Low-energy enhancement in theγ-ray strength functions ofGe73,74

The $\ensuremath{\gamma}$-ray strength functions and level densities of $^{73,74}\mathrm{Ge}$ have been extracted up to the neutron-separation energy ${S}_{n}$ from particle-$\ensuremath{\gamma}$ coincidence data using the Oslo method. Moreover, the $\ensuremath{\gamma}$-ray strength function of $^{74}\mathrm{Ge}$ above ${S}_{n}$ has been determined from photoneutron measurements; hence these two experiments cover the range of ${\mathrm{E}}_{\ensuremath{\gamma}}\ensuremath{\approx}1$--13 MeV for $^{74}\mathrm{Ge}$. The obtained data show that both $^{73,74}\mathrm{Ge}$ display an increase in strength at low $\ensuremath{\gamma}$ energies. The experimental $\ensuremath{\gamma}$-ray strength functions are compared with $M1$ strength functions deduced from average $B(M1)$ values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in $^{73,74}\mathrm{Ge}$ are adopted in the calculations of the $^{72,73}\mathrm{Ge}(n,\ensuremath{\gamma})$ cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.