High-resolution study of Gamow-Teller excitations in thê {42}Ca(^{3}He,t)^{42}Sc reaction and the observation of a "low-energy super-Gamow-Teller state"Phys. Rev. C 91, 064316

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A ¼ 42, 46, 50, and 54 "f-shell" nuclei in ( 3 He, t) charge-exchange reactions. In the 42 Ca → 42 Sc reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the 54 Fe → 54 Co reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei.

The nature of 0 + excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying 158 Gd, we investigated a large group of nuclei in the rare-earth region with the (p, t) pickup reaction using the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. Outgoing tritons were recorded at various lab angles, and their angular distributions are compared to those calculated using the distorted-wave Born approximation. Using the unique shape of the L = 0 angular distribution, more than double the number of 0 + states than were previously known are identified. The distribution of 0 + energies and cross sections is discussed in terms of collective and noncollective degrees of freedom, and the density of low-lying 0 + states is discussed as a corroboration of a characteristic feature of phase transition regions. The degree of level mixing, as extracted from Brody distribution fits to the energy spacings of adjacent 0 + levels, is also explored.

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