Abstract
We study the spin magnetic dipole transitions in sd-shell even-even nuclei with N=Z and a p-shell nucleus 12C by modern shell model calculations. The shell model wave functions take into account enhanced isoscalar (IS) spin-triplet pairing as well as the effective spin operators. We point out that the IS pairing and the effective spin operators give a large quenching effect on the isovector (IV) spin transitions to be consistent with observed data by (p, p′) experiments. On the other hand, the observed IS spin strengths do not show large quenching effect suggested by the calculated investigation. The IS pairing gives a substantial quenching effect on the spin magnetic dipole transitions, especially on the IV ones.
Highlights
The spin-isospin response and spin-isospin dependent interactions in nuclei are fundamental important subjects in nuclear physics and astrophysics
The quenching phenomena of magnetic moments and Gamow–Teller (GT) strengths have been extensively studied by taking into account the mixings of particle-hole (p−h) configurations, meson exchange currents and the coupling to the ∆ resonances [1,2,3,4,5]
On the quenching of the GT sum rule, 3(N−Z) [6], there have been a serious question whether the effect of many-particle many hole states or the coupling to ∆-isobar states is a dominant contribution [4, 5]
Summary
The spin-isospin response and spin-isospin dependent interactions in nuclei are fundamental important subjects in nuclear physics and astrophysics. After a long debate [7], experimental investigations by chargeexchange (p, n) and (n, p) reactions on 90Zr have revealed that about 90% of the GT sum rule strength exists in the energy region up to Ex=50 MeV [8, 9] This demonstrates the importance of higher order effect beyond the mean-field approximations; mostly, the two-particle twohole (2p−2h) configuration mixings due to the central and tensor forces [3], the coupling to ∆ is not completely excluded. One clear observation of IS spin excitation is 1+ state at 12.71 MeV in 12C by electron scattering This observation shows no quenching in IS M1 strength compared with the shell model calculation, but indicating a large enhancement. The former corresponds to the ferromagnet limit of the spin alignment, while the latter is the antiferromagnetic one
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