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Description
The presence at low energy of pair of nuclear levels differing in orbital angular momentum by two units, which can be ascribed to single-particle states in the shell model, is common place in many odd-mass nuclei located near closed shells. Such single-particle states can be labelled with the radial quantum number n$_r$, the orbital angular momentum $l$ and the total angular momentum $j$, and would correspond to |n$_r$ l j=l+1/2> and |n$_r$–1 l+2 j´=l+3/2>, respectively. The pairs s$_{1/2}$ – d$_{3/2}$, p$_{3/2}$ – f$_{5/2}$ and g$_{7/2}$ – d$_{5/2}$ are examples of such orbitals. They are experimentally observed as the ground state and low-lying first-excited state in many odd-A nuclei across the nuclear chart.
Since the magnetic dipole isovector operator does not change the orbital angular momentum, magnetic dipole M1 $\Delta$l=2 transitions between pairs of states of this kind are $l$-forbidden in the extreme shell model picture [1]. Nonetheless these transitions still occur, although with rates typically smaller than those of allowed transitions, or even below the single-particle limit. Consequently, it is anticipated that these transitions arise from the breakdown of $l$-forbiddeness due to nuclear dynamic effects such as core polarization and meson exchange mechanisms [2]. Therefore the investigation of $l$-forbidden M1 transitions may provide insight into the role of these effects within the atomic nucleus [3].
This study is a part of a systematic investigation of $l$-forbidden M1 transitions in semimagic nuclei, making use of available data and our own experimental results. We focus on odd-A N=50 nuclei in the vicinity $^{78}$Ni [4] and neutron-rich odd-A Z=50 Sn isotopes [5,6]. The experimental M1 transitions probabilities are obtained from excited level lifetime measurements employing fast-timing methods.
Regarding the N=50 isotopes new results will be presented for $^{83}$As and $^{85}$Br, obtained from experiments performed at ISOLDE/CERN and ILL, respectively. They will be discussed in the context of other available data for the region. In the case of tin (Z=50), the systematic study of $l$-forbidden transitions in several odd-mass isotopes just below $^{132}$Sn will be presented.
[1] I.M. Govil and C.S. Kurana, Systematics of l-forbidden M1 transitions, Nuclear Physics 60 (1964) 666-671.
[2] W. Andrejtscheff, L. Zamick, N.Z. Marupov et al., Core polarization of l-forbidden M1 transitions in light nuclei, Nuclear Physics A 351 (1981) 54-62.
[3] P. von Neumann-Cosel and J. N. Ginocchio, l-forbidden M1 transitions and pseudospin symmetry, Phys. Rev. C 62 (2000) 014308.
[4] V. Paziy, L.M. Fraile, H. Mach et al., Fast-timing study of $^{81}$Ga from the $\beta$ decay of $^{81}$Zn, Phys. Rev. C 102 (2020) 014329.
[5] R. Lica, H. Mach, L.M. Fraile, et al., Fast-timing study of the l-forbidden 1/2$^+$ $\to$ 3/2$^+$ M1 transition in $^{129}$Sn, Phys. Rev. C 93 (2016) 044303.
[6] J. Benito et al., submitted to Phys. Rev. C (2024).
