The strength function Sβ(Е) governs [1,2] the nuclear energy distribution of elementary charge-exchange excitations and their combinations like proton particle (πp)–neutron hole (νh) coupled into a momentum Iπ : [πp νh]Iπ and neutron particle (νp)–proton hole (πh) coupled into a momentum Iπ : [νp πh)]Iπ. The strength function of Fermi-type β-transitions takes into account excitations [πp νh]0+ or [νp πh]0+. Since isospin is a quite good quantum number, the strength of the Fermi-type transitions is concentrated in the region of the isobar-analogue resonance (IAR). The strength function for β-transitions of the Gamow–Teller (GT) type describes excitations [πp νh]1+ or [νp πh]1+. At excitation energies E smaller than Qβ (total β-decay energy), Sβ(E) determines the characters of the β-decay. For higher excitation energies that cannot be reached with the β-decay, Sβ(E) determines the charge exchange nuclear reaction cross sections, which depend on the nuclear matrix elements of the β-decay type. From the macroscopic point of view, the resonances in the GT β-decay strength function Sβ(E) are connected with the oscillation of the spin–isospin density without change in the shape of the nucleus [1,3].
When the nuclear parent state has the two-neutron Borromean halo structure, than IAR and configuration states (CSs) can simultaneously have nn, np Borromean halo components in their wave functions . After M1 γ-decay of IAR with np Borromean halo structure or GT β- - decay of parent nuclei with nn Borromean halo structure, the states with np halo structure of tango type may be populated [5-7].
In this work the structure of resonances in the GT β-decay strength function Sβ(E) for halo nuclei is discussed. It is shown that when the parent nucleus has nn Borromean halo structure, then after GT β- -decay of parent state or after M1 γ-decay of IAR the states with np tango halo structure or mixed np tango + nn Borromean halo structure can be populated. Or, in other words, resonances in the GT β-decay strength function Sβ(E) of halo nuclei, may have np tango halo structure or mixed np tango + nn Borromean halo structure. Structure of Sβ(E) may be studied both in experiments on M1 γ-decay of (or on) IAR and in experiments on Sβ(E) measurements in charge exchenge nuclear reactions or in β-decay [1,2]. Since the operators of GT β-decay and M1 γ-decay have no spatial components, GT β-transitions and M1 γ-transitions between states with similar spatial shapes are favoured. Data of 6He (Borromean nn halo) ground state (g.s., Iπ=0+) GT β-decay and M1 gamma decay of its IAR (Borromean np halo, resonans in 6Li, E=3.56 MeV, Iπ=0+) were analysed. The enhancement of the M1 gamma transition from the IAR to the ground state of the 6Li nucleus (Iπ=1+) complies the presence of an np tango halo in 6Li g.s.
1.Yu.V. Naumov, A.A. Bykov, I.N. Izosimov, Sov.J.Part.Nucl., 14, 175 (1983)
2.I.N. Izosimov, V.G. Kalinnikov, A.A. Solnyshkin, Physics of Particles and Nuclei, 42, 1804 (2011). DOI: 10.1134/S1063779611060049
3.I.N. Izosimov, A.A. Solnyshkin, J.H. Khushvaktov, Yu.A. Vaganov, Joint Institute for Nuclear Research Preprint E6-2017-29, Dubna (2017)
4.I.N. Izosimov, Procceedings of the International Symposium on Exotic Nuclei (EXON2012), Vladivostok, Russia, 2012 (World Scientific, 2013), p.129.
5.I.N. Izosimov, AIP Conference Proceedings, 1681, 030006 (2015).
6.I.N. Izosimov, EPJ Web of Conferences, 107, 09003 (2016)
7.I.N. Izosimov, Phys. of At. Nucl., 80, 867 (2017). DOI:10.1134/S1063778817050118