Magnetic dipole strength functions are deduced from averages of a large number of M1 transition strengths calculated within the shell model for the nuclides 90Zr, 94Mo, 95Mo, and 96Mo. An enhancement of M1 strength toward low transition energy is found for all nuclides considered. Large M1 strengths appear for transitions between close-lying states with configurations including proton as well as neutron high-j orbits that recouple their spins and add up their magnetic moments coherently. The M1 strength function deduced from the calculated M1 transition strengths is compatible with the low-energy enhancement found in (3He, 3He') and (d, p) experiments. The Letter presents an explanation of the experimental findings.

A low-energy enhancement of the γ-ray strength function in several light and medium-mass nuclei has been observed recently in 3 He-induced reactions. The effect of this enhancement on (n, γ) cross-sections is investigated for stable and unstable neutron-rich Fe, Mo and Cd isotopes. Our results indicate that the radiative neutron capture cross sections may increase considerably due to the low-energy enhancement when approaching the neutron drip line. This could have non-negligible consequences on r-process nucleosynthesis calculations.

Nuclear level density and $\gamma$-ray strength functions of $^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He,$^3$He$^\prime\gamma$) and ($^3$He,$\alpha \gamma$) reactions. The level densities of $^{121,122}$Sn display step-like structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for $E_\gamma \gtrsim 5.2 $ MeV. This enhancement is compatible with pygmy resonances centered at $\approx 8.4(1)$ and $\approx 8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\approx1.8^{+1}_{-5}%$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in $^{116-119}$Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in $^{116-122}$Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths

A novel technique has been developed, which will open exciting new opportunities for studying the very neutron-rich nuclei involved in the r-process. As a proof-of-principle, the γ-spectra from the β -decay of 76 Ga have been measured with the SuN detector at the National Superconducting Cyclotron Laboratory. The nuclear level density and γ-ray strength function are extracted and used as input to Hauser-Feshbach calculations. The present technique is shown to strongly constrain the 75 Ge(n, γ) 76 Ge cross section and reaction rate.One of the most important questions in Nuclear Astrophysics is the origin of the elements heavier than iron. It is well known that there are three main processes responsible for the nucleosynthesis of the heavier elements: two neutroninduced processes (s-and r-process) that create the majority of these nuclei and a third process (p-process), which is called upon to produce the small number of neutron-deficient isotopes that are not reached by the other two processes. Although the general characteristics of these processes were proposed already more than fifty years ago [1], they are far from understood.Despite the fact that the r-process is responsible for producing roughly half of the isotopes of the heavy elements, its astrophysical site has not yet been unambiguously identified. Multiple sites have been proposed and investigated, however, to date, no firm conclusion has been drawn for where the rprocess takes place. Nevertheless, it is thought to occur in environments with a high density of free neutrons, where neutron capture reactions push the matter flow to very neutronrich nuclei, while subsequent β -decays bring the flow back to the final stable nuclei (e.g. [2]). One of the limiting factors in being able to determine the r-process site are the large uncertainties in the nuclear physics input. Because the nuclei involved in the r-process are many mass units away from the valley of stability, it is difficult, and sometimes even impossible to measure the relevant quantities directly. A large effort has been devoted to the measurement of masses, β -decay half-lives, and β -delayed neutron emission probabilities (e.g. recently [3][4][5]), however, the majority of the r-process nuclei are still not accessible. In addition, although in many environments the neutron-capture reaction rates do not play significant role in the r-process flow due to (n, γ)-(γ, n) equilib- * spyrou@nscl.msu.edu † liddick@nscl.msu.edu ‡ a.c.larsen@fys.uio.no rium, recent studies have shown significant sensitivity to the neutron-capture reaction rates in certain conditions [6]. A major recognized challenge in the field is the measurement of the relevant neutron-capture reactions since all of the participating nuclei are unstable with short half-lives. The direct determination of the (n, γ) cross sections that dominate in many cases the astrophysical r-process is not currently possible. It is therefore of paramount importance to develop indirect techniques to extract these critical reaction rates.Many differ...

We have analyzed primary γ-ray spectra of the odd-odd 238 Np nucleus extracted from 237 Np(d, pγ) 238 Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary γ spectra cover an excitation-energy region of 0 ≤ E i ≤ 5.4 MeV, and allowed us to perform a detailed study of the γ-ray strength as function of excitation energy. Hence, we could test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the γ strength. In this work, using the available highquality 238 Np data, we show that the γ-ray strength function is to a very large extent independent on the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis has been experimentally verified for γ transitions between states in the quasi-continuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.PACS numbers: 24.30. Gd, 21.10.Ma, 25.40.Hs Sixty years ago, David M. Brink proposed in his PhD thesis [1] that the photoabsorption cross section of the giant electric dipole resonance (GDR) is independent of the detailed structure of the initial state. In his thesis, he expressed his hypothesis as follows: "If it were possible to perform the photo effect on an excited state, the cross section for absorption of a photon of energy E would still have an energy dependence given by (15)", where equation (15) refers to a Lorentzian shape of the photoabsorption cross section. Brink's original idea, the Brink hypothesis, was first intended for the photoabsorption process on the GDR, but has been further generalized, applying the principle of detailed balance, to include absorption and emission of γ rays between resonant states [2,3]. In addition to assuming independence of excitation energy, there is no explicit dependence of initial and final spins except the obvious dipole selection rules, implying that all levels exhibit the same dipole strength regardless of their initial spin quantum number. We will refer to this as the generalized Brink-Axel (gBA) hypothesis. A review of the history of the hypothesis was given by Brink in Ref. [4].The gBA hypothesis has implications for almost any situation where nuclei are brought to an excited state above ≈ 2∆, where ∆ ≈ 1 MeV is the pair-gap parameter. Here, the nucleus will typically de-excite via γ-ray emission and/or by emission of particles. In this context, it is usual to translate the γ-ray cross section σ (E γ ) into γ-ray strength function (γSF) by f (E γ ) = (3π 2h2 c 2 ) −1 σ (E γ )/E γ .To describe and model the electric dipole part of the γ-decay channel, the gBA hypothesis is frequently used, applying in particular the assumption of spin independence [5]. For example, a rather standard approach to calculating E1 strength is to apply some variant of the quasi-particle random-phase approximation (QRPA) to obtai...