S. Chmel

Magnetic rotation in 197Pb and 198Pb

Abstract High-spin states in 197Pb and 198Pb were populated in the 186W(18O ,x n) reactions. In-beam γ -ray coincidences were measured in two experiments using the Gammasphere and the Eurogam II spectrometer arrays, respectively. In both nuclei new bands of enhanced magnetic dipole transitions were found and the known cascades were partly reordered and extended to higher spins. In most cases, γ -ray transitions connecting the magnetic rotational bands to lower-lying states have been identified. Configuration assignments are suggested for the bands. The systematic behavior confirms the shears mechanism. An effective interaction between the main high-spin proton and neutron orbitals is derived.

Shears bands and their links to spherical states in 197Pb

Abstract High-spin states in 197 Pb have been investigated by in-beam γ-ray spectroscopy. Two previously known high-spin magnetic-dipole sequences have been connected to the spherical level scheme at lower spins. A reordering of the transitions in the previously reported irregular band leads to a regular sequence with a band crossing. The properties of the dipole sequences are interpreted in terms of “shears bands” within the tilted-axis cranking model.

Shears bands in 201Pb and 202Pb

Abstract High-spin states in 201 Pb and 202 Pb have been investigated using in-beam γ-ray spectroscopic techniques. Seven regular sequences of enhanced dipole transitions, with weak E2 crossovers in some cases, have been found, one of which is firmly connected to low-lying levels in 201 Pb. These bands are interpreted to be built on high-spin proton-particle neutron-hole excitations. Tilted-axis cranking calculations show that the angular momentum along the bands is predominantly generated by a continuous and simultaneous reorientation of the proton and neutron spins into the direction of the total angular momentum (shears effect).

Quadrupole moments of the 11− intruder isomers in 194,196Pb in the particle-core coupling model

Abstract The recently measured spectroscopic quadrupole moments of the 11 − intruder isomers in 194,196 Pb are being discussed in the particle-core coupling model. The essential physics behind the nature of these isomers is revealed by a simple two-level-mixing calculation, showing that the coupling of the valence proton particles with vibrations of the underlying Hg core induces a factor of two increase of the quadrupole moments.

Two-Particle Separation Energies in the Superdeformed Well

The location of nuclear closed shells, as evidenced through discontinuities in binding energy and one- and two-particle separation energy systematics, remains one of the simplest tests of global nuclear models. How shell gaps evolve, whether with increasing mass, increasing neutron:proton ratio or increasing deformation, is still uncertain, and it has recently been suggested that one must go beyond a static meanfield picture to include the effects of dynamic fluctuations in the nuclear shape even in the ground state. The identification of key properties which may distinguish between competing approaches is thus vital. Comparison of the binding energies of superdeformed nuclei in the A ≈ 190 region shows that two-proton separation energies are higher in the superdeformed state than in the normal state, despite the probably lower Coulomb barrier and lower total binding energy. Possible reasons for this difference are discussed. This somewhat counterintuitive result provides a critical test for global nuclear models.

Shears bands in Pb nuclei – a new nuclear structure effect

In nuclei in the mass region around A = 190-200 a large number of regular dipole sequences have recently been found. In these bands the gain in angular momentum may not be created by collective rotation as is usual for more deformed nuclei, but stems from aligning the proton and neutron spins in the direction of the total spin axis. Calculations within the frame-work of the tilted axis cranking model reproduce the available experimental data and support the alignment picture.

Deformation parameters and transition probabilities for shears bands in Pb isotopes

Tilt angles, deformation parameters, angular momenta, and reduced magnetic dipole and electric quadrupole transition probabilities are calculated within the framework of the tilted-axis cranking model for shears bands in the neutron-deficient Pb isotopes {sup 193}Pb to {sup 202}Pb. The self-consistently calculated quadrupole-deformation parameters for the various configurations lie around {epsilon}{sub 2}=-0.1, and the triaxiality is small. The tilt angle, i.e., the angle between the angular momentum and the nuclear symmetry axis, remains almost constant within each band, showing only a small increase toward higher angular momenta. The angular momentum increases mainly due to the shears effect, which is a step-by-step alignment of high-j proton and neutron orbitals into the direction of the total angular momentum. Calculated and experimental M1 transition rates are in good agreement. They decrease with increasing spin within the bands as expected for the shears effect. The calculated B(E2) values show only a weak spin dependence. However, the experimental B(E2) values have large errors, and more accurate data are needed to decide between the present model and a recently suggested geometrical approach.

Shape Coexistence in Pb Nuclei Probed by Static Electromagnetic Moments

Spectroscopic quadrupole moment measurements for high‐spin isomeric states in 192,193,194Pb nuclei have been performed by the time‐differential perturbed γ‐ray angular distribution method. The quadrupole moments of the isomers described by quasineutron configurations in neutron‐deficient Pb nuclei are well reproduced by the pairing plus quadrupole model, which predicts for these states a very small prolate deformation β2 ≈0.02. The quadrupole moment of the 11− isomeric state in 192Pb involving the π(3s1/2−2 1h9/2 1i13/2) configuration corresponds to a deformation β2=−0.11(1). The quadrupole moments of the proton intruder states in Pb nuclei are compared with theoretical predictions of tilted‐axis cranking, mean‐field and interacting boson models.

Magnetic Rotation In 196Pb

A detailed spectroscopic study of high‐spin states in 196Pb has been carried out using the EUROBALL spectrometer array. Five new magnetic‐rotational bands have been found and the previously known bands have been extended. With one exception, all the bands have been connected to lower‐lying states and excitation energies, spins and parities have been determined. Doppler‐broadend lineshapes have been analyzed and lifetimes of states in four of the bands have been determined. The B(M1) values show the decrease with increasing spin that is expected for shears bands. Configurations of proton‐particle excitations coupled to neutron‐hole states have been assigned to the bands.

Quadrupole moments of the 11− intruder isomers in 194,196Pb

The spectroscopic quadrupole moments of the 11− isomers in 194Pb and 196Pb have been measured by applying the Level Mixing Spectroscopy technique. These isomers have the π(s1/2−2h9/2i13/2) proton configuration. The implications of these results for the influence of the intruder character of the isomers on their deformation is given. We will especially focus on a simple 2-dimensional model, taking into account the coupling of the valence nucleons with the vibrations of the underlying core, which gives theoretical insight in the obtained experimental results for the quadrupole moments of the 11− isomers.