The photo-neutron cross sections of 162,163 Dy have been measured for the first time in an energy region from the neutron threshold (S n ) up to ≈ 13 MeV. The (γ,n) reaction was induced with quasi-monochromatic laser Compton-scattered γ rays, produced at the NewSUBARU laboratory. The corresponding γ-ray strength functions (γSF) have been calculated from the photo-neutron cross sections. The data are compared to reanalyzed γSFs of 160−164 Dy, which are measured below S n . The excellent agreement with the photo-neutron data at S n confirms the principle of detailed balance. Thus, a complete γSF is established covering in total the energy region of 1 MeV ≤ E γ ≤ 13 MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as (γ, γ ) data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the γSFs as input in the reaction code TALYS, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined γSFs.

We have made a thorough study of the low-energy behaviour of the γ-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, considering 283 nuclei in total, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for M1 transitions. There are clear trends in the calculations: From being nearly absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find support for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with an LEE, and indicate that the slope varies as function of mass number.

Nuclear level densities and γ-ray strength functions of 56,57Fe have been extracted from proton-γ coincidences. A low-energy enhancement in the γ-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in 57Fe indicate its dipole nature, in agreement with findings for 56Fe. The high statistics and the excellent energy resolution of the large-volume LaBr3(Ce) detectors allowed for a thorough analysis of γ 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 γ-ray strength function, in support of the generalized Brink–Axel hypothesis.

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their γ-emission probability at very low γ energies. In this work, we present measurements of the γ-decay strength of 70 Ni over the wide range 1.3 ≤ E γ ≤ 8 MeV. A significant enhancement is found in the γ-decay strength for transitions with E γ < 3 MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed E1-strength calculations within the quasiparticle time-blocking approximation, which describe our data above E γ 5 MeV very well. Moreover, large-scale shell-model calculations indicate an M1 nature of the low-energy γ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich 72,74,76 Ni.