV but increase strongly with increasing inelasticity.

C. Lauterbach

Papers

Coulomb excitation of 226Ra

High-spin states in 224,226,228Th and the systematics of octupole effects in even Th isotopes

Evidence for Negative Deflection Angles in Ar 40 + Ag Deep-Inelastic Reactions from γ -Ray Circular Polarization Measurements

Yrast levels of 220Ra populated in the 208Pb(14C, 2n) reaction

Anisotropy of yrast gamma transitions from /sup 16/O on /sup 48/Ti: Angular momentum dissipation and alignment in deep-inelastic reactions