R. F. Casten

Large deformation of the very neutron-rich nucleus 32Mg from intermediate-energy Coulomb excitation

Abstract The Coulomb excitation of a very neutron-rich nucleus 32Mg to its 2+ state was studied using an unstable nuclear beam of 32Mg at 49.2 MeV/u with a 208Pb target. The B(E2) value of 454±78 e2fm4 was extracted by a coupled channel analysis. The value is in good agreement with recent predictions. It confirms a large deformation for 32Mg suggested by the low excitation energy of the 2+ state, and points to the vanishing of the N = 20 shell gap.

An extensive region of O(6)-like nuclei near A = 130

Abstract Evidence for an extensive region of nuclei near A = 130 resembling the O(6) symmetry of the IBA is presented. Beyond the O(6)-like character of the A = 130 and Pt regions, other similarities between these two regions are found, in particular, a common relation of the energy scales of the O(6) and O(5) groups. Deviations from the O(6) symmetry in both regions are interpreted in terms of the interplay of γ-softness and triaxiality.

NpNn systematics in heavy nuclei

Abstract The systematics of the nuclear phase transitional regions near A = 100, 130, 150 and 190 are studied from the standpoint of the valence nucleon product N p N n . The use of this construction or parametrization leads to a substantial simplification of these complex regions. The results are discussed in terms of the proton-neutron interaction in different shell-model orbits and are used to interpret the effects of subshell closures. The mechanism for the onset of deformation near A = 100, involving the eradication of such a subshell closure in one type of particle as a function of the number of valence nucleons of the other type, is shown to be of more general applicability, not only for the A = 150 region, but also for the A = 190 region. The N p N n plots for five mass regions ( A = 100, 130, 150 ( Z ⩽ 64), 150 ( Z ⩾ 66) and 190) are compared. It is found that the quantity N p N n provides an excellent unit to assess the rapidity of different transition regions: that is, the slopes of different observables plotted against this quantity are related to the average interaction strength, per proton-neutron pair, in the crucial highly overlapping orbits whose occupation induces the phase transition. The five regions studied are found to display a remarkable similarity which results from similar strengths of this interaction in the crucial orbits.

Signatures of γ softness or triaxiality in low energy nuclear spectra

Abstract Signatures of γ softness or rigid triaxiality in low energy, low spin nuclear spectra are discussed. Two classes of signatures, relating to γ-band energy staggering, are found to provide clear distinctions between these shapes. The data for even-even nuclei are compared to predictions for potentials with varying γ dependence. It is found that nuclei with large asymmetries can be characterized by potentials that are nearly γ flat, with, at most, a few percent deviation from γ independence.

Identification of all intrinsic excitations below 2 MeV in 168Er

The levels and gamma decay in168Er have been investigated following thermal neutron capture on enriched targets of 167Er2O3. Precision measurements of the gamma spectrum with curved-crystal spectrometers revealed over 700 transitions up to an energy of 2.5 MeV, with intensities spanning five orders of magnitude. A list of primary gamma transitions populating levels in 168Er up to an excitation energy of 3.1 MeV resulted from measurements with the pair spectrometer. Information on the gamma spectrum in the energy region 1400<E<2600 keV was obtained using a Ge(Li) spectrometer. The conversion electron spectrum up to 2.2 MeV electron energy was measured with a beta spectrometer, whence multipolarities of many of the transitions were derived. Average resonance capture measurements at neutron energies of 2 keV and 24 keV were made which ensured that the set of spin 2-5 levels below about 2.2 MeV was complete. From all these data, employing the Ritz combination principle, a level scheme was developed and the levels were arranged into a complete set of 20 low-K rotational bands. This is the most complete and extensive test of the limits of applicability of models for low-lying collective excitations.

The interplay of γ-softness and triaxiality in O(6)-like nuclei

Abstract It is shown that the empirical deviations from the O(6) limit of the IBA, in the Pt and Xe, Ba regions, are nearly identical and that both can be interpreted very simply by introducing explicitly triaxial degrees of freedom to the IBA potential via an additional cubic interaction in the hamiltonian. This simultaneously improves the energy staggering in the γ-band, the energy of the first excited 0 + state, and the spacings among high ρ-states. A physical interpretation of the results in terms of the effective mean γ and the dynamic fluctuations in γ (zero-point motion) is presented.

Empirical proton-neutron interaction energies. Linearity and saturation phenomena

Abstract A method to extract empirical proton-neutron interaction energies for individual protons and neutrons as well as integrated valence p-n interaction strenghts is discussed. The results give direct experimental support to the N p N n scheme, verifying the linearity of the p-n interaction against N p N n early in a shell and its subsequent saturation. They also reflect the behavior i of recently calculated quadrupole p-n strengths which account for the empirical saturation in B (E2: 0 1 + → 2 1 + ) values, and point to the importance of the monopole p-n interaction as well.

A shell-model interpretation of intruder states and the onset of deformation in even-even nuclei

Abstract By simple consideration of the relative magnitude of the proton-neutron interaction (especially the monopole and quadrupole components) and the energy gaps between spherical shell-model states near shell and subshell closures, a shell-model interpretation of both the occurrence of low-lying intruder states in certain mass regions and of the onset of stable deformation in the ground state in others, is obtained. It is shown that in the Cd region near A ≅ 114, spherical ground states with low-lying intruder 0 + configurations result whereas deformation should occur in the A ≅ 100 ( Z = 40) and the A ≅ 150 ( Z = 64) mass regions.

A simple approach to nuclear transition regions

Abstract A scheme for the interpretation of extended nuclear transition regions is introduced which is based on the importance of the neutron-photon interaction among valence nucleons. It substantially simplifies both the empirical patametrization and theoretical calculation of such regions. Data from near A ∼ 100, ∼130, and ∼150 are discussed. In the first and last cases, information on the valence proton shell structure is extracted. In the A ∼ 130region an IBA calculation is presented which exploits the proposed method.

The enigma of 114Cd. A classical case of ambiguities in quantum mechanical state mixing

Abstract Lifetime measurements of levels near 2 MeV in 114 Cd give quantitative evidence for collective 3-phonon vibrational states. An analysis of the level scheme reveals a remarkable ambiguity with origins in the basic properties of quantum mechanical mixing.