Low-energy enhancement in the γ-ray strength functions of ${}^{73,74}$Ge

Abstract

The ${\hat{I}}^3$-ray strength functions and level densities of ${}^{73,74}$Ge have been extracted up to the neutron separation energy S${}_n$ from particle-${\hat{I}}^3$ coincidence data using the Oslo method. Moreover, the ${\hat{I}}^3$-ray strength function of ${}^{74}$Ge above S${}_n$ has been determined from photo-neutron measurements, hence these two experiments cover the range of E${}_{\hat{I}}^3\approx$ 1-13 MeV for ${}^{74}$Ge. The obtained data show that both ${}^{73,74}$Ge display an increase in strength at low ${\hat{I}}^3$ energies. The experimental ${\hat{I}}^3$-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 ${}^{73,74}$Ge are adopted in the calculations of the ${}^{72,73}$Ge(n,${\hat{I}}^3$) 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.