.
We analyze the relation between isoscalar toroidal modes and so-called pygmy dipole resonance (PDR), which both appear in the same region of low-energy dipole excitations. To this end, we use a ...theoretical description within the fully self-consistent Skyrme quasiparticle random-phase approximation (QRPA). Test cases are spherical nuclei
40, 48
Ca,
58, 72
Ni,
90, 100
Zr, and
100, 120, 132
Sn which cover four different elements and for each element at least two isotopes with different neutron excess, one small and another large. The structure of the modes is investigated in terms of strength functions, transition densities (TD) and transition currents (TC). For all considered nuclei, we see that, independently on whether PDR strength exists or not, the flow pattern in the lower part of the “PDR energy region” is basically an isoscalar vortical toroidal motion with a minor irrotational fraction. A one-to-one correspondence between calculated TD and TC is established. The toroidal flow appears already in the uncoupled two-quasiparticle (2qp) excitations and becomes definitively strong for the QRPA modes. Altogether, we find that low-lying dipole strength often denoted as isoscalar PDR is actually an oversimplified imitation of the basically toroidal motion in nuclei with sufficient neutron excess.
.
We investigate effects of pairing and of quadrupole deformation on two sorts of nuclear excitations,
γ
-vibrational
K
π
=
2
+
states and dipole resonances (isovector dipole, pygmy, compression, ...toroidal). The analysis is performed within the quasiparticle random phase approximation (QRPA) based on the Skyrme energy functional using the Skyrme parametrization SLy6. Particular attention is paid to i) the role of the particle-particle (pp) channel in the residual interaction of QRPA, ii) comparison of volume pairing (VP) and surface pairing (SP), iii) peculiarities of deformation splitting in the various resonances. We find that the impact of the pp-channel on the considered excitations is negligible. This conclusion applies also to any other excitation except for the
K
π
=
0
+
states. Furthermore, the difference between VP and SP is found small (with exception of peak height in the toroidal mode). In the low-energy isovector dipole (pygmy) and isoscalar toroidal modes, the branch
K
π
=
1
-
is shown to dominate over the
K
π
=
0
-
one in the range of excitation energy
E
<
8-10 MeV. The effect becomes impressive for the toroidal resonance whose low-energy part is concentrated in a high peak of almost pure
K
π
=
1
-
nature. This peculiarity may be used as a fingerprint of the toroidal mode in future experiments. The interplay between pygmy, toroidal and compression resonances is discussed, the interpretation of the observed isoscalar giant dipole resonance is partly revised.
The 9.2 keV nuclear transition in 227Th was studied in the β−-decay of 227Ac by means of the internal conversion electron spectroscopy to clarify the spin-parity assignment of the ground state and ...the two lowest excited states of 227Th. The transition multipolarity was proved to be of mixed character M1 + E2 and the spectroscopic admixture parameter δ2(E2/M1)=0.695±0.248 (|δ(E2/M1)|=0.834±0.149) was determined. Nonzero value of δ(E2/M1) questioned the present theoretical interpretation of low-lying levels of 227Th. Calculations performed prefer the 1/2+, 3/2+, and 3/2+ sequence instead of the adopted 1/2+, 5/2+ and 3/2+ one for the 0.0, 9.2, and 24.3 keV levels, respectively.
The low-energy
1 states in deformed
Dy and spherical
Ni are explored in the framework of fully self-consistent Quasiparticle Random-Phase Approximation (QRPA) with various Skyrme forces. The main ...attention is paid to orbital and spin
1 excitations. The obtained results are compared with the prediction of the low-energy
spin-scissors
1 resonance suggested within Wigner Function Moments (WFM) approach. A possible relation of this resonance to low-energy spin-flip excitations is analyzed. In connection with recent WFM studies, we consider evolution of the low-energy spin-flip states in
Dy with deformation (from the equilibrium value to the spherical limit). The effect of tensor forces is briefly discussed. It is shown that two groups of
states observed at 2.4–4 MeV in
Dy are rather explained by fragmentation of the orbital
1 strength than by the occurrence of the collective spin-scissors resonance. In general, our calculations do not confirm the existence of this resonance.
We review a recent progress in investigation of the isoscalar toroidal dipole resonance (TDR). A possible relation of the TDR and low-energy dipole excitations (also called a pygmy resonance) is ...analyzed. It is shown that the dipole strength in the pygmy region can be understood as a local manifestation of the collective vortical toroidalmotion at the nuclear surface. Application of the TDR as a measure of the nuclear dipole vorticity is discussed. An anomalous splitting of the TDR in deformed nuclei is inspected.
Abstract
The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) is a centre of expertise and innovation in the field of nuclear-structure physics and is a leader in several high-impact ...studies. One of the highlights of these nuclear-structure experiments is the study of the broad structure of the IsoVector Giant Dipole Resonance (IVGDR) in the rare-earth region. Proton inelastic scattering experiments with
E
p
= 200 MeV were performed on the even-even Nd isotope chain and
152
Sm at very forward scattering angles including zero degrees with the K600 magnetic spectrometer. The evolution of the shape of the IVGDR in the transition from spherical to deformed nuclei was investigated. One of the goals of this highlighted study was to confirm the
K
-splitting observed in previous photo-absorption measurements from Saclay. Significant discrepancies were found between the direct (γ, xn) data obtained at Saclay and the equivalent photo-absorption cross sections obtained using (p, p′) data from the K600. Furthermore, discrepancies exist for several nuclei between photo-absorption data taken at the Saclay and Livermore laboratories. These discrepancies, possible reasons for them and future investigations will be presented and discussed.
The phenomenon of fine structure of the Isoscalar Giant Quadrupole Resonance (ISGQR) has been studied with high energy-resolution proton inelastic scattering at iThemba LABS in the chain of stable ...even-mass Nd isotopes covering the transition from spherical to deformed ground states. A wavelet analysis of the background-subtracted spectra in the deformed 146, 148, 150Nd isotopes reveals characteristic scales in correspondence with scales obtained from a Skyrme RPA calculation using the SVmas10 parameterization. A semblance analysis shows that these scales arise from the energy shift between the main fragments of the K=0,1 and K=2 components.
Despite a great success of the Skyrme mean-field approach in the exploration of nuclear dynamics, it seems to fail in the description of the spin-flip M1 giant resonance. The results for different ...Skyrme parameterizations are contradictory and poorly agree with experiment. In particular, there is no parameterization which simultaneously describes the one-peak gross structure of M1 strength in doubly magic nuclei and two-peak structure in heavy deformed nuclei. The reason of this mismatch could lie in an unsatisfactory treatment of spin correlations and spin--orbit interaction. We discuss the present status of the problem and possible ways of its solution. In particular, we inspect (i) the interplay of the collective shift and spin--orbit splitting, (ii) the isovector M1 response versus isospin-mixed responses and (iii) the role of tensor and isovector spin--orbit interaction.
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