3,063 research outputs found

    Concentration of electric dipole strength below the neutron separation energy in N = 82 nuclei

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    The semi-magic nuclei Ba-138, Ce-140, and Sm-144 have been investigated in photon scattering experiments up to an excitation energy of about 10 MeV. The distribution of the electric dipole strength shows a resonance like structure at energies between 5.5 and 8 MeV exhausting up to 1% of the isovector E1 Energy Weighted Sum Rule.Comment: 10 pages, 3 figure

    Photon-induced Nucleosynthesis: Current Problems and Experimental Approaches

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    Photon-induced reactions play a key role in the nucleosynthesis of rare neutron-deficient p-nuclei. The paper focuses on (gamma,alpha), (gamma,p), and (gamma,n) reactions which define the corresponding p-process path. The relation between stellar reaction rates and laboratory cross sections is analyzed for photon-induced reactions and their inverse capture reactions to evaluate various experimental approaches. An improved version S_C(E) of the astrophysical S-factor is suggested which is based on the Coulomb wave functions. S_C(E) avoids the apparent energy dependence which is otherwise obtained for capture reactions on heavy nuclei. It is found that a special type of synchrotron radiation available at SPring-8 that mimics stellar blackbody radiation at billions of Kelvin is a promising tool for future experiments. By using the blackbody synchrotron radiation, sufficient event rates for (gamma,alpha) and (gamma,p) reactions in the p-process path can be expected. These experiments will provide data to improve the nuclear parameters involved in the statistical model and thus reduce the uncertainties of nucleosynthesis calculations.Comment: 13 pages, 6 figures, EPJA, accepte

    The s-process branching at 185W

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    The neutron capture cross section of the unstable nucleus 185W has been derived from experimental photoactivation data of the inverse reaction 186W(gamma,n)185W. The new result of sigma = (687 +- 110) mbarn confirms the theoretically predicted neutron capture cross section of 185W of sigma = 700 mbarn at kT = 30 keV. A neutron density in the classical s-process of n_n = (3.8 +0.9 -0.8} * 1e8 cm-3 is derived from the new data for the 185W branching. In a stellar s-process model one finds a significant overproduction of the residual s-only nucleus 186Os.Comment: ApJ, in pres

    Fragmentation and systematics of the Pygmy Dipole Resonance in the stable N=82 isotones

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    The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe has been measured which finalizes the systematic survey to investigate the so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with the method of nuclear resonance fluorescence using real photons in the entrance channel. In all cases, a fragmented resonance-like structure of E1 strength is observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation of the strength reveals that the degree of fragmentation decreases towards the proton-deficient isotones while the total integrated strength increases indicating a dependence of the total strength on the neutron-to-proton ratio. The experimental results are compared to microscopic calculations within the quasi-particle phonon model (QPM). The calculation includes complex configurations of up to three phonons and is able to reproduce also the fragmentation of the E1 strength which allows to draw conclusions on the damping of the PDR. Calculations and experimental data are in good agreement in the degree of fragmentation and also in the integrated strength if the sensitivity limit of the experiments is taken into account

    The decay of quadrupole-octupole 11^- states in 40^{40}Ca and 140^{140}Ce

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    Background: Two-phonon excitations originating from the coupling of two collective one-phonon states are of great interest in nuclear structure physics. One possibility to generate low-lying E1E1 excitations is the coupling of quadrupole and octupole phonons. Purpose: In this work, the γ\gamma-decay behavior of candidates for the (21+31)1(2_1^+\otimes 3_1^-)_{1^-} state in the doubly-magic nucleus 40^{40}Ca and in the heavier and semi-magic nucleus 140^{140}Ce is investigated. Methods: (γ,γ)(\vec{\gamma},\gamma') experiments have been carried out at the High Intensity γ\gamma-ray Source (HIγ{\gamma}S) facility in combination with the high-efficiency γ\gamma-ray spectroscopy setup γ3\gamma^3 consisting of HPGe and LaBr3_3 detectors. The setup enables the acquisition of γ\gamma-γ\gamma coincidence data and, hence, the detection of direct decay paths. Results: In addition to the known ground-state decays, for 40^{40}Ca the decay into the 313^-_1 state was observed, while for 140^{140}Ce the direct decays into the 21+2^+_1 and the 02+0^+_2 state were detected. The experimentally deduced transition strengths and excitation energies are compared to theoretical calculations in the framework of EDF theory plus QPM approach and systematically analyzed for N=82N=82 isotones. In addition, negative parities for two J=1J=1 states in 44^{44}Ca were deduced simultaneously. Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the 111^-_1 excitation in the light-to-medium-mass nucleus 40^{40}Ca as well as in the stable even-even N=82N=82 nuclei.Comment: 11 pages, 6 figures, as accepted in Phys. Rev.

    187^{187}Re(\gamm,n) cross section close to and above the neutron threshold

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    The neutron capture cross section of the unstable nucleus 186^{186}Re is studied by investigating the inverse photodisintegration reaction 187^{187}Re(γ\gamma,n). The special interest of the {\it s}-process branching point 186^{186}Re is related to the question of possible {\it s}-process contributions to the abundance of the {\it r}-process chronometer nucleus ^{187}Re.Weusethephotoactivationtechniquetomeasurephotodisintegrationrates.Ourexperimentalresultsareingoodagreementwithtwodifferentstatisticalmodelcalculations.AlthoughthecrosssectionspredictedbybothmodelsfortheinversereactionRe. We use the photoactivation technique to measure photodisintegration rates. Our experimental results are in good agreement with two different statistical model calculations. Although the cross sections predicted by both models for the inverse reaction ^{186}Re(n,Re(n,\gamma)istoolowtoremovetheoverproductionof) is too low to remove the overproduction of ^{186}$Os; the two predicted neutron-capture cross sections differ by a factor of 2.4; this calls for future theoretical study.Comment: Phys. Rev. C, in pres
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