Nuclear reaction rates for the s-process
Sala B
Centro Congressi Partenope
The slow neutron capture process (s-process) is one of the main mechanisms for the production of heavy elements in stars. In the recent years great effort was devoted on improving our knowledge on the nuclear reactions primarily responsible for s-process, i.e.13C(a,n)16O and 22Ne(a,n)25Mg.
This conference has the main goal of bringing together the international community of nuclear astrophysicists working on s-process reactions to share the present state of the art and coordinate future efforts which are still necessary for a more accurate understanding of s-process. As main follow up, a review article on this topic is expected to be coordinated by the participants.
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Greetings
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Main s-process: 13C(α,n)16O contribution
Info about main s-process
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Direct Measurement of 13C(α,n)16O cross section at LUNA
The 13C(α,n)16O reaction is the main neutron sources for the weak s-process, responsible for the nucleosynthesis of about half of the elements heavier than iron. It takes place in thermally pulsing low mass AGB stars at about 90 MK, corresponding to a Gamow window between 140 - 230 keV. These energies are well below the Coulomb barrier. In last decades several direct and indirect measurements of the low energy cross section of 13C(α, n)16O have been performed. Going down in energy, the environmental background strongly hampers direct measurement, important also for the normalization of indirect measurements.
The LUNA collaboration has performed a measurement of the 13C(α,n)16O cross section in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS), where the natural neutron background is reduced by over three orders of magnitude with respect to the surface laboratories. This unique location, combined with a high-efficiency low background detector based on 3He counters, a highly stable intense alpha beam (< I >= 200 μA) and a pulse shape discrimination technique for the rejection of the intrinsic detector background, allowed to cover the energy range 230-300 keV, with drastically reduced uncertainties over previous measurements and for the first time reaching the high-energy edge of the s-process Gamow window with a direct measurement.
In this talk the experimental techniques and the final results of the recent LUNA campaign will be presented, together with the astrophysical implication of our revised reaction rate. In particular, for stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, such as, the two radioactive nuclei 60Fe and 205Pb, as well as 152Gd.Speaker: Dr Giovanni Francesco Ciani (Istituto Nazionale di Fisica Nucleare) -
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13C(α,n)160 with the Trojan Horse Method
More than ∼ 50% of nuclei with A ≥ 56 are produced by the s-process [1], a succes-sion of neutron captures slower than β-decay rates, constraining the nucleosynthesis path close to the stability valley. The neutron source feeding the s-process has been identified in the 13C(α, n)16O reaction taking place in asymptotic giant branch (AGB) stars. In this contribution, I will focus on the indirect measurement of its S-factor by means of the Trojan Horse Method (THM), and in particular on the concordance scenario we have reached by the concurrent application of the THM and the ANC and the reanalysis of the available direct data.
Indeed, due to its astrophysical importance, many direct and indirect determina-tions of the 13C(α, n)16O S-factor have been carried out (see, e.g., [2] for a review). The low center-of-mass energies of astrophysical relevance (≤ 230 keV) make direct measurements extremely challenging due to the small cross sections, and especially due to the interplay between the rise in the S-factor linked to the 17O 6.356 MeV 1/2+ threshold level and the enhancement produced by the electron screening. An additional problem is the scatter of the absolute normalizations of existing data, as large as a factor of 2 [2], which could be attributed to systematic errors on neutrons detection efficiency.
The indirect ANC approach lead to several coherent measurements of the ANC of 6.356 MeV 17O threshold level (see [3] for a list). Therefore, we decided to change the paradigm usually adopted in THM applications and normalized the THM S-factor to the ANC of the 6.356 MeV state. This approach lead to a concordance scenario for the 13C(α, n)16O S-factor, where both direct and indirect data accurately agree.
The results, extensively discussed in [3], have been recently confirmed by [4], where an accurate measurement down to 230 keV (upper edge of the Gamow window) has been reported. This shows the importance of the interplay between direct (espe-cially underground) and indirect measurements to reach accurate reactions rates for astrophysical applications.Speaker: Dr Marco La Cognata (Istituto Nazionale di Fisica Nucleare)
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11:00
Coffee break
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Main s-process: 13C(α,n)16O contribution
Info about main s-process
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Deep Underground Laboratory Measurement of 13C(α,n)16O at JUNA
The 13C(a,n)16O reaction is the main neutron source for the s-process in AGB stars. Meanwhile, this reaction is also believed to be the neutron source for the i-process although its astrophysical site is still unclear. Direct measurement of its cross section at astrophysical energies is challenging in above-ground laboratories due to the vast cosmic-ray induced background. Underground laboratories have orders-of-magnitude lower background and open new opportunities for such measurements. We performed consistent measurements of the cross section covering a wide energy range of Ec.m. = 0.24 – 1.9 MeV at JUNA and Sichuan University. Our measurement covers almost the entire i-process Gamow window in which the large uncertainty of the previous experiments has been reduced from 60% down to 15%, eliminates the large systematic uncertainty in the extrapolation arising from the inconsistency of existing data sets, and provides a more reliable reaction rate for the studies of the s- and i-processes. Detailed description of our measurement, including experimental setups and data analysis, will be presented in this talk. A short outlook and future plans on the new measurements of this reaction at JUNA will be shown.
Speaker: Dr Bingshui Gao -
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13C(α,n)16O and 22Ne(α,n)25Mg at the University of Notre DameSpeaker: Dr Richard James Deboer
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12:30
Lunch break
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Weak s process: 22Ne(α,n)25Mg contribution
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The Big Three Reaction for Nuclear Astrophysics: the forthcoming workshop in Strasbourg
A forthcoming workshop (29th May) will be held in Strasbourg also in the context of the ChETEC-INFRA project, to discuss the The Big Three Reaction for the nuclear astrophysicists community. The main goal of the workshop is to network the existing and forthcoming research programs around the 12C(α,γ), 12C+12C and 22Ne(α,n) fusion reactions
Speaker: Dr Roberta Sparta' (Istituto Nazionale di Fisica Nucleare) - 7
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Neutron spectroscopy of 26Mg states: Constraining the stellar neutron source 22Ne(α,n)25Mg
This study presents accurate and high-resolution measurements of the 25Mg(n,γ)26Mg and 25Mg(n,tot) cross sections, from thermal energies up to approximately 300 keV. Through a combined R-matrix analysis of the experimental data, the pertinent neutron resonances for the interaction with 25Mg were characterized. Consequently, this analysis led to an improved set of reaction widths, along with a definitive spin/parity assignment for the corresponding excited states in 26Mg.
Speaker: Dr Cristian Massimi (Istituto Nazionale di Fisica Nucleare) -
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Decay properties of 22Ne + α resonances and their impact on s-process nucleosynthesisSpeaker: Dr Shuya Ota
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15:30
Coffee break
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Weak s process: 22Ne(α,n)25Mg contribution
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22Ne(α.n)25Mg at CASPARSpeaker: Dr Frank Strieder Strieder (South Dakota School of Mines & Technology)
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Re-evaluation of the 22Ne+alpha reaction rates
There has been a large amount of new nuclear data on the 26Mg compound system, some of which we will discuss at this workshop. Combining these data into reaction rates requires dedicated work to combine the different data, resolve discrepancies and provide updated estimates with, if possible, well-motivated uncertainties. In this talk, I will discuss some of the indirect measurements which went into the ChETEC 22Ne+alpha evaluation published in 2021 (Physical Review C 103 015805). In addition, I will identify the current largest contributors to the uncertainties in the 22Ne+alpha reaction rates with some recommendations for future experimental and theoretical efforts, as well as possible inconsistencies between past datasets which could be revisited to try to improve the situation.
Speaker: Dr Philip Adsley (IPN Orsay)
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20:00
Social dinner
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Perspectives: Facilities
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Nuclear Astrophysics at JUNA
The 13C(a,n)16O reaction is the main neutron source for the s-process in AGB stars. Meanwhile, this reaction is also believed to be the neutron source for the i-process although its astrophysical site is still unclear. Direct measurement of its cross section at astrophysical energies is challenging in above-ground laboratories due to the vast cosmic-ray induced background. Underground laboratories have orders-of-magnitude lower background and open new opportunities for such measurements. We performed consistent measurements of the cross section covering a wide energy range of Ec.m. = 0.24 – 1.9 MeV at JUNA and Sichuan University. Our measurement covers almost the entire i-process Gamow window in which the large uncertainty of the previous experiments has been reduced from 60% down to 15%, eliminates the large systematic uncertainty in the extrapolation arising from the inconsistency of existing data sets, and provides a more reliable reaction rate for the studies of the s- and i-processes. Detailed description of our measurement, including experimental setups and data analysis, will be presented in this talk. A short outlook and future plans on the new measurements of this reaction at JUNA will be shown.
Speaker: Dr Bingshui Goa -
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Nuclear Astrophysics at CASPAR
This talk will provide a short overview of the development of the CASPAR laboratory and its scientific program over the last decade. Because of the installation of the DUNE detector, CASPAR is presently not accessible. The talk will report on the present status of CASPAR and the scientific priorities after the laboratory reopens in summer 2024.
Speaker: Prof. Michael Wiescher (University of Notre Dame)
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11:00
Coffee break
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Perspectives: Young researchers
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22Ne(α,n)25Mg with SHADES
The talk is a summary of the characterisation work done for the understanding, calibration and then commissioning of the EJ-309 organic liquid scintillators used by the ERC funded SHADES project lead by prof. Best. A new nuclear astrophysics experiment aiming to perform, for the first time, a direct cross section measurements for the 22Ne(α,n)25Mg reaction at very low energy. The SHADES experiment will be held in the deep-underground INFN’s facility of LGNS (Gran Sasso – AQ). This will help to minimize the background activity by several orders of magnitude, and therefore, make the collected data of a much higher quality.
SHADES uses a hybrid detection system that consists of a first circular row of organic liquid scintillators surrounded 02 other rows of 3He counters. The ingenuity of this system lies in the double role of the scintillators: firstly, as a moderating material, thermalizing high energy neutrons that can’t be detected by the 3He counters and secondly, the data provided by counters can be time-matched with the one of the liquid scintillators to effectively filtrate “bad” events that can’t be emanating from the studied reaction. Moreover, steel-made 3He counters have a lower intrinsic radioactivity, which helps to further push the detection precision.
Speaker: Chemseddine Ananna (Università degli Studi di Napoli "Federico II") -
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22Ne(α,γ)26Mg with EASγ
The reaction 22Ne(α,γ)26Mg is associated with several questions in nuclear astrophysics like the Mg isotope ratio in stellar atmospheres and its competition with the neutron source 22Ne(α,n)25Mg.
Due to very low stellar energies and therefore very low cross section, direct experiments have been only able to provide upper limits below a strong resonance at 832 keV.
The purpose of the EASγ project is to perform the first direct measurement of the 22Ne(α,γ)26Mg in the range of astrophysical interest below 600-800 and the remeasurement of the 832 keV resonance.
The measurement will be carried out using the new LUNA MV accelerator at Laboratori Nazionali del Gran Sasso, which provides a high and stable α particle current. Moreover, its position underground and additional passive shielding will reduce the γ-background. The γ-rays produced in the reaction will be detected by a NaI scintillator array surrounding a windowless, recirculating gas target.
Additional information on the excited state of 26Mg near the alpha threshold will be provided by an indirect measurement via 7Li(22Ne, t)26Mg in inverse kinematics, scheduled at the TRIUMF laboratory in Vancouver.
We present the current status of the project and an overview of the planned TRIUMF experiment.Speaker: Daniela Mercogliano (Istituto Nazionale di Fisica Nucleare)
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12:00
Lunch break
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Round table
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