The neutron capture cross sections of $^{152}\mathrm{Gd}$, $^{154}\mathrm{Gd}$, $^{155}\mathrm{Gd}$, $^{156}\mathrm{Gd}$, $^{157}\mathrm{Gd}$, and $^{158}\mathrm{Gd}$ were measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$(p,n${)}^{7}$Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4\ensuremath{\pi} Barium Fluoride Detector, which was improved by replacing crystals with high \ensuremath{\alpha} background and by introducing a pierced crystal at zero degrees with respect to the beam axis. These changes resulted in a significantly increased efficiency for capture events. The main experimental problem was that the samples of the two s isotopes $^{152}\mathrm{Gd}$ and $^{154}\mathrm{Gd}$ showed only relatively low enrichment. Nevertheless, the spectroscopic quality of the ${\mathrm{BaF}}_{2}$ detector allowed evaluation of the corresponding corrections for isotopic impurities reliably. The cross section ratios could be determined with an overall uncertainty of typically 1%, an improvement by factors of five to ten compared to existing data. Severe discrepancies were found with respect to previous results. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT=10 keV and 100 keV. The new stellar cross sections were used for an updated analysis of the s-process reaction flow in the mass region between samarium and gadolinium, which is characterized by branchings at $^{151}\mathrm{Sm}$, $^{154}\mathrm{Eu}$, and $^{155}\mathrm{Eu}$. With the classical approach, the s-process temperature could be constrained corresponding to a range of thermal energies between kT=28 and 33 keV. The $^{152}\mathrm{Gd}$ production in low mass stars was found to depend strongly on the neutron freeze-out at the end of the helium shell burning episodes.

/pdf/stellar-neutron-capture-cross-sections-of-the-cd-isotopes-45xy638q4f.pdf

Stellar neutron capture cross sections of the Cd isotopes

Neutron captures produce the vast majority of abundances of elements heavier than iron in the Universe. Beyond the classical slow ( s) and rapid ( r) processes, there is observational evidence for neutron-capture processes that operate at neutron densities in between, at different distances from the valley of β stability. Here, we review the main properties of the s process within the general context of neutron-capture processes and the nuclear physics input required to investigate it. We describe massive stars and asymptotic giant branch stars as the s-process astrophysical sites and discuss the related physical uncertainties. We also present current observational evidence for the s process and beyond, which ranges from stellar spectroscopic observations to laboratory analysis of meteorites. 

The s Process and Beyond

Measurements of the $^{197}$Au(n, $\gamma $) cross section up to 100 keV at the CSNS Back-n facility

We present the status of nucleosynthesis beyond Sr, using up-to-date nuclear inputs for both the slow (s-process) and rapid (r-process) scenarios of neutron captures. It is now widely accepted that at least a crucial part of the r-process distribution is linked to neutron star merger (NSM) events. However, so far, we have found only a single direct observation of such a link, the kilonova GW170817. Its fast evolution could not provide strict constraints on the nucleosynthesis details, and in any case, there remain uncertainties in the local r-process abundance patterns, which are independent of the specific astrophysical site, being rooted in nuclear physics. We, therefore, estimate the contributions from the r-process to solar system (S.S.) abundances by adopting the largely site-independent waiting-point concept through a superposition of neutron density components normalized to the r-abundance peaks. Nuclear physics inputs for such calculations are understood only for the trans-Fe nuclei; hence, we restrict our computations to the Sr–Pr region. We then estimate the s-process contributions to that atomic mass range from recent models of asymptotic giant branch stars, for which uncertainties are known to be dominated by nuclear effects. The outcomes from the two independent approaches are then critically analyzed. Despite the remaining problems from both sides, they reveal a surprisingly good agreement, with limited local discrepancies. These few cases are then discussed. New measurements in ionized plasmas are suggested as a source of improvement, with emphasis on β-decays from unstable Cs isotopes. For heavier nuclei, difficulties grow as r-process progenitors lie far off experimental reach and poorly known branchings affect s-processing. This primarily concerns nuclei that are significantly long-lived in the laboratory and have uncertain decay rates in stars, e.g., L u 176 and R e 187 . New measurements are urgently needed for them, too.

/pdf/production-of-solar-abundances-for-nuclei-beyond-sr-the-s-1nvypcdg.pdf

Production of solar abundances for nuclei beyond Sr: The s- and r-process perspectives


 
 74Se is one of 35 p-nuclei, and 82Se is a r-only nucleus, and their (n,γ) cross sections are vital input parameters for nuclear astrophysics reaction network calculations. The neutron capture cross section in the resonance range of isotopes and even natural selenium samples has not been measured. Prompt γ-rays originating from neutron-induced capture events were detected by four C6D6 liquid scintillator detectors at the Back-n facility of China spallation neutron source (CSNS). The pulse height weighting technique (PHWT) was used to analyze the data in the 1 eV to 100 keV region. The deduced neutron capture cross section was compared with ENDF/B-VIII.0, JEFF-3.2, and JENDL-4.0, and some differences were found. Resonance parameters were extracted by the R-Matrix code SAMMY in the 1 eV - 1 keV region. All the cross sections of natSe and resonance parameters are given in the datasets. The datasets are openly available at http://www.doi.org/10.11922/sciencedb.j00113.00019 or https://www.scidb.cn/en/s/feiER3.

New experimental measurement of natSe(n,γ) cross section between 1 eV to 1 keV at the CSNS Back-n facility

Neutron capture cross sections are one of the most important nuclear inputs to models of stellar nucleosynthesis of the elements heavier than iron. The activation technique and the time-of-flight method are mostly used to determine the required data experimentally. Recent developments of experimental techniques allow for new experiments on radioactive isotopes. Monte Carlo based analysis methods give new insights into the systematic uncertainties of previous measurements. We present an overview over the state-of-the-art experimental techniques, a detailed new evaluation of the 197Au(n,
 $\gamma$
 ) cross section in the keV-regime and the corresponding re-evaluation of 63 more isotopes, which have been measured in the past relative to the gold cross section.

Neutron-induced cross sections From raw data to astrophysical rates

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Neutron-induced cross sections From raw data to astrophysical rates