96 research outputs found

    Stellar neutron capture cross sections of ⁴¹K and ⁴⁵Sc

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    The neutron capture cross sections of light nuclei (

    Neutron Capture Cross Sections for the Weak s Process

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    In past decades a lot of progress has been made towards understanding the main s-process component that takes place in thermally pulsing Asymptotic Giant Branch (AGB) stars. During this process about half of the heavy elements, mainly between 90<=A<=209 are synthesized. Improvements were made in stellar modeling as well as in measuring relevant nuclear data for a better description of the main s process. The weak s process, which contributes to the production of lighter nuclei in the mass range 56<=A<=90 operates in massive stars (M>=8Msolar) and is much less understood. A better characterization of the weak s component would help disentangle the various contributions to element production in this region. For this purpose, a series of measurements of neutron-capture cross sections have been performed on medium-mass nuclei at the 3.7-MV Van de Graaff accelerator at FZK using the activation method. Also, neutron captures on abundant light elements with A<56 play an important role for s-process nucleosynthesis, since they act as neutron poisons and affect the stellar neutron balance. New results are presented for the (n,g) cross sections of 41K and 45Sc, and revisions are reported for a number of cross sections based on improved spectroscopic information

    Stellar (n,γ) cross sections of ²³Na

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    The cross section of the ²³Na(n,γ)²⁴Na reaction has been measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at kT = 5.1 and 25 keV produced via the ¹⁸O(p,n)¹⁸F and ⁷Li(p,n)⁷Be reactions, respectively. The derived capture cross sections (σ)kT=5keV = 9.1 ± 0.3mb and (σ)kT=25keV = 2.03 ± 0.05 mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first ²³Na resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of s-process nucleosynthesis are discussed

    Structure of 10N in 9C+p resonance scattering

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    The structure of exotic nucleus 10N was studied using 9C+p resonance scattering. Two L=0 resonances were found to be the lowest states in 10N. The ground state of 10N is unbound with respect to proton decay by 2.2(2) or 1.9(2) MeV depending on the 2- or 1- spin-parity assignment, and the first excited state is unbound by 2.8(2) MeV.Comment: 6 pages, 4 figures, 1 table, submitted to Phys. Lett.

    Proton Capture on ^{17}O and its astrophysical implications

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    The reaction 17^{17}O(p,γ)18(p,\gamma)^{18}F influences hydrogen-burning nucleosynthesis in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars and classical novae. In the relevant temperature range for these environments (T9=0.010.4),themaincontributionstotherateofthisreactionarethedirectcaptureprocess,twolowlyingnarrowresonances(T_{9}=0.01-0.4), the main contributions to the rate of this reaction are the direct capture process, two low lying narrow resonances (E_{r}=65.1and183keV)andthelowenergytailsoftwobroadresonances( and 183 keV) and the low-energy tails of two broad resonances (E_{r}=557and677keV).Previousmeasurementsandcalculationsgivecontradictoryresultsforthedirectcapturecontributionwhichinturnincreasestheuncertaintyofthereactionrate.Inaddition,veryfewpublishedcrosssectiondataexistforthehighenergyregionthatmightaffecttheinterpretationofthedirectcaptureandthecontributionsofthebroadresonancesinthelowerenergyrange.Thisworkaimstoaddresstheseissues.Thereactioncrosssectionwasmeasuredinawideprotonenergyrange( and 677 keV). Previous measurements and calculations give contradictory results for the direct capture contribution which in turn increases the uncertainty of the reaction rate. In addition, very few published cross section data exist for the high energy region that might affect the interpretation of the direct capture and the contributions of the broad resonances in the lower energy range. This work aims to address these issues. The reaction cross section was measured in a wide proton energy range (E_{c.m.}=3451700keV)andatseveralangles( - 1700 keV) and at several angles (\theta_{lab}=0^{\circ},45^{\circ},90^{\circ},135^{\circ}).Theobservedprimary). The observed primary \gammatransitionswereusedasinputinan-transitions were used as input in an Rmatrixcodeinordertoobtainthecontributionofthedirectcaptureandthetwobroadresonancestothelowenergyregion.TheextrapolatedSfactorfromthepresentdataisingoodagreementwiththeexistingliteraturedatainthelowenergyregion.AnewreactionratewascalculatedfromthecombinedresultsofthisworkandliteratureSfactordeterminations.Resonancestrengthsandbranchingsarereportedforseveral-matrix code in order to obtain the contribution of the direct capture and the two broad resonances to the low-energy region. The extrapolated S-factor from the present data is in good agreement with the existing literature data in the low-energy region. A new reaction rate was calculated from the combined results of this work and literature S-factor determinations. Resonance strengths and branchings are reported for several ^{18}Fstates.WewereabletoextrapolatetheastrophysicalSfactorofthereactionF states. We were able to extrapolate the astrophysical S-factor of the reaction ^{17}OO(p,\gamma)^{18}$F at low energies from cross section data taken at higher energies. No significant changes in the nucleosynthesis are expected from the newly calculated reaction rate.Comment: Accepted in Physical Review

    Nuclear structure beyond the neutron drip line: the lowest energy states in 9^9He via their T=5/2 isobaric analogs in 9^9Li

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    The level structure of the very neutron rich and unbound 9^9He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy 9^9He states exist with spins Jπ=1/2+J^\pi=1/2^+ and Jπ=1/2J^\pi=1/2^- and widths on the order of hundreds of keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the 8^8He+n+n system, performed via a search for the T=5/2 isobaric analog states in 9^9Li populated through 8^8He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad Jπ=1/2+J^{\pi}=1/2^+ state in 9^9He located approximately 3 MeV above the neutron decay threshold

    The C12(α,γ)O16 reaction and its implications for stellar helium burning

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    The creation of carbon and oxygen in our Universe is one of the forefront questions in nuclear astrophysics. The determination of the abundance of these elements is key to our understanding of both the formation of life on Earth and to the life cycles of stars. While nearly all models of different nucleosynthesis environments are affected by the production of carbon and oxygen, a key ingredient, the precise determination of the reaction rate of C12(α, 3)O16, has long remained elusive. This is owed to the reaction's inaccessibility, both experimentally and theoretically. Nuclear theory has struggled to calculate this reaction rate because the cross section is produced through different underlying nuclear mechanisms. Isospin selection rules suppress the E1 component of the ground state cross section, creating a unique situation where the E1 and E2 contributions are of nearly equal amplitudes. Experimentally there have also been great challenges. Measurements have been pushed to the limits of state-of-the-art techniques, often developed for just these measurements. The data have been plagued by uncharacterized uncertainties, often the result of the novel measurement techniques that have made the different results challenging to reconcile. However, the situation has markedly improved in recent years, and the desired level of uncertainty ‰10% may be in sight. In this review the current understanding of this critical reaction is summarized. The emphasis is placed primarily on the experimental work and interpretation of the reaction data, but discussions of the theory and astrophysics are also pursued. The main goal is to summarize and clarify the current understanding of the reaction and then point the way forward to an improved determination of the reaction rate
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