Nuclear spin and isospin excitations

Abstract

A review is given of our present knowledge of collective spin-isospin excitations in nuclei. Most of this knowledge comes from intermediate-energy charge-exchange reactions and from inelastic electron-and proton-scattering experiments. The nuclear-spin dynamics is governed by the spin-isospin-dependent two-nucleon interaction in the medium. This interaction gives rise to collective spin modes such as the giant Gamow-Teller resonances. An interesting phenomenon is that the measured total Gamow-Teller transition strength in the resonance region is much less than a model-independent sum rule predicts. Two physically different mechanisms have been discussed to explain this so-called quenching of the total Gamow-Teller strength: coupling to subnuclear degrees of freedom in the form of A-isobar excitation and ordinary nuclear configuration mixing. Both detailed nuclear structure calculations and extensive analyses of the scattering data suggest that the nuclear configuration mixing effect is the more important quenching mechanism, although subnuclear degrees of freedom cannot be ruled out. The quenching phenomenon occurs for nuclear-spin excitations at low excitation energies {co~ 10-20 MeV) and smallmomentum transfers {q < 0.5 fm~ *). A completely opposite effect is anticipated in the high (&>,#)-transfer region (0 < (o < 500 MeV, 0.5 < q < 3 fm~1). The nuclear spin-isospin response might be enhanced due to the attractive pion field inside the nucleus. Charge-exchange reactions at GeV incident energies have been used to study the quasifree peak region and the A-resonance region. An interesting result of these experiments is that the A excitation in the nucleus is shifted downwards in energy relative to the A excitation of the free proton. The physical origin of this shift is discussed, and it is shown that it may be related to the energy-dependent, attractive one-pion exchange interaction in the medium.

Tensor part of the Skyrme energy density functional. III. Time-odd terms at high spin

“…Note also that empirical values for g 0 ≈ 0.4 [91] and 1.4 ≤ g ′ 0 ≤ 1.6 [92][93][94] have been derived from M1 and Gamow-Teller response. The values quoted in Table I and II are in all cases except one very different from the empirical values.…”

Isospin-breaking corrections to superallowed Fermiβdecay in isospin- and angular-momentum-projected nuclear density functional theory

“…The most likely origin of this compensation mechanism is due to the time-odd spin-isospin mean fields, which are poorly constrained by the standard fitting protocols of Skyrme EDFs [41][42][43]. For instance, if one compares the Landau-Migdal parameters characterizing the spin-isospin time-odd channels [41][42][43] To illustrate the compensation mechanism related to the bulk symmetry energy and g 0 , in Fig. 10 we plot δ C for the 14 O→ 14 N→ 14 C super-allowed 0 + → 0 + transitions as functions of the bulk symmetry parameter a sym for a set of SV-based Skyrme forces with systematically varied x 0 parameter.…”

“…This is an intriguing result, which indicates that an increase of the bulk symmetry energy -that tends to restore the isospin symmetry -is partly compensated by other effects. The most likely origin of this compensation mechanism is due to the time-odd spin-isospin mean fields, which are poorly constrained by the standard fitting protocols of Skyrme EDFs [41][42][43]. For instance, if one compares the Landau-Migdal parameters characterizing the spin-isospin time-odd channels [41][42][43] To illustrate the compensation mechanism related to the bulk symmetry energy and g 0 , in Fig.…”

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

“…The spin and orbital currents-and their interference invoked by M1 transitions provide insight into a number of important problems. The spin part is closely linked to the Gamow-Teller (GT) strength from β decay or charge exchange processes because it is mediated by the same one-body operator [1]. A quenching of the spin g-factor is observed in analogy to the renormalization of the axial-vector coupling constant (see, e.g., [2,3] and references therein).…”