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1. The s process in massive stars, a benchmark for neutron capture reaction rates

   https://doi.org/10.1140/epja/s10050-023-01206-1  

   Pignatari, Marco; Gallino, Roberto; Reifarth, Rene (December 2023, The European Physical Journal A)

   Abstract A clear definition of the contribution from the slow neutron-capture process (s process) to the solar abundances between Fe and the Sr-Zr region is a crucial challenge for nuclear astrophysics. Robust s-process predictions are necessary to disentangle the contribution from other stellar processes producing elements in the same mass region. Nuclear uncertainties are affecting s-process calculations, but most of the needed nuclear input are accessible to present nuclear experiments or they will be in the near future. Neutron-capture rates have a great impact on the s process in massive stars, which is a fundamental source for the solar abundances of the lighter s-process elements heavier than Fe (weak s-process component). In this work we present a new nuclear sensitivity study to explore the impact on the s process in massive stars of 86 neutron-capture rates, including all the reactions between C and Si and between Fe and Zr. We derive the impact of the rates at the end of the He-burning core and at the end of the C-burning shell, where the$$^{22}$$ ${}^{22}$Ne($$\alpha $$ $\alpha$,n)$$^{25}$$ ${}^{25}$Mg reaction is is the main neutron source. We confirm the relevance of the light isotopes capturing neutrons in competition with the Fe seeds as a crucial feature of the s process in massive stars. For heavy isotopes we study the propagation of the neutron-capture uncertainties, finding a clear difference of the impact of Fe and Co isotope rates with respect to the rates of heavier stable isotopes. The local uncertainty propagation due to the neutron-capture rates at the s-process branching points is also considered, discussing the example of$$^{85}$$ ${}^{85}$Kr. The complete results of our study for all the 86 neutron-capture rates are available online. Finally, we present the impact on the weak s process of the neutron-capture rates included in the new ASTRAL library (v0.2). 

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2. Nucleosynthesis of "Light" Heavy Nuclei in Neutrino-driven Winds. Role of ( α,n ) reactions

   https://doi.org/10.1088/1742-6596/1668/1/012033  

   Pereira, J.; Arcones, A.; Bliss, J.; Montes, F. (January 2020, ArXivorg)

   Neutrino-driven winds following core collapse supernovae have been proposed as a suitable site where the so-called light heavy elements (between Sr to Ag) can be synthetized. For moderately neutron-rich winds, ( α,n ) reactions play a critical role in the weak r process, becoming the main mechanism to drive nuclear matter towards heavier elements. In this paper we summarize the sensitivity of network-calculated abundances to the astrophysical conditions, and to uncertainties in the ( α,n ) reaction rates. A list of few ( α,n ) reactions were identified to dominate the uncertainty in the calculated elemental abundances. Measurements of these reactions will allow to identify the astrophysical conditions of the weak r process by comparing calculated/observed abundances in r-limited stars. 

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3. Investigation of the ⁷ Li(p,n) neutron fields at high energies

   https://doi.org/10.1051/epjconf/202226011005  

   Brückner, Benjamin; Erbacher, Philipp; Göbel, Kathrin; Heftrich, Tanja; Khasawneh, Kafa; Kurtulgil, Deniz; Langer, Christoph; Nolte, Ralf; Reich, Markus; Reifarth, René; et al (January 2022, EPJ Web of Conferences)

   Liu, W.; Wang, Y.; Guo, B.; Tang, X.; Zeng, S. (Ed.)

   The neutron activation method is well-suited to investigate neutron-capture cross sections relevant for the main s-process component. Neutrons can be produced via the 7 Li(p,n) reaction with proton energies of 1912 keV at e.g. Van de Graaff accelerators, which results in a quasi-Maxwellian spectrum of neutrons corresponding to a temperature of k B T = 25 keV. However, the weak s-process takes place in massive stars at temperatures between 25 and 90 keV. Simulations using the PINO code [2] suggest that a Maxwellian spectrum for higher energies, e.g. k B T = 90 keV, can be approximated by a linear combination of different neutron spectra. To validate the PINO code at proton energies E p ≠ 1912 keV, neutron time-of-flight measurements were carried out at the PTB Ion Accelerator Facility (PIAF) at the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. 

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4. The impact of (n,γ) reaction rate uncertainties of unstable isotopes on the i -process nucleosynthesis of the elements from Ba to W

   https://doi.org/10.1093/mnras/stab772  

   Denissenkov, Pavel A; Herwig, Falk; Perdikakis, Georgios; Schatz, Hendrik (April 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The abundances of neutron (n)-capture elements in the carbon-enhanced metal-poor (CEMP)-r/s stars agree with predictions of intermediate n-density nucleosynthesis, at Nn ∼ 1013–1015 cm−3, in rapidly accreting white dwarfs (RAWDs). We have performed Monte Carlo simulations of this intermediate-process (i-process) nucleosynthesis to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131I to 189Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of Nn = 3.16 × 1014 and 3.16 × 1013 cm−3 and on a multizone model based on a realistic stellar evolution simulation of He-shell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser–Feshbach computations and the predicted abundances, with the Pearson product–moment correlation coefficients |rP| > 0.15. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-i star CS 31062−050 are significantly diminished if the rate of 137Cs(n,γ)138Cs is reduced and the rates of 141Ba(n,γ)142Ba or 141La(n,γ)142La increased. The uncertainties of temperature-dependent β-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances. One-zone Monte Carlo simulations can be used instead of computationally time-consuming multizone Monte Carlo simulations in reaction rate uncertainty studies if they use comparable values of Nn. We discuss the key challenges that RAWD simulations of i process for CEMP-i stars meet by contrasting them with recently published low-Z asymptotic giant branch (AGB) i process. 

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5. The 22Ne($$\alpha $$,n)25Mg reaction - state of the art, astrophysics, and perspectives

   https://doi.org/10.1140/epja/s10050-025-01572-y  

   Best, Andreas; Adsley, Philip; Amberger, Ryan; Battino, Umberto; Chillery, Thomas; La_Cognata, Marco; deBoer, Richard_James; Mercogliano, Daniela; Ota, Shuya; Rapagnani, David; et al (May 2025, The European Physical Journal A)

   Abstract One of the most important stellar neutron sources is the²²Ne($$\alpha ,n$$ $\alpha ,n$)²⁵Mg reaction, which gets activated both during the helium intershell burning in asymptotic giant branch stars and in core helium and shell carbon burning in massive stars. The²²Ne($$\alpha ,n$$ $\alpha ,n$)²⁵Mg reaction serves as the main neutron producer for the weaks-process and provides a short but strong neutron exposure during the helium flash phase of the mains-process, significantly affecting the abundances at thes-process branch points. The cross section needs to be known at very low energies, as close as possible to the neutron threshold at$$E_\alpha =$$ $E_{\alpha }=$562 keV (Q= −478 keV), but both direct and indirect measurements have turned out to be very challenging, leading to significant uncertainties. Here we discuss the current status of the reaction, including recent and upcoming measurements, and provide a discussion on the astrophysical implications as well as an outlook into the near future. 

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