115 research outputs found
Stellar neutron capture cross sections of ⁴¹K and ⁴⁵Sc
The neutron capture cross sections of light nuclei (
Neutron activation of natural zinc samples at kT = 25 keV
The neutron-capture cross sections of 64Zn, 68Zn, and 70Zn have been measured
with the activation technique in a quasistellar neutron spectrum corresponding
to a thermal energy of kT = 25 keV. By a series of repeated irradiations with
different experimental conditions, an uncertainty of 3% could be achieved for
the 64Zn(n,g)65Zn cross section and for the partial cross section
68Zn(n,g)69Zn-m feeding the isomeric state in 69Zn. For the partial cross
sections 70Zn(n,g)71Zn-m and 70Zn(n,g)71Zn-g, which had not been measured so
far, uncertainties of only 16% and 6% could be reached because of limited
counting statistics and decay intensities. Compared to previous measurements on
64,68Zn, the uncertainties could be significantly improved, while the 70Zn
cross section was found to be two times smaller than existing model
calculations. From these results Maxwellian average cross sections were
determined between 5 and 100 keV. Additionally, the beta-decay half-life of
71Zn-m could be determined with significantly improved accuracy. The
consequences of these data have been studied by network calculations for
convective core He burning and convective shell C burning in massive stars
Neutron Capture Cross Sections for the Weak s Process
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
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
Stellar neutron capture cross sections of ²⁰ ²¹ ²²Ne
The stellar (n,γ) cross sections of the Ne isotopes are important for a number of astrophysical quests, i.e., for the interpretation of abundance patterns in presolar material or with respect to the s-process neutron balance in red giant stars. This paper presents resonance studies of experimental data in the keV range, which had not been fully analyzed before. The analyses were carried out with the R-matrix code sammy. With these results for the resonant part and by adding the components due to direct radiative capture, improved Maxwellian-averaged cross sections (MACS) could be determined. At kT=30keV thermal energy we obtain MACS values of 240±29,1263±160, and 53.2±2.7 μbarn for ²⁰Ne,²¹Ne, and ²²Ne, respectively. In earlier work the stellar rates of ²⁰Ne and ²¹Ne had been grossly overestimated. ²²Ne and ²⁰Ne are significant neutron poisons for the s process in stars because their very small MACS values are compensated by their large abundances
Structure of 10N in 9C+p resonance scattering
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
The reaction OF 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 (E_{r}=65.1E_{r}=557E_{c.m.}=345\theta_{lab}=0^{\circ},45^{\circ},90^{\circ},135^{\circ}\gammaR^{18}^{17}(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
Opportunities for Nuclear Astrophysics at FRANZ
The "Frankfurter Neutronenquelle am Stern-Gerlach-Zentrum" (FRANZ), which is
currently under development, will be the strongest neutron source in the
astrophysically interesting energy region in the world. It will be about three
orders of magnitude more intense than the well-established neutron source at
the Research Center Karlsruhe (FZK)
Nuclear structure beyond the neutron drip line: the lowest energy states in He via their T=5/2 isobaric analogs in Li
The level structure of the very neutron rich and unbound He nucleus has
been the subject of significant experimental and theoretical study. Many recent
works have claimed that the two lowest energy He states exist with spins
and 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
He system, performed via a search for the T=5/2 isobaric analog states
in Li populated through He+p elastic scattering. The present data show
no indication of any narrow structures. Instead, we find evidence for a broad
state in He located approximately 3 MeV above the neutron
decay threshold
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 excited state and the ground state of . The 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 to 3.6 MeV for both the ground state and the 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- 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 -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 factors of the 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 transition, large inconsistencies in both the -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
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