Search

Nuclear level densities and $��$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$��$ coincidences. A low-energy enhancement in the $��$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $��$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $��$-ray strength function, in support of the generalized Brink-Axel hypothesis., 24 pages, 17 figures, accepted for publication in J. Phys. G: Nucl. Phys; Special Issue

The $��$-ray strength functions and level densities of $^{73,74}$Ge have been extracted up to the neutron separation energy S$_n$ from particle-$��$ coincidence data using the Oslo method. Moreover, the $��$-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$_��\approx$ 1-13 MeV for $^{74}$Ge. The obtained data show that both $^{73,74}$Ge display an increase in strength at low $��$ energies. The experimental $��$-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,$��$) 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.

The $��$-ray strength function of $^{56}$Fe has been measured from proton-$��$ coincidences for excitation energies up to $\approx 11$ MeV. The low-energy enhancement in the $��$-ray strength function, which was first discovered in the ($^3$He,$����$)$^{56}$Fe reaction, is confirmed with the ($p,p^\prime��$)$^{56}$Fe experiment reported here. Angular distributions of the $��$ rays give for the first time evidence that the enhancement is dominated by dipole transitions., 5 pages, 4 figures. Version accepted for publication in Phys. Rev. Letters

The level densities and $��$-ray strength functions of $^{105,106,111,112}$Cd have been extracted from particle-$��$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The $��$-ray strength functions display no strong enhancement for low $��$ energies. However, more low-energy strength is apparent for $^{105,106}$Cd than for $^{111,112}$Cd. For $��$ energies above $\approx$ 4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to $E1$ and $M1$ transitions originating from neutron skin oscillations or the spin-flip resonance, respectively., 10 pages, 11 figures