Observation of Low- and High-Energy Gamow-Teller Phonon Excitations in Nuclei

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 $A=42$, 46, 50, and 54 “$f$-shell” nuclei in (${}^3\mathrm{He}$, $t$) charge-exchange reactions. In the ${}^{42}\mathrm{Ca}\to {}^{42}\mathrm{Sc}$ 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 ${}^{54}\mathrm{Fe}\to {}^{54}\mathrm{Co}$ 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.

Figure 1

High energy-resolution spectra of (${}^3\mathrm{He}$, $t$) reaction on $A=42–54$, $T_z=+1$ target nuclei. The vertical scales are normalized so that the heights of all GT peaks are approximately proportional to the $B(\mathrm{GT})$ values. The GT strength is concentrated in one low-energy state in ${}^{42}\mathrm{Sc}$ [panel (a)]. The fine structures of GTRs in the 6–11 MeV region are observed in panels (c) and (d).

Figure 2

(a) Cumulative-sum strengths of the experimental $B(\text{GT})$ values in the final nuclei ${}^{42}\mathrm{Sc}$, ${}^{46}\mathrm{V}$, ${}^{50}\mathrm{Mn}$, and ${}^{54}\mathrm{Co}$. They are shown by dotted-dashed, dashed, dotted, and solid lines, respectively. (b) Cumulative sums from SM calculations [28] including the quenching factor of ${(0.74)}^2$.

Figure 3

The SM configurations before and after the ${\beta }^{-}$-type GT transitions in $A=42–54$, $f$-shell nuclei. Positions occupied by protons ($\pi$) and neutrons ($\nu$) are shown by open crosses. Positions that are newly occupied by protons and unoccupied by neutrons after making GT transitions (shown by the arrows) are indicated by filled crosses and open circles, respectively. The shell closures at $Z=N=20$ and 28 are indicated by thick solid lines.