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Presolar grain isotopic ratios as constraints to nuclear physics inputs for s-process calculations
(2023)
The isotopic abundances in presolar SiC grains of AGB origin provide important and precise constraints to those star nucleosynthesis models. By comparing the values of the s-element abundances resulting from calculations with the ones measured in these dust grains, it turns out that new measurements of weak-interaction rates in ionized plasmas, as well as of neutron-capture cross sections, are needed, especially in the region near the neutron magic numbers 50 and 82.
We show examples of the impact of the Maxwellian averaged capture cross sections determined at n_TOF over the past 20 years on AGB stellar nucleosynthesis models. In particular, we developed an automated procedure to derive MACSs from evaluated data libraries, which are subsequently used as input to stellar models computed by means of the FuNS code. In this contribution, we present a number of s-process abundances obtained using different data libraries as input to stellar models, with a focus on the role of n_TOF data.
Present nuclear reaction network computations for astrophysical simulations involve many different types of rates, including neutron-capture reactions of interest for the modeling of heavy-element nucleosynthesis. While for many of them we still have to rely on theoretical calculations, an increasing number of experimentally-determined cross sections have now become available. In this contribution, we present “ASTrophysical Rate and rAw data Library” (ASTRAL), a new online database for neutron-capture cross sections based on experimental results, mainly obtained through activation and timeof-flight measurements. For the evaluation process, cross sections were re-calculated starting from raw data and by considering recent changes in physical properties of the involved isotopes (e.g., half-life and γ-ray intensities). We show the current status of the database, the techniques adopted to derive the recommended Maxwellian-averaged cross sections, and future developments.
Asymptotic giant branch (AGB) stars are responsible for the production of the main component of the solar s-process distribution. Despite enormous progress in the theoretical modeling of these objects over the last few decades, many uncertainties remain. The still-unknown mechanism leading to the production of 13C neutron source is one example. The nucleosynthetic signature of AGB stars can be examined in a number of stellar sources, from spectroscopic observations of intrinsic and extrinsic stars to the heavy-element isotopic composition of presolar grains found in meteorites. The wealth of available observational data allows for constraining the processes occurring in AGB interiors. In this view, we discuss recent results from new AGB models including the effects of mixing triggered by magnetic fields, and show comparisons of the related s-process nucleosynthesis with available observations.
Mixing and magnetic fields in asymptotic giant branch stars in the framework of FRUITY models
(2021)
In the last few years, the modeling of asymptotic giant branch (AGB) stars has been much investigated, both focusing on nucleosynthesis and stellar evolution aspects. Recent advances in the input physics required for stellar computations made it possible to construct more accurate evolutionary models, which are an essential tool to interpret the wealth of available observational and nucleosynthetic data. Motivated by such improvements, the FUNS stellar evolutionary code has been updated. Nonetheless, mixing processes occurring in AGB stars’ interiors are currently not well-understood. This is especially true for the physical mechanism leading to the formation of the 13C pocket, the major neutron source in low-mass AGB stars. In this regard, post-processing s-process models assuming that partial mixing of protons is induced by magneto-hydrodynamics processes were shown to reproduce many observations. Such mixing prescriptions have now been implemented in the FUNS code to compute stellar models with fully coupled nucleosynthesis. Here, we review the new generation of FRUITY models that include the effects of mixing triggered by magnetic fields by comparing theoretical findings with observational constraints available either from the isotopic analysis of trace-heavy elements in presolar grains or from carbon AGB stars and Galactic open clusters.
Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies.
Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest.
Methods: The γ-ray spectra from neutron-capture reactions on the 234U, 236 U, and 238 U nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV.
Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths.
Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section.
Although the 12C(n,p)12B and 12C(n,d)11B reactions are of interest in several fields of basic and applied Nuclear Physics the present knowledge of these two cross-sections is far from being accurate and reliable, with both evaluations and data showing sizable discrepancies. As part of the challenging n_TOF program on (n,cp) nuclear reactions study, the energy differential cross-sections of the 12C(n,p)12B and 12C(n,d)11 B reactions have been measured at CERN from the reaction thresholds up to 30 MeV neutron energy. Both measurements have been recently performed at the long flight-path (185 m) experimental area of the n_TOF facility at CERN using a pure (99.95%) rigid graphite target and two silicon telescopes. In this paper an overview of the experiment is presented together with a few preliminary results.
i-TED is an innovative detection system which exploits Compton imaging techniques to achieve a superior signal-to-background ratio in (n,γ) cross-section measurements using time-of-flight technique. This work presents the first experimental validation of the i-TED apparatus for high-resolution time-of-flight experiments and demonstrates for the first time the concept proposed for background rejection. To this aim both 197Au(n,γ) and 56Fe(n,γ) reactions were measured at CERN n\_TOF using an i-TED demonstrator based on only three position-sensitive detectors. Two \cds detectors were also used to benchmark the performance of i-TED. The i-TED prototype built for this study shows a factor of ∼3 higher detection sensitivity than state-of-the-art \cds detectors in the ∼10~keV neutron energy range of astrophysical interest. This paper explores also the perspectives of further enhancement in performance attainable with the final i-TED array consisting of twenty position-sensitive detectors and new analysis methodologies based on Machine-Learning techniques.
Fission program at n_TOF
(2019)
Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown.
The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely (n,γ) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how, advances in instrumentation have led, over the years, to the assessment and discovery of many new aspects of s-process nucleosynthesis and to the progressive refinement of theoretical models of stellar evolution. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with γ-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.