Magnetic dipole excitations in nuclei: Elementary modes of nucleonic motion

Abstract

The nucleus is one of the most multi-faceted many-body systems in the universe. It exhibits a multitude of responses depending on the way one 'probes' it. With increasing technical advancements of beams at the various accelerators and of detection systems the nucleus has, over and over again, surprised us by expressing always new ways of 'organized' structures and layers of complexity. Nuclear magnetism is one of those fascinating faces of the atomic nucleus we discuss in the present review. We shall not just limit ourselves to presenting the by now very large data set that has been obtained in the last two decades using various probes, electromagnetic and hadronic alike and that presents ample evidence for a low-lying orbital scissors mode around 3 MeV, albeit fragmented over an energy interval of the order of 1.5 MeV, and higher-lying spin-flip strength in the energy region 5 − 9 MeV in deformed nuclei, nor to the presently discovered evidence for low-lying proton-neutron isovector quadrupole excitations in spherical nuclei. To the contrary, we put the experimental evidence in the perspectives of understanding the atomic nucleus and its various structures of well-organized modes of motion and thus enlarge our discussion to more general fermion and bosonic many-body systems.

Meaning of antiparallel proton and neutron angular momenta at low spins

“…The B(M1) strength to the scissors mode has in fact been analyzed in Ref. [4] in connection to the present J p -J n correlation, and the resultant B(M1) value (∼4(μ 2 N ) for 156 Sm [4]) is in a reasonable agreement with the systematic behavior of the observed B(M1) sum rule [8,14].…”

“…The scissors mode can be of high interest in neutron-rich exotic nuclei as discussed later in this section. Furthermore, the importance of the scissors mode is underlined by the fact that it is also found in other many-body systems such as trapped Bose-Einstein condensates [8]. A consistent picture of the scissors mode and the above-mentioned J p -J n correlation will be shown in some detail below.…”

“…The photon scattering facilities both at the S-DALINAC and HIGS at Duke University have produced and are still producing a wealth of data on the scissors mode and related phenomena. One finds many past and on-going developments in a review [8], as well as recent theoretical works (for example, Refs. [9,10]).…”

“…[8]. This idea is based on the proportionality of the summed M1 strength to the square of the nuclear deformation parameter [15].…”

The high-efficiency spectroscopy setup at

“…Until now γ-γ coincidence experiments have been performed mainly in combination with particle-induced reactions [1,2]. In such reactions high-lying J = 1 states (in even-even nuclei) are often not or only very weakly excited, which strongly limits the investigation of the decay behavior of excitation modes such as the nuclear scissors mode [3][4][5] or the Pygmy Dipole Resonance [6][7][8][9]. Consequently, for these excitation modes the decay behavior was not studied in detail so far.…”