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...
The β-decay half-lives of 110 neutron-rich isotopes of the elements from 37 Rb to 50 Sn were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell effects. The new data have direct implications for r-process calculations and reinforce the notion that the second (A ≈ 130) and the rare-earth-element (A ≈ 160) abundance peaks may result from the freeze-out of an ðn; γÞ ⇄ ðγ; nÞ equilibrium. In such an equilibrium, the new half-lives are important factors determining the abundance of rare-earth elements, and allow for a more reliable discussion of the PRL 114, 192501 (2015) P H Y S I C A L R E V I E W L E T T E R S week ending 15 MAY 2015 0031-9007=15=114(19)=192501 (7) 192501-1 © 2015 American Physical Society r process universality. It is anticipated that universality may not extend to the elements Sn, Sb, I, and Cs, making the detection of these elements in metal-poor stars of the utmost importance to determine the exact conditions of individual r-process events. Introduction.-The origin of the heavy elements from iron to uranium is one of the main open questions in science. The slow neutron-capture (s) process of nucleosynthesis [1,2], occurring primarily in helium-burning zones of stars, produces about half of the heavy element abundance in the universe. The remaining half requires a more violent process known as the rapid neutron-capture (r) process [3][4][5][6]. During the r process, in environments of extreme temperatures and neutron densities, a reaction network of neutron captures and β decays synthesizes very neutron-rich isotopes in a fraction of a second. These isotopes, upon exhaustion of the supply of free neutrons, decay into the stable or semistable isotopes observed in the solar system. However, none of the proposed stellar models, including explosion of supernovae [7][8][9][10][11][12] and merging neutron stars [13][14][15][16], can fully explain abundance observations. The mechanism of the r process is also uncertain. At temperatures of one billion degrees or more, photons can excite unstable nuclei which then emit neutrons, thus, counteracting neutron captures in an ðn; γÞ ⇄ ðγ; nÞ equilibrium that determines the r process. These conditions may be found in the neutrino-driven wind following the collapse of a supernova core and the accreting torus formed around the black hole remnant of merging neutron stars. Alternatively, recent r-process models have shown that the r process is also possible at lower temperatures or higher neutron densities where the contribution from ðγ; nÞ reactions is minor. These conditions are expected in supersonically expanding neutrino-driven outflow in low-mass supernovae progenitors (e.g., 8 − 12 M ⊙ ) or prompt ejecta from neutron star mergers [17]. The final abundance distribution may also be dominated by postprocessing effects such as fission of heavy nuclei (A ≳ 280) possibly produced in merging neutron stars [18].New clues about the r process have come from the discovery of de...
Classical novae are expected to contribute to the 1809-keV Galactic γ-ray emission by producing its precursor 26Al, but the yield depends on the thermonuclear rate of the unmeasured 25Al(p,γ)26Si reaction. Using the β decay of 26P to populate the key J(π)=3(+) resonance in this reaction, we report the first evidence for the observation of its exit channel via a 1741.6±0.6(stat)±0.3(syst) keV primary γ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured γ-ray energy and intensity with other experimental data on 26Si, we find the center-of-mass energy and strength of the resonance to be E(r)=414.9±0.6(stat)±0.3(syst)±0.6(lit.) keV and ωγ=23±6(stat)(-10)(+11)(lit.) meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model 26Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic 26Al.
The half-lives of 20 neutron-rich nuclei with Z ¼ 27-30 have been measured at the RIBF, Atomic nuclei are quantum many-body systems consisting of two distinct types of fermions-protons and neutrons. Analogous to atomic physics, the concept of nuclear shell structure was triggered by the discovery of particularly stable nuclei with specific numbers of proton and neutron, such as 2, 8,20,28, 50, 82, and 126 along the β-stability line [1]. By assuming a strong spin-orbit interaction within a mean field potential, these magic numbers were correctly interpreted and regarded to be immutable throughout the nuclear chart [2,3]. However, with the development of experimental techniques exploiting radioactive ion beams, many nuclei with extreme neutron-to-proton ratios (N=Z), so-called exotic nuclei, have been produced and studied in the last few decades. The results obtained heretofore have demonstrated that the shell structure established for nuclei near the β-stability line may change drastically in these exotic nuclei. For instance, classical magic numbers in 12 Be (N ¼ 8), 32 Mg (N ¼ 20), and 42 Si (N ¼ 28) were found to disappear [4-6], whereas new magic numbers emerged in 24 O (N ¼ 16) and 54 Ca (N ¼ 34) [7][8][9]. To address the origins of shell evolution in heavier mass regions, it is of particular interest to investigate the properties of nuclei in the vicinity of 78 Ni, which has the proton number Z ¼ 28 and the neutron number N ¼ 50 with a large neutron excess N=Z ≈ 1.8.To study the shell evolution around 78 Ni, many experimental efforts have been made. One of the interesting phenomena related to the proton Z ¼ 28 shell gap is the monopole migration in Cu isotopes. A sudden drop of the excited 5=2− state relative to the ground 3=2 − state was observed in 71;73 Cu [10,11]. These two states are characterized by a single-particle nature [12] and their order was
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