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High-resolution study of Gamow-Teller excitations in thê {42}Ca(^{3}He,t)^{42}Sc reaction and the observation of a "low-energy super-Gamow-Teller state"Phys. Rev. C 91, 064316

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...