A. Gelberg

IBM-2 calculations of even-even Pd nuclei

Abstract Excitation energies, electromagnetic transition strengths and static moments in even-even Pd nuclei have been described systematically by using the proton-neutron interacting boson model (IBM-2). Parameters are fitted so as to be consistent with microscopic calculations. Mixed symmetry 2 + states have been identified using the M1 transition strength. The F -spin impurity of the first 2 + state is investigated by means of g -factors and M1 transitions. The calculations have been extended to neutron-rich Pd isotopes with A > 116 for which excited states are still not known.

Proton-neutron symmetry in boson models of nuclear structure

The present review deals with nuclear models which consider collective proton and neutron motion separately in terms of bosons. Extensions of the geometric Bohr-Mottelson model are sketched, followed by a more extensive discussion of the algebraic interacting boson approximation (IBA-2). In the latter the concept of F spin plays a central role, and detailed attention is paid to its tensorial properties. A general projection method is presented whereby any IBA-2 calculation for states of maximal F spin can be transformed into a simpler one in IBA-1, where the proton-neutron distinction is ignored. Data are presented on F-spin multiplets, and global fits with the IBA are discussed. A major portion of the review is concerned with M1 transitions. Strong M1s in deformed and spherical nuclei, with emphasis on the latter, are discussed as resulting from a change in proton-neutron symmetry. Weak M1s are considered as reflecting such symmetry changes in small amplitudes of the wave function. Attention is given to one-fluid boson models (IBA-1, geometric) simulating these two-fluid effects.

A study of the low-lying states in 178Hf through the (n, γ) reaction☆

The nucleus 178Hf was studied through thermal neutron and averaged resonance neutron capture reactions. The γ-ray and conversion electrons were measured with high resolution spectrometers. A level scheme up to an excitation energy of ∼2.1 MeV was constructed. It includes ∼65 levels, most of which are ordered into 18 rotational bands. The level scheme is complete up to about 1800keV for spins between 2 and 5. The neutron binding energy was established to be at 7626.3 (3) keV. The consistent Q form of the IBA-1 (CQF) was used to describe the low-lying collective γ and Kπ = 0+ bands. The agreement with the data was found to be excellent for the energies and B(E2) ratios of the ground and γ bands, whereas the agreement was poor for the Kπ = 0+ bands.

Excited states in 130Ba

Abstract Excited states of 130Ba have been studied by means of in-beam γ-ray spectroscopy and electron-spectroscopy via the reaction 120Sn(13C, 3n). Spin and parity assignments have been made for 28 new levels. A comparison with a cranking-model calculation shows that the negative-parity band based on the 5− state is a two-quasiproton band.

Davydov-filippov limit of the IBM

Abstract The O(6) limit of the interacting boson model (IBM) is studied in connection to the rigid-triaxial-rotor model of Davydov and Filippov (D–F), and the γ unstable model of Wilets and Jean (W–J). The IP π = 0 + deformation potential is calculated as a function of axial asymmetry γ by projecting a -γ-fixed intrinsic state onto the good angular momentum I π = 0 + . Although the projected potential has already been reported to have a minimum at γ = 30° for small values of the boson number N , it is shown that the potential resembles a square well at the classical limit: N → ∞. This indicates that the rigid triaxial (D–F) wave function differs at N → ∞ from the γ unstable (W–J) wave function which is the exact solution of the O(6), although these two types of wave functions are known to be identical for smaller N . The projected spectrum with γ = 30° is found to converge at N → ∞ to the D–F spectrum with γ = 30°. Thus the D–F limit of the IBM can be introduced as the “projected” classical limit, although it can no longer be a good approximation to the exact O(6) solution.

On the parity of the yrast 8+ level in138Ce and an anomaly in theB(E2) values

On the basis of conversion electron andγ-angular distribution measurements new spin and parity assignments in138Ce have been made. They lead together with the already known lifetime of the 10+ yrast state whose parity was unclear to an extremely low transition probability of the 10+→8+ transition. A comparison with neighbouring even isotopes is made.

Levels in 138Ce by in-beam spectroscopy

Abstract Gamma ray and conversion electrons from levels in 138Ce have been studied following the 139La(p, 2n)138Ce and 136Ba(α, 2n)138Ce reactions. A level scheme, based on the measurement of conversion coefficients, of gamma-ray angular distributions and excitation functions and of gamma-gamma coincidences, is proposed and compared with findings by other authors. Some transitions observed lead to levels not previously identified in gamma-ray work; in other cases new spin and/or parity assignments have been given. An indication on the nature of some of the observed states is obtained by comparing the experimental data with the predictions of the ALAGA model and of the IBM-2 model.

A study of the low‐lying states in 178Hf through the neutron capture reaction

The decay of the low‐lying states of 178Hf was investigated using: (1) High‐resolution curved crystal spectrometry of the secondary γ‐rays using the GAMS‐1 and GAMS 2/3 facilities at the ILL, (2) Measurements of the secondary (n,e−) transitions using the Electron Spectrometer BILL at the ILL, (3) Measurements of the primary γ‐transitions following thermal neutron capture with the pair‐spectrometer at the ILL and (4) Average Resonance Capture (ARC) measurements at the neutron energies of 2 keV and 24 keV, using the tailored beam facilities at BNL.

Generation of Irreducible Representations in the O(6) Limit of the IBM

An operator that can generate excited irreducible representations in the O(6) limit of the interacting boson model is proposed. This operator is a quasispin vector of the boson realization of the SU(1, 1) algebra. This operator is coupled to the |[N ], σ = N〉 ground state of the O(6) system by using an SU(1, 1) Wigner coefficient so as to generate the excited irreducible representation |[N ], σ = N − 2〉. Any representation of an O(6)-type system can be generated by repeated actions of this operator, and any state of this representation can be generated by the subsequent repeated actions of the quadrupole operator on the state generated by the former operator. The general form of the operator for any value of σ has been given. Another approach, which uses the commutator of the O(6) Casimir operator with the boson number operators as well as the double commutator, is also presented.

SUPERSYMMETRIC OPERATOR FOR THE U(6/4) DYNAMICAL SYMMETRY

Abstract An explicit form of the supersymmetric operator is constructed algebraically for the case of the U(6/4) dynamical symmetry. It converts the eigenstates of the even nucleus into corresponding eigenstates of the odd SUSY partner nucleus. It gives an explicit form of the wave function of the states of the odd nucleus in terms of the supersymmetric operator. A comparison with the particle–core model is given.