Observation of low- and high-energy Gamow-Teller phonon excitations in nuclei

Structure of beta-decay strength function Sβ(E) and quenching of axial-vector weak interaction constant in halo nuclei

Resonance structure of the beta decay strength function Sβ (E) for GT β--decay of halo nuclei 6He and 11Li is analyzed. Compare experimental total strength for β-transitions in gv2/4π units with the Ikeda sum rule (in (gAeff)2/4π units) one can determine the squared ratio of axial-vector and vector weak interaction constants value (g eff/gV)2. We obtained (g eff/gv)1 = 1.272 ± 0.010 for 6He and (gA eff/gv)2 = 1.5±0.2 for 11Li β--decays. Quenching of the weak axial-vector constant gAeff in halo nuclei is discussed. Analysis of the evolution of EGTR - EIAS toward neutron-rich nuclei was done. It is shown that the value Z/N ≈ 0.6 corresponds to the SU(4) Wigner's spin-isospin symmetry region.

Analogous Gamow-Teller and M1 Transitions in Tz = ±½ Mirror Nuclei and in Tz = ±1, 0 Triplet Nuclei relevant to Low-energy Super GT state

Nuclei have spin- and isospin-degrees of freedom. Therefore, Gamow-Teller (GT) transitions caused by the στ operator (spin-isospin operator) are unique tools for the studies of nuclear structure as well as nuclear interactions. They can be studied in β decays as well as charge-exchange (CE) reactions. Similarly, M1 γ decays are mainly caused by the στ operator. Combined studies of these transitions caused by Weak, Strong, and Electro-Magnetic interactions provide us a deeper understanding of nuclear spin-isospin-type transitions. We first compare the strengths of analogous GT and M1 transitions in the A = 27, Tz = ±½ mirror nuclei 27Al and 27Si. The comparison is extended to the Tz = ±1, 0 nuclei. The strength of GT transition from the ground state (g.s.) of 42Ca to the 0.611 MeV first Jπ = 1+ state in 42Sc is compared with that of the analogous M1 transition from the 0.611 MeV state to the T = 1, 0+ g.s. (isobaric analog state: IAS) in 42Sc. The 0.611 MeV state has the property of Low-energy Super GT (LeSGT) state, because it carries the main part of the GT strength of all available transitions from the g.s. of 42Ca (and 42Ti) to the Jπ = 1+ GT states in 42Sc.

Isoscalar and isovector spin responses in p-shell and sd-shell nuclei

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.

Effects of three-nucleon forces and two-body currents on Gamow-Teller strengths

We optimize chiral interactions at next-to-next-to leading order to observables in two- and three-nucleon systems and compute Gamow-Teller transitions in 14C and (22,24)O using consistent two-body currents. We compute spectra of the daughter nuclei 14N and (22,24)F via an isospin-breaking coupled-cluster technique, with several predictions. The two-body currents reduce the Ikeda sum rule, corresponding to a quenching factor q2≈0.84-0.92 of the axial-vector coupling. The half-life of 14C depends on the energy of the first excited 1+ state, the three-nucleon force, and the two-body current.

Hindered Gamow-Teller decay to the odd-odd N=Z (62)Ga: absence of proton-neutron T=0 condensate in A=62

Search for a new kind of superfluidity built on collective proton-neutron pairs with aligned spin is performed studying the Gamow-Teller decay of the T=1, J(π)=0+ ground state of (62)Ge into excited states of the odd-odd N=Z nucleus (62)Ga. The experiment is performed at GSI Helmholtzzentrum für Shwerionenforshung with the (62)Ge ions selected by the fragment separator and implanted in a stack of Si-strip detectors, surrounded by the RISING Ge array. A half-life of T1/2=82.9(14)  ms is measured for the (62)Ge ground state. Six excited states of (62)Ga, populated below 2.5 MeV through Gamow-Teller transitions, are identified. Individual Gamow-Teller transition strengths agree well with theoretical predictions of the interacting shell model and the quasiparticle random phase approximation. The absence of any sizable low-lying Gamow-Teller strength in the reported beta-decay experiment supports the hypothesis of a negligible role of coherent T=0 proton-neutron correlations in (62)Ga.