Role of T=0 pairing in Gamow–Teller states in N=Z nuclei

Abstract

Gamow-Teller (GT) states in N=Z nuclei with the mass number A from 48 to 64 are studied by using Hartree-Fock-Bogoliubov + quasi-particle random phase approximation (HFB+QRPA) with Skyrme interactions. The isoscalar spin-triplet (T=0,S=1) pairing interaction is taken into account in QRPA calculations. It is found in the context of SU(4) symmetry in the spin-isospin space that the GT strength of lower energy excitations is largely enhanced by the T=0 pairing interaction which works cooperatively with the T=1 pairing interaction in the ground state. A two-peaked structure observed recently in (p, n) reaction on 56 Ni can be considered as a manifestation of the role of T=0 pairing in the GT collective states. Keywords:Gamow Teller states, T=0 pairing interaction, SU(4) supermultipletThe most prominent evidence of pairing correlation in nuclei is found in the odd-even staggering in binding energies and the gap in the excitation spectrum of even-even nuclei in contrast to the compressed quasi-particle spectrum in odd-A nuclei [1,2,3]. There are also dynamical effects of pairing correlations seen in the moment of inertia associated with nuclear rotation and large amplitude collective motion [3,4,5]. The Hartree-Fock (HF)+BCS method and HartreeFock-Bogoliubov (HFB) method have been commonly used to study the ground state properties of superfluid nuclei in a broad mass region [6,7,8,9]. For the study of excited spectra, quasiparticle random phase approximation (QRPA) has often been adopted as a basic method [10,11,12,13].The strong attraction between nucleons is the basic ingredient for the pairing correlations. So far, the pairing interactions of like-nucleons with the isovector spin-singlet (T=1, S=0) channel has been mainly discussed. In fact, the attraction between protons and neutrons is even stronger in the isoscalar spin-triplet (T=0, S=1) channel [14], which gives rise to the deuteron bound state. However the role of T=0 pairing is limited in nuclei because of large imbalance between neutron and proton numbers, and also the two-body spin-orbit interaction which breaks the S=1 pair more effectively than the S=0 pair [4,15,16]. Nevertheless, the isoscalar pairing causes extra binding energies in nuclei with N=Z and has been considered as one of the origins of the Wigner energy [17].Gamow-Teller (GT) states have been studied both experimentally and theoretically intensively in the last three decades. Many interesting nuclear structure information has been reEmail address: sagawa@u-aizu.ac.jp (H. Sagawa) vealed by these studies, for example, the quenching of sum rule strength [18] and the role of GT strength in the astrophysical processes such as neutrino-nucleus reactions [19]. Because of recent development of modern radioactive beam accelerator, it becomes feasible to observe GT states in exotic nuclei near the proton and neutron drip lines. Recently, the GT transition strength was studied in a N=Z nucleus 56 Ni which has an important impact on late stellar evolution through electron capture and β deca...

Self-consistent relativistic quasiparticle random-phase approximation and its applications to charge-exchange excitations

“…Moreover, the important ingredient of nuclear force -the tensor force was found to play a crucial role in describing the nuclear charge-exchange excitations and β-decay half-lives within the QRPA approaches [40][41][42][43], which inspires much interest to explore the nature of nuclear tensor force [44,45].…”

Low-energy collective Gamow-Teller states and isoscalar pairing interaction

“…[32], a two-peaked structure was observed in the N = Z nucleus 56 Ni and this can provide some insightful information on the isoscalar pairing. Then, the GT transitions in 56 Ni together with other N = Z nuclei were studied by employing the HFB + QRPA theory [33], and this study has highlighted the formation of low-energy GT states in several f -shell nuclei, induced by the isovector and isoscalar pairing interactions and having reasonably strong strength. Recently, it was reported that strong GT states are experimentally observed, only slightly higher than the IAR, in some N = Z + 2 nuclei with mass number A = 42-54 [34].…”

High-resolution study ofTz=+1→0Gamow-Teller transitions in theMg26(He

“…Fine structures are also reported in pf -shell nuclei 50 Mn, 54 Co, and 58 Cu (T z = 0) [35][36][37][38], 47 V (T z = +1/2) [39], 48 V and 60 Cu (T z = +1) [5,40], 62 Cu (T z = +2) [5], and 64 Cu (T z = +3) [5,41]. However, the GTR structure was not found in 42 Sc (T z = 0) [26] and was not clear in 46 V (T z = 0) [38,42] as well as 44 Sc (T z = +1) [43]. Note that the observed features of the strength distributions and the fragmented GTR structures are distinctively different in these sd-and pf -shell nuclei, showing a marked individuality of their structures (see the discussion in Refs.…”

“…[44]). Owing to the repulsive nature, it is expected that the GT strength is mainly pushed up to the higher-E x region and forms the GTR structure (see, e.g., Refs.…”

“…On the other hand, in open-shell nuclei, the isoscalar (IS)-type residual pairing interaction is active in proton-particle-neutron-particle (πp-νp) configurations in GT excitations. Owing to the attractive nature of the IS interaction, it is expected that the GT strength is pulled down to the lower-E x region [26,38,44,45]. In fact, in the measurement of GT transitions from 42 Ca to 42 Sc having mainly πp-νp configurations [i.e., the πf 7/2 -νf 7/2 and πf 5/2 -νf 7/2 configurations] on the 40 Ca core, it has been observed that the GT strength mostly concentrates in the lowest 1 + GT state at 0.611 MeV in 42 Sc [which is called the low-energy super Gamow-Teller state (LeSGT state)] and only a little GT strength remains in the GTR region [26,38].…”