Alpha-induced cross sections of 106Cd for the astrophysical p-process

The 106Cd(alpha,gamma)110Sn reaction cross section has been measured in the energy range of the Gamow window for the astrophysical p-process scenario. The cross sections for 106Cd(alpha,n)109Sn and for 106Cd(alpha,p)109In below the (alpha,n) threshold have also been determined. The results are compared with predictions of the statistical model code NON-SMOKER using different input parameters. The comparison shows that a discrepancy for 106Cd(alpha,gamma)110Sn when using the standard optical potentials can be removed with a different alpha+106Cd potential. Some astrophysical implications are discussed.Comment: 10 pages, 9 figures, accepted for publication in Phys. Rev

Region of hadron-quark mixed phase in hybrid stars

Hadron--quark mixed phase is expected in a wide region of the inner structure of hybrid stars. However, we show that the hadron--quark mixed phase should be restricted to a narrower region to because of the charge screening effect. The narrow region of the mixed phase seems to explain physical phenomena of neutron stars such as the strong magnetic field and glitch phenomena, and it would give a new cooling curve for the neutron star.Comment: to be published in Physical Review

Cross section measurement of N 14 ( p , γ ) O 15 in the CNO cycle

Background: The CNO cycle is the main energy source in stars more massive than our sun; it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. In our sun about 1.6% of the total solar neutrino flux comes from the CNO cycle. The largest uncertainty in the prediction of this CNO flux from the standard solar model comes from the uncertainty in the ^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O} reaction rate; thus, the determination of the cross section at astrophysical temperatures is of great interest.Purpose: The total cross section of the ^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O} reaction has large contributions from the transitions to the ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ excited state and the ground state of $^{15}\mathrm{O}$. The ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ transition is dominated by radiative direct capture, while the ground state is a complex mixture of direct and resonance capture components and the interferences between them. Recent studies have concentrated on cross-section measurements at very low energies, but broad resonances at higher energy may also play a role. A single measurement has been made that covers a broad higher-energy range but it has large uncertainties stemming from uncorrected summing effects. Furthermore, the extrapolations of the cross section vary significantly depending on the data sets considered. Thus, new direct measurements have been made to improve the previous high-energy studies and to better constrain the extrapolation.Methods: Measurements were performed at the low-energy accelerator facilities of the nuclear science laboratory at the University of Notre Dame. The cross section was measured over the proton energy range from ${E}_{p}=0.7$ to 3.6 MeV for both the ground state and the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transitions at {\ensuremath{\theta}}_{\text{lab}}={0}^{\ensuremath{\circ}}, {45}^{\ensuremath{\circ}}, {90}^{\ensuremath{\circ}}, {135}^{\ensuremath{\circ}}, and {150}^{\ensuremath{\circ}}. Both TiN and implanted-$^{14}\mathrm{N}$ targets were utilized. \ensuremath{\gamma} rays were detected by using an array of high-purity germanium detectors.Results: The excitation function as well as angular distributions of the two transitions were measured. A multichannel $R$-matrix analysis was performed with the present data and is compared with previous measurements. The analysis covers a wide energy range so that the contributions from broad resonances and direct capture can be better constrained.Conclusion: The astrophysical $S$ factors of the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ and the ground-state transitions were extrapolated to low energies with the newly measured differential-cross-section data. Based on the present work, the extrapolations yield {S}_{6.79}(0)=1.29\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.09(\mathrm{syst})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b} and {S}_{\text{g.s.}}(0)=0.42\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b}. While significant improvement and consistency is found in modeling the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transition, large inconsistencies in both the $R$-matrix fitting and the low-energy data are reaffirmed for the ground-state transition. Reflecting this, a systematic uncertainty of {}_{\ensuremath{-}0.19}^{+0.09}\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b} is recommended for the ground-state transition

AGB yields and Galactic Chemical Evolution : last updated

We study the s-process abundances at the epoch of the Solar-system formation as the outcome of nucleosynthesis occurring in AGB stars of various masses and metallicities. The calculations have been performed with the Galactic chemical evolution (GCE) model presented by [1, 2]. With respect to previous works, we used updated solar meteoritic abundances, a neutron capture cross section network that includes the most recent measurements, and we implemented the s-process yields with an extended range of AGB initial masses. The new set of AGB yields includes a new evaluation of the $^{22}$Ne(α, n)$^{25}$Mg rate, which takes into account the most recent experimental information

Probing astrophysically important states in the ²⁶Mg nucleus to study neutron sources for the s process

Background: The ²²Ne(α,n) ²⁵Mg reaction is the dominant neutron source for the slow neutron capture process (s process) in massive stars, and contributes, together with C¹³(α,n)O¹⁶, to the production of neutrons for the s process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with ²²Ne(α,γ)²⁶Mg radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of ²⁶Mg near the α and neutron separation energies. These uncertainties affect the s-process nucleosynthesis calculations in theoretical stellar models. Purpose: Indirect studies in the past have been successful in determining the energies and the γ-ray and neutron widths of the Mg26 states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the α widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the α widths by α-transfer techniques. Methods: The α-inelastic scattering and α-transfer measurements were performed on a solid ²⁶Mg target and a ²²Ne gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The (α,α′) measurements were performed at 0.45°, 4.1°, 8.6°, and 11.1° and the (⁶Li,d) measurements at 0° and 10°. The scattered α particles and deuterons were detected by the focal plane detection system consisting of multiwire drift chambers and plastic scintillators. The focal plane energy calibration allowed the study of ²⁶Mg levels from Eₓ = 7.69–12.06 MeV in the (α,α′) measurement and Eₓ = 7.36–11.32 MeV in the (⁶Li,d) measurement. Results: Six levels (Eₓ = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the α threshold in the region of interest (10.61–11.32 MeV). The α widths were calculated for these states from the experimental data. The results were used to determine the α-capture induced reaction rates. Conclusion: The energy range above the α threshold in ²⁶Mg was investigated using a high resolution spectrometer. A number of states were observed for the first time in α-scattering and α-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in ²⁶Mg. From the observed resonance levels the Eₓ = 10717 keV state has a negligible contribution to the α-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at Ex = 10951, 11167, and 11317 keV. The Eₓ = 11167 keV state has the most appreciable impact on the (α,γ) rate and therefore plays an important role in the prediction of the neutron production in s-process environments

Cd110,116(α,α)Cd110,116 elastic scattering and systematic investigation of elastic α scattering cross sections along the Z=48 isotopic and N=62 isotonic chains

The elastic scattering cross sections for the reactions Cd110,116(α,α)Cd110,116 at energies above and below the Coulomb barrier are presented to provide a sensitive test for the α-nucleus optical potential parameter sets. Additional constraints for the optical potential are taken from the analysis of elastic scattering excitation functions at backward angles which are available in literature. Moreover, the variation of the elastic α scattering cross sections along the Z=48 isotopic and N=62 isotonic chain is investigated by the study of the ratios of the Cd106,110,116(α,α)Cd106,110,116 scattering cross sections at E cm15.6and18.8 MeV and the ratio of the Cd110(α,α)Cd110 and Sn112(α,α)Sn112 reaction cross sections at Ecm18.8 MeV, respectively. These ratios are sensitive probes for the α-nucleus optical potential parametrizations. The potentials under study are a basic prerequisite for the prediction of α-induced reaction cross sections (e.g., for the calculation of stellar reaction rates in the astrophysical p or γ process). © 2011 American Physical Society.This work was supported by the EUROGENESIS research program, by the Hungarian Office of the National Scientific Research Fund (OTKA), Grants No. NN83261 and No. K068801, by the European Research Council, Grant No. 203175, and by the Joint Institute for Nuclear Astrophysics (NSF Grant No. PHY0822648). G.G.K. and D.G. acknowledge the support of the Spanish Interministerial Commission of Science and Technology, under Project No. FPA2005-02379, and the Ministry of Education and Science (MEC) Consolider, Project No. CSD2007-00042. G.G. acknowledges support from the Bolyai grant. D.G. acknowledges support from the Spanish Ministry of Science Juan de la Cierva grant. This work was also supported by the Scientific and Technological Research Council of Turkey (TUBITAK), Grants No. 108T508 (TBAG1001) and No. 109T585 (under the EUROGENESIS research program). Fruitful discussions with M. Avrigeanu are gratefully acknowledged.Peer Reviewe

The impact of the 18F(a,p)21Ne reaction on asymptotic giant branch nucleosynthesis

We present detailed models of low and intermediate-mass asymptotic giant branch (AGB) stars with and without the 18F(a,p)21Ne reaction included in the nuclear network, where the rate for this reaction has been recently experimentally evaluated for the first time. The lower and recommended measured rates for this reaction produce negligible changes to the stellar yields, whereas the upper limit of the rate affects the production of 19F and 21Ne. The stellar yields increase by ~50% to up to a factor of 4.5 for 19F, and by factors of ~2 to 9.6 for 21Ne. While the 18}F(a,p)21Ne reaction competes with 18O production, the extra protons released are captured by 18O to facilitate the 18O(p,a)15N(a,g)19F chain. The higher abundances of 19F obtained using the upper limit of the rate helps to match the [F/O] ratios observed in AGB stars, but only for large C/O ratios. Extra-mixing processes are proposed to help to solve this problem. Some evidence that the 18F(a,p)21Ne rate might be closer to its upper limit is provided by the fact that the higher calculated 21Ne/22Ne ratios in the He intershell provide an explanation for the Ne isotopic composition of silicon-carbide grains from AGB stars. This needs to be confirmed by future experiments of the 18F(a,p)21Ne reaction rate. The availability of accurate fluorine yields from AGB stars will be fundamental for interpreting observations of this element in carbon-enhanced metal-poor stars.Comment: 9 pages, accepted for publication in Ap

Photoexcitation of astrophysically important states in Mg 26 . II. Ground-state-transition partial widths

The level structure of Mg-26 near the neutron-separation energy, which is of interest for s-process nucleosynthesis, was studied at the High Intensity Gamma-Ray Source of the Triangle Universities Nuclear Laboratory using the method of nuclear resonance fluorescence. A nearly monoenergetic and linearly polarized gamma-ray beam was used to scan the excitation energy range from 10.5 to 11.7 MeV. For the five states observed, the total widths and partial widths are determined. Precise measurement of these widths is necessary for the prediction of neutron production for the s-process

First measurement of the 14N(p,gamma)15O cross section down to 70 keV

In stars with temperatures above 20*10^6 K, hydrogen burning is dominated by the CNO cycle. Its rate is determined by the slowest process, the 14N(p,gamma)15O reaction. Deep underground in Italy's Gran Sasso laboratory, at the LUNA 400 kV accelerator, the cross section of this reaction has been measured at energies much lower than ever achieved before. Using a windowless gas target and a 4pi BGO summing detector, direct cross section data has been obtained down to 70 keV, reaching a value of 0.24 picobarn. The Gamow peak has been covered by experimental data for several scenarios of stable and explosive hydrogen burning. In addition, the strength of the 259 keV resonance has been remeasured. The thermonuclear reaction rate has been calculated for temperatures 90 - 300 *10^6 K, for the first time with negligible impact from extrapolations