Orbital magnetic dipole strength inSm148,150,152,154<

Abstract

Nuclear-resonance-fluorescence spectra have been measured in the chain of l48l50 ' 152154 Sm isotopes. Together with supplementary information from inelastic electron scattering and other reaction studies, orbital Ml transition strengths have been deduced from a number of 1 + states located around an excitation energy of 3 MeV. The systematic study, carried out for the first time, for nuclei within a large range of the deformation parameter 8 shows that the orbital Ml strength varies quadratically with 8. This result is interpreted in terms of models containing explicitly neutron and proton degrees of freedom.PACS numbers: 21.10. Re, 23.20.Qz, 25.20.Dc, 27.70.+q The role of neutron-proton interactions on collective nuclear excitations has been a subject of investigations for over four decades. Soon after the discovery of giant dipole resonances, they were interpreted 1 as isovector volume vibrations with neutrons as a whole oscillating out of phase against protons. More recently, a so-called "scissors mode" of oscillations based on a macroscopic two-rotor model (TRM) was suggested, 2 which predicts that 1 + levels with strong ground-state Ml transitions occur in even-even deformed nuclei. The discovery 3 of A/1 excitations in heavy deformed nuclei by highresolution inelastic electron scattering led to a series of detailed investigations on experimental and theoretical fronts. 4 These excitations today still constitute also the best proof for the so-called "mixed-symmetry states" in the neutron-proton interacting-boson model (IBM-2). 5 Macroscopic calculations predict that the orbital Ml strength is to be found in one or a few states, while the experiments indicate that it is generally fragmented into more levels (see, e.g., Refs. 4 and 6). This disparity with macroscopic descriptions is attributed to two-quasiparticle excitations. 7 Very recently, a systematic study 8 of Ml strength in the rare-earth region within the Nilsson model has shown quantitatively a direct correlation between the quadrupole ground-state deformation and the orbital magnetic dipole strength. This finding is also in agreement with predictions 5,9 of the interacting-boson model where in both the SU(3) and the 0(6) limits, i.e., the case where nonspherical nuclei rotate and vibrate, respectively, the reduced Ml transition strength is proportional to N X NJ(N X +N V ), with N K (N v ) being the number of valence proton (neutron) bosons, and is thus within a given series of isotopes not only a function of the number of neutrons present but predominantly dependent upon the neutron-proton interaction responsible for the quadrupole deformation of nuclear ground states. 10 Besides these two classes of models, other calculations exist where the deformation parameter 5 is explicitly contained in the analytic expressions for the reduced Ml transition strength. Some of them are random-phase-approximation predictions; 11 " 13 others re-sult from the TRM, sum-rule, and so-called giantangle-dipole approaches. 14 " 16 These predictions, which are listed...

Low-energy magnetic dipole response in 56Fe from high-resolution electron scattering

“…M1 transitions are also sensitive to non-nucleonic degrees of freedom in elementary nuclear excitations [4][5][6]. Furthermore, a collective orbital magnetic dipole mode-the scissors mode-has been identified [7] at low excitation energies whose strength and location are intimately related to rotational properties of the nucleus (see, e.g., [8][9][10][11] and references therein).…”

Photon strength functions in Gd isotopes studied from radiative capture of resonance neutrons

“…The low density of J = 1 states allowed observation of a majority of the dipole transition strength at these energies. In these experiments the total observed reduced M1 strength B(M1)↑ in the energy range E γ ≈ 2.5 − 4.0 MeV was proportional to the square of the nuclear deformation [28,29] and for well deformed nuclei reached B(M1)↑≈ 3μ 2 N . This strength is usually distributed over only a few states.…”

Meaning of antiparallel proton and neutron angular momenta at low spins

“…[8]. This idea is based on the proportionality of the summed M1 strength to the square of the nuclear deformation parameter [15]. A deeper understanding of the present issue may provide us with a way to understand this proportionality.…”