Nature of low-lying electric dipole resonance excitations in ${}^{74}\mathrm{Ge}$

Isospin properties of dipole excitations in ${}^{74}\mathrm{Ge}$ are investigated using the $\left(\alpha ,{\alpha }^{\prime }\gamma \right)$ reaction and compared to $\left(\gamma ,{\gamma }^{\prime }\right)$ data. The results indicate that the dipole excitations in the energy region of 6 to 9 MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar $E1$ strength with an increase in excitation energy, which is consistent with the measurement.

Figure 1

Spectrum of $\alpha$ particles detected in coincidence with $\gamma$ rays. Solid and dashed curves are representing data with and without the subtraction of uncorrelated events, respectively. Visible peaks (solid curve) are strongly populated discrete states in ${}^{74}\mathrm{Ge}$. ${\mathrm{S}}_n$ indicates the location of the neutron separation energy.

Figure 2

Spectrum of $\gamma$-ray transitions decaying directly to the ground state from defined excitation energies. Blue and red spectra correspond to correlated and uncorrelated events, respectively. Inset: the lower energy part of the spectrum where the arrow indicates the position of the unobserved 3558-keV transition, known from $\left(\gamma ,{\gamma }^{\prime }\right)$ experiments [39, 40].

Figure 3

Angular distributions of (a) the first-excited state $l=2$ 596-keV transition and (b) $l=1$ transitions from the known 2690- and 3648-keV states together with the total strength of resolved and unresolved transitions for $6.5 <E_x<$ 8 MeV in ${}^{74}\mathrm{Ge}$.

Figure 4

In panel (a) relative cross sections of $E1$ transitions from the $\left(\alpha ,{\alpha }^{\prime }\gamma \right)$ data are plotted, while in panel (b) the relative integrated scattering cross sections $I_s$ obtained from $\left(\gamma ,{\gamma }^{\prime }\right)$ data [40] are shown. Numbers next to some transitions indicate the total value of relative cross section. In panel (a), the sensitivity limit is shown by the black solid curve and was determined using the procedure outlined in Ref. [11]. Uncertainties on the cross sections in panel (a) are $\sim 50$% for weakly populated states and decrease to $\sim 15\%$ for strongly populated states.

Figure 5

Reduced transition probabilities in ${}^{74}\mathrm{Ge}$ from RQTBA calculations plotted for the isoscalar (a) and electromagnetic (isovector) (b) dipole operators.

Figure 6

Transition densities for two calculated RQTBA states at $E_x=4.55$ (a) and 7.05 MeV (b) in ${}^{74}\mathrm{Ge}$.

Figure 7

Cross sections of inelastic scattering of $\alpha$ particles are plotted as a function of scattering angle in the center-of-mass frame for the $2^{+}$ state in ${}^{74}\mathrm{Ge}$ at 596 keV and dipole states at (a) 4.55 and (b) 7.05 MeV. The blue shaded areas represent the angular coverage of scattered $\alpha$ particles in the present measurement.