The nucleus is one of the most multifaceted 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 discussed in the present review. We shall not just limit ourselves to presenting the by now large data set that has been obtained in the past 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, the experimental evidence is put in the perspectives of understanding the atomic nucleus and its various structures of well-organized modes of motion and thus enlarges the discussion to more general fermion and bosonic many-body systems.

/pdf/magnetic-dipole-excitations-in-nuclei-elementary-modes-of-1lz9bk3at0.pdf

Magnetic dipole excitations in nuclei: Elementary modes of nucleonic motion

The nuclear physics of muon capture

Spin-isospin excitations probed by strong, weak and electro-magnetic interactions

The effects of varying the spin-orbit and tensor components of a realistic interaction on $M1$ excitation rates and $B(E2)'s$ are studied on nuclei in the $0p$ and $1s-0d$ shells. Not only the total $M1$ but also the spin and orbital parts separately are studied. The single-particle energies are first calculated with the same interaction that is used between the valence nucleons. Later this stringent condition is relaxed somewhat and the $1s$ level is raised relative to $0d$. For nuclei up to $^{28}Si$, much better results i.e stronger $B(M1)$ rates are obtained by increasing the strength of the spin-orbit interaction relative to the free value. This is probably also true for $^{32}S$, but $^{36}Ar$ presents some difficulties. The effects of weakening the tensor interaction are also studied. On a more subtle level, the optimum spin-orbit interaction in the lower half of the $s-d$ shell, as far as $M1$ excitations are concerned, is substantially larger than the difference $E(J=3/2^+)_1-E(J=5/2^+)_1=5.2~MeV$ in $^{17}O$. A larger spin-orbit splitting is also needed to destroy the triaxiality in $^{22}Ne$. Also studied are how much $M1$ orbital and spin strength lies in an observable region and how much is buried in the grass at higher energies. It is noted that for many nuclei the sum $B(M1)_{orbital}+B(M1)_{spin}$ is very close to $B(M1)_{total}$, indicating that the summed cross terms are very small.

Effects of the Spin-Orbit and Tensor Interactions on the $M1$ and $E2$ Excitations in Light Nuclei

Erratum to ``Dipole excitations in the semi-magic nucleus 51 V studied with the (/γ,γ') reaction'' [Nucl. Phys. A 660 (1999) 41-53]

Meson exchange currents (MEC{close_quote}s) can induce an enhancement of the B(M1) strength in nuclei over the corresponding Gamow-Teller strength. Such an increase was observed in {sup 28}Si [C. L{umlt u}ttge {ital et al.}, Phys. Rev. C {bold 53}, 127 (1996)], but the experimental ratio was much larger than shell model predictions using the unified sd-shell interaction and corrections to the g factors which incorporate MEC effects. It is pointed out that, when taking the Gamow-Teller strength from a reanalysis of the {sup 28}Si(p,n) data as well as from a recent {sup 28}Si({sup 3}He,t) experiment, the derived ratio is now in good agreement with the shell model results. This finding provides strong support that the observed enhancement is due to meson exchange currents. {copyright} {ital 1997} {ital The American Physical Society}

Enhancement of the isovector M1 strength in {sup 28}Si due to meson exchange currents

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Enhancement of the isovector M1 strength in {sup 28}Si due to meson exchange currents