Effect of nuclear deformation on the observation of a low-energy super-Gamow-Teller state

A well-known structure with concentrated Gamow-Teller (GT) transitions is the Gamow-Teller resonance, which has been observed at higher-energy regions (usually >6 MeV) of nuclear excitation. It has been found that the GT strength can also concentrate in the lowest J(pi) = 1(+) GT state named the low-energy super-GT (LeSGT) state when the initial even-even nucleus has the structure of LS-closed-shell core nucleus + 2 neutrons (or 2 protons) and the final nucleus LS-closed-shell core nucleus + 1 proton and 1 neutron. Such concentrations are realized with the core nuclei He-4, O-16, and Ca-40, corresponding to the shell closures of s, p, and sd shells, respectively. It is natural to speculate that the LeSGT state may also be observed in the A = 82 systems if the N = Z = 40 shell gap is significant and Zr-80 represents a good core nucleus corresponding to the pf shell closure. Possible conditions that allow the formation of the LeSGT state in the Zr-82 -> Nb-82 charge-exchange reaction (or Mo-82 -> Nb-82 beta decay) are discussed by evaluating the results of projected shell model calculations, which are based on deformed model space. Our calculations show that with increasing deformation, the LeSGT feature found in the spherical limit (zero deformation) evolves gradually into a broad distribution in the higher-energy region. This lets us conclude that no LeSGT state is expected in Nb-82 because the shape of the Zr-80 core nucleus is ellipsoidal.

Automated summary

A well-known structure in nuclear excitation is the Gamow-Teller resonance, where concentrated Gamow-Teller transitions occur at higher-energy regions. The low-energy super-GT (LeSGT) state, which is the lowest J(pi) = 1(+) GT state, can also show concentrated GT strength under specific conditions. This phenomenon is observed when the initial and final nuclei have LS-closed-shell core structures and the core nuclei belong to certain shell closures.