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Virtual workshop on (α,n) reactions for astrophysics

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Thanassis Psaltis (TU Darmstadt), Almudena Arcones (TU Darmstadt/GSI), Alison Laird (University of York), Zach Meisel (Ohio University)
Description

(α,n) reactions play a pivotal role in a variety of astrophysical sites and mass regions, and they can help us understand the origin of the elements. Their astrophysical rates are the main nuclear physics uncertainty in the weak r-process (also known as the α-process), which occurs in the neutrino driven ejecta of core-collapse supernovae and can explain the production of the lighter heavy elements, that are observed in metal poor stars. The 22Ne(α,n)25Mg and 13C(α,n)16O reactions are the main neutron sources for the s- and the i-processes. In addition, (α,n) reactions on 17< A <34 nuclei could affect nucleosynthesis in Type Ia supernova explosions. As far as the low mass regime is concerned, the 9Be(α,n)12C is a key reaction for the s- and  r-processes, as well as for primordial nucleosynthesis.

Under the light of recent advancements in astronomical observations (e.g. more metal poor stars), stellar modelling and nuclear physics experiments, there is an intense interest in (α,n) reactions both from an experimental and a theoretical perspective. With this virtual workshop we aim on bringing experts from the international nuclear astrophysics  community together to discuss and set future directions for the study of (α,n) reactions for astrophysics. We hope to provide a setting where the community will meet, exchange ideas and build and strengthen international collaborations. We highly encourage students, postdocs and early career scientists to give talks on their projects related to the workshop topic.

Some interesting questions that we look forward to discussing are: How different experimental techniques can help us constrain the alpha-optical potential? Is there a need for a database of such quantities? Can the stellar modeling provide additional sites where (α,n) reactions play a key role? How could advancements in nuclear reaction theory help us understand these alpha-induced reactions?

Abstract submission deadline: June 27th, 2021

This workshop is supported by IReNA, the International Research Network for Nuclear Astrophysics . IReNA is a US National Science Foundation AccelNet Network of Networks. It ​connects six interdisciplinary research networks across 17 countries to foster collaboration, complement and enhance research capabilities in the US and abroad, and thus greatly accelerate progress in science.

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Participants
  • Aikaterini Zyriliou
  • Akash Hingu
  • Alba Formicola
  • Alexander Voinov
  • Alison Laird
  • Almudena Arcones
  • Alvaro Tolosa-Delgado
  • Andras Vitez-Sveiczer
  • Andreas Best
  • Anu Kankainen
  • Axel Boeltzig
  • Bruce Millar
  • Caleb Marshall
  • Cameron Angus
  • Camilla Juul Hansen
  • Carl Brune
  • Carlos Alberto Mirez Tarrillo
  • Chad Ummel
  • Chiaki Kobayashi
  • Christine Hampton
  • Cristina Bordeanu
  • Dan Bardayan
  • Daniel Cano Ott
  • David Rapagnani
  • Drew Blankstein
  • Eleni Vagena
  • Eliana Masha
  • Erin White
  • Fernando Montes
  • Gerald (Gerry) Hale
  • Gianluca Imbriani
  • Giovanni Francesco Ciani
  • Gábor Kiss
  • Heshani Jayatissa
  • Hirokazu Sasaki
  • Jaideep Singh
  • James deBoer
  • Jaspreet Singh Randhawa
  • Jeena S K
  • Jorge Pereira
  • Jose L. Tain
  • Joseph Derkin
  • Kristyn Brandenburg
  • Laszlo Csedreki
  • Mansi Saxena
  • Marco Pignatari
  • mariko segawa
  • Matthew Williams
  • Melina Avila
  • Michael Smith
  • mohammed akram zermane
  • Nabin Rijal
  • Nobuya Nishimura
  • Noritaka Kitamura
  • Paraskevi Dimitriou
  • Patryk Liniewicz
  • Philip Adsley
  • Pierre Capel
  • Pooja Airee
  • Ragandeep Singh Sidhu
  • Rebecca Toomey
  • Rene Reifarth
  • Richard Longland
  • Shahina Shahina
  • Shunji Nishimura
  • Soham Chakraborty
  • Sophia Florence Dellmann
  • Stylianos Nikas
  • Sándor Kovács
  • Terese Hansen
  • Thanassis Psaltis
  • Thomas Massey
  • Tibor Norbert Szegedi
  • Toshihiko Kawano
  • Tsung-Han Yeh
  • Umberto Battino
  • Vibhuti Vashi
  • Victoria Durant
  • Wei Jia Ong
  • Yenuel Jones-Alberty
  • Yong-Zhong Qian
  • Zach Meisel
  • zhenpeng chen
  • Zsolt Fulop
  • Wednesday, 14 July
    • 10:00 → 10:10
      Welcome Talk 10m
    • 10:10 → 12:30
      The weak r-process
      • 10:10
        Core-Collapse Supernovae Neutrino-Driven Winds: A Possible Site for the Nucleosynthesis of “Light” Heavy Nuclei 50m

        Neutrino-driven winds following core-collapse supernovae explosions have been proposed as a possible scenario where the synthesis of the so-called “light” r-process nuclei(between Fe and Ag)might occur.Steady-state model calculations, combined with nucleosynthesis reaction networks indicate a substantial sensitivity of the element abundances to (α,n) reaction rates and the astrophysical conditions (e.g. alpha-to-seed and neutron-to-seed ratios). In this presentation, I will summarize the most relevant aspects of our study.

        Speaker: Jorge Pereira and Fernando Montes (NSCL/FRIB)
      • 11:00
        Validation of statistical Hauser-Feshbach Theory for the (a,n) reactions in the light and medium mass range 20m

        We calculate the (a,n) reaction cross sections with the statistical Hauser-Feshbach theory and validate such theory in the light and medium mass range. Since the Hauser-Feshbach theory assumes that the level density at a compound nuclear state is high enough, it is not so trivial to apply the statistical theory to the calculation of (a,n) reactions safely in the light mass region. To calculate the cross sections of (a,n) reactions based on the Hauser-Feshbach theory, we use the CoH3 code. Our numerical results of the cross sections on stable nuclei are in good agreement with available experimental data, while the resonance structure cannot be reproduced by the Hauser-Feshbach theory. We also calculate alpha-induced reactions on some unstable nuclei. Our results suggest that the Hauser-Feshbach theory is useful to calculate the (a,n) reactions when only energy-averaged properties of the cross section are important for the nucleosynthesis inside explosive astrophysical phenomena.

        Speaker: Hirokazu Sasaki
        presentation_virtual_workshop_a_n_astro.pdf
      • 11:20
        From chiral EFT NN interaction to nucleus-nucleus optical potentials 20m

        One of the long-standing challenges in the study and description of nuclear reactions is the determination of the interaction between projectile and target. These interactions are important inputs to compute reaction observables, which have applications in various fields of nuclear physics, including nuclear astrophysics. Typically, they are modeled using phenomenological potentials whose parameters are fitted to reproduce elastic-scattering data. Optical potentials are constructed as complex functions whose imaginary part simulates absorption into non-elastic channels. This approach gives precise reproduction of experimental data, but lacks predictive power and cannot be used at energies where there are no previous measurements.
        Double-folding potentials are nucleus-nucleus interactions constructed using the densities of the reacting nuclei and the interaction between their nucleons as input. They present a way of determining potentials relevant for nuclear reactions based on more fundamental inputs.
        We determine nucleus-nucleus potentials applying a double-folding technique. To this end, we use chiral EFT nucleon-nucleon interactions at N$^2$LO. Since this technique naturally leads to an energy dependence, we consider dispersion relations to constrain the imaginary part of the optical potential. With this approach we study reactions involving light nuclei such as $\alpha$, $^{12}$C, $^{16}$O, and $^{20}$O. We present results for elastic scattering off various targets, as well as for low-energy fusion relevant for stellar evolution. Our analysis has enabled us to study the impact of the nuclear density and the nucleon-nucleon interaction on the corresponding cross sections. Thanks to the predictive power of this technique, we can calculate various reaction observables without adjusting any parameter, obtaining excellent agreement with data.

        Speaker: Victoria Durant (JGU Mainz)
        Durant_slides.pdf
      • 11:40
        Low energy measurement of the $^{96}$Zr($\alpha$,n) and $^{100}$Mo($\alpha$,n) reactions for studying the weak r-process nucleosynthesis 20m

        Light neutron-rich isotopes are thought to be synthesized in the neutrino-driven ejecta of core-collapse supernova explosions via the weak r-process [1]. Recent nucleosynthesis studies have demonstrated that (α,n) reactions play a particularly important role in the production of these isotopes [1-4].
        The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, comes from the α+nucleus optical potential. There are several sets of parameters to calculate the α+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude.
        To constrain the parameters of the α+nucleus optical potential, recently the cross sections of the $^{96}$Zr($\alpha$,n)$^{99}$Mo and $^{100}$Mo($\alpha$,n)$^{103}$Ru reactions were measured at the astrophysically relevant energy regions [5,6]. The high precision experimental data have been analyzed in the statistical model and it was found that only the calculations performed with the Atomki-V2 potential [7] could reproduce the experimental data.
        Details on the experimental approach and the theoretical analysis will be presented and an outlook into the ongoing measurement of the $^{86}$Kr($\alpha$,n)$^{99}$Sr will be given.

        [1] A. Arcones and F. Montes, Astrophys. J. 731 5 (2011).
        [2] J. Pereira and F. Montes, Phys. Rev. C 93 034611 (2016).
        [3] P. Mohr, Phys. Rev. C 94 035801 (2016).
        [4] J. Bliss, A. Arcones, F. Montes, and J. Pereira, J. Phys. G 44 054003 (2017).
        [5] G. G. Kiss et al., Astrophysical J. 908 202 (2021).
        [6] T. N. Szegedi et al., Phys. Rev. C – submitted
        [7] P. Mohr et al., Phys. Rev. Lett. 124 252701 (2020).

        Speaker: Gabor Kiss (Atomki)
        GGKiss_Irena.pdf
      • 12:00
        Discussion/Break in Gather.Town 30m

        https://gather.town/app/qEmfdSensNrdQW6t/Alphan%20Workshop

    • 12:30 → 14:20
      The 22Ne(α,n) reaction
      • 12:30
        The 22Ne(α,n)25Mg reaction rate and its key role in low metallicity AGB stellar nucleosynthesis 20m

        The slow neutron-capture process (s-process) is one of the two main processes forming elements heavier than iron in stars. Its efficiency critically depends on key (α,n) reactions, which represent the main sources of neutrons to trigger the neutron-capture chain, producing all elements up to bismuth. In this work, we compute the evolution and s-process nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses M$_{ini}$/M$_{\odot}$ = 2 and 3 for initial metallicities around one-tenth that of the Sun. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. We compared our theoretical predictions with observed surface abundances on low-metallicity stars, finding that the $^{22}$Ne(α,n)$^{25}$Mg reaction rate plays a critical role in the s-process at low metallicities, shaping the abundance distribution in the whole atomic mass range. In particular, our results indicate that recent re-evaluation incorporating indirect measurements of the $^{22}$Ne(α,n)$^{25}$Mg reaction rate strongly impact our stellar nucleosynthesis calculations, bringing them into much better agreement with observations. We finally discuss which resonances need future complementary studies and the impact of current rate uncertainties on the s-process.

        Speaker: Umberto Battino (University of Edinburgh)
        Battino_Ne22an.pdf
      • 12:50
        SHADES - $\rm ^{22}Ne(\alpha,n)^{25}Mg$ in the Gamow window 20m

        Neutron capture reactions are the main contributors to the synthesis of the heavy elements. Besides $\rm ^{13}C(\alpha,n)^{16}O$, which has recently been measured by the LUNA collaboration in an energy region inside the Gamow peak, $\rm ^{22}Ne(\alpha,n)^{25}Mg$ is the second main neutron source in stars and its cross section is mostly unknown in the relevant stellar energy (450 keV < 750 keV), where only upper limits from direct experiments and highly uncertain estimates from indirect sources exist. The ERC project SHADES (UniNa/INFN) aims to provide for the first time direct cross section data in this region and to reduce the uncertainties of higher energy resonance parameters. High sensitivity measurements will be performed in the new LUNA-MV accelerator in the INFN-LNGS laboratory in Italy: the energy sensitivity of the hybrid neutron detector, together with the low background environment of the LNGS and the high beam current of the new accelerator, promises to improve the sensitivity by over 2 orders of magnitude over the state of the art, allowing to finally probe the unexplored low-energy cross section.

        An overview of the project and first results on the setup characterization will be presented in this talk.

        Speaker: Dr David Rapagnani (University of Naples "Federico II")
        SHADES - 22Ne(a,n)25Mg in the Gamow window.pdf
      • 13:10
        Sub-Coulomb alpha-transfer technique to constrain the 22Ne(alpha,n)25Mg reaction rate 20m

        The $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction is a very important neutron source reaction for the slow neutron capture process (s-process) in asymptotic giant branch stars. Direct measurements are extremely difficult to carry out at Gamow energies due to the extremely small reaction cross section. The large uncertainties introduced when extrapolating direct measurements at high energies down to the Gamow energies can be overcome by determining the partial $\alpha$-width of the relevant states in indirect measurements. This can be done using $\alpha$-transfer reactions at sub-Coulomb energies to reduce the dependence on optical model parameters. Two $\alpha$-transfer reaction measurements of $^{22}$Ne($^6$Li,d)$^{26}$Mg were carried out at the Cyclotron Institute at Texas A$\&$M University to study this reaction. It appears that the widths of the near $\alpha$-threshold resonances of $^{26}$Mg are quite different for similar $^{22}$Ne($^6$Li,d)$^{26}$Mg measurements carried out previously using different energies, which affects the final $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction rate and thus the final abundances of the s-process isotopes.

        Speaker: Heshani Jayatissa (Argonne National Laboratory)
      • 13:30
        Prospects for a Single Atom Microscope for Nuclear Astrophysics 20m

        The Single Atom Microscope (SAM) project sets out to measure rare, low-yield nuclear reactions with the pinnacle reaction of interest being $^{22}$Ne(α, n)$^{25}$Mg, a key source of neutrons in the s-process. The novel detector technique involves capturing product atoms in a cryogenically frozen and optically transparent noble gas solid, and then counting the embedded atoms via laser-induced fluorescence and optical imaging. Due to the unique absorption and emission wavelengths of the product atoms—enabled by the lattice of noble gas atoms—optical filters can distinguish between them to select the wavelength range of interest, making single-atom sensitivity feasible. The prototype Single Atom Microscope (pSAM) houses the films during growth and analysis. A refined procedure has been developed for growing clear noble gas films thick enough to stop recoiling ions with energies of astrophysical relevance. Moreover, pSAM attains cryogenic temperatures despite the large amount of optical access. Preliminary studies of laser-friendly Rubidium atoms in solid Krypton showed evidence that a large fraction of the ions neutralized. Calibrating the brightness of embedded atoms is the final step before initiating single atom sensitivity studies. The work is supported by U.S. National Science Foundation (NSF) under grant number 1654610. Additionally, EEW is supported as a 2020 NSF Graduate Research Fellowship Program recipient via grant DGE-1848739.

        Speaker: Ms Erin White (Michigan State University)
      • 13:50
        Discussion/Break in Gather.Town 30m

        https://gather.town/app/qEmfdSensNrdQW6t/Alphan%20Workshop

  • Thursday, 15 July
    • 10:00 → 11:50
      Experimental Techniques
      • 10:00
        Direct measurements of (α,n) cross sections and their impact in nuclear astrophysics 40m

        Several (α,n) reactions on stable and radioactive isotopes play a crucial role in nuclear astrophysics. For instance, some (α,n) reactions have been found to be important for the nucleosynthesis of the lightest elements in the rapid neutron-capture process (r-process) in neutrino-driven winds after a core collapse supernova. Direct measurements of these reactions at relevant astrophysical energies are experimentally challenging. This is due to the typically small cross sections and the experimental difficulties associated with low-intensity radioactive beams needed to study them. As a consequence, most of these reaction rates are still unknown. However, recent advances in the capabilities of radioactive ion beam facilities and experimental techniques have opened up new possibilities for the study of these astrophysically important reactions. In this talk I will review recent experimental efforts and future possibilities for the measurement of such reactions.

        Speaker: Melina Avila (ANL)
        Melina_Avila_(a,n)_Workshop.pdf
      • 10:40
        Non-standard techniques to determine ($\alpha$,n) cross sections 20m

        I want to report and discuss 2 techniques to determine ($\alpha$,n) cross sections.

        The first technique is based on the detailed balance principle and requires significant support from nuclear reaction theory. The investigation of Y(n,$\alpha$)X instead of X($\alpha$,n)Y can have advantages depending on the availability of sample material and need for energy resolution. I will present a recently commissioned detector, which is optimized for the detection of neutron-induced charge particle emission (NICE).

        Depending on the isotopes of interest, the only option investigating the desired ($\alpha$,n) reaction might be using a radioactive beam in inverse kinematics. Under such circumstances a ion storage ring might be a good solution. I will present options and discuss potential pros and cons of this approach.

        Speaker: Rene Reifarth (Goethe University Frankfurt (DE))
      • 11:00
        (alpha,n) Measurements With HeBGB at Ohio University 20m

        Understanding ($\alpha$,n) reaction yields is important for several applications in nuclear science and astrophysics. In particular, measured cross sections involving low Z nuclides are increasingly necessary, as many of the reactions of interest have little to no existing experimental data and theoretical models disagree by orders of magnitude in some cases due to limitations in the validity of statistical models for low to medium A nuclides. The $\alpha$-optical potential is also very uncertain, which leads to large uncertainties in predictions of reaction rates. Ascertaining the origin of the ~zinc to ~tin elements, for example, requires improved nuclear data through the measurement of several key ($\alpha$,n) cross sections at alpha energies in the 0.01-10MeV range. Many of the key reactions at these energies coincide with data required for applications involving inertial confinement fusion, molten salt reactors, and special nuclear materials. In all cases, a lack of certainty in ($\alpha$,n) reactions creates a potential hazard due to the unpredictable yields of neutrons produced via ($\alpha$,n). Improving nuclear data sets is therefore crucial and will be done in part by a newly developed neutron long counter, HeBGB, at the Ohio University Edwards Accelerator Lab.

        Speaker: Ms Kristyn Brandenburg (Ohio University)
      • 11:20
        Discussion/Break in Gather.Town 30m

        https://gather.town/app/qEmfdSensNrdQW6t/Alphan%20Workshop

    • 11:50 → 13:20
      The 13C(α,n) reaction
      • 11:50
        $^{13}$C$(\alpha,n)^{16}$O studies at the University of Notre Dame 20m

        The $^{13}$C$(\alpha,n)^{16}$O reaction is a common, naturally occurring, reaction in our universe because of its high cross section and the large natural abundance of carbon. In stars, it acts as a neutron source for the $s$-process during core helium burning, fueling the production of many of the heavy elements. On earth, because decaying actinides and carbon are often present together in rock and construction material, it is one of the main sources of neutron background in underground environments. Further, its inverse reaction, $^{16}$O$(n,\alpha)^{13}$C, is important in many environments where large neutron fluxes are present. Therefore, we want to have a precise and accurate evaluation of this cross section over a wide energy range (essentially $\approx$0 up to $\approx$9~MeV).

        Because of the experimental challenges associated with neutron detection, most measurements have been made using counting detectors. Yet for many of these applications, we need to know the angular distributions and partial cross sections of the reaction. At the University of Notre Dame, we've made several studies using a variety of experimental techniques. We've used the ORNL Deuterated Spectroscopic Array (ODeSA) to measure angular distributions and partial cross sections over regions where the neutron energies exceed $\approx$1~MeV. We've used the Hybrid Array of Gamma Ray Detectors (HAGRiD) and HPGes from the LANL GEANIE array to measure angular distributions of secondary $\gamma$-rays, and we've used a $^3$He spectrometer to measure low energy neutrons near thresholds. In this talk I'll give updates on the analysis of these different measurements and talk about plans for future studies.

        Speaker: Richard deBoer (University of Notre Dame)
      • 12:10
        Technical aspect of the direct measurement of the $^{13}$C$(\alpha$,n$)^{16}$O reaction in its Gamow window 20m

        The $^{13}$C$(\alpha$,n$)^{16}$O reaction is the dominant neutron source for the synthesis of the main $s$-process component of heavy elements, taking place in thermally pulsing, low-mass asymptotic giant branch stars. For the first time, the LUNA collaboration have performed the direct measurement of the $^{13}$C$(\alpha$,n$)^{16}$O reaction cross-section towards the Gamow window (150-230 keV) in the LNGS Underground Laboratory thanks to the reduction of the neutron background by 3 orders of magnitude.
        To reach the required low uncertainty over the experimental data it was mandatory to minimize the sources of systematic uncertainty. For this reason, a new approach of target monitoring and characterization; and a neutron detection setup with high and well known efficiency combined with low intrinsic neutron background have been performed.
        In this talk, we will give an overview of the different technical aspect of this LUNA experiment.

        Speaker: Laszlo Csedreki (Atomki)
      • 12:30
        Underground measurement of $^{13}$C($\alpha$,n)$^{16}$O reaction cross-section at low energies at LUNA 20m

        About half of the elements heavier than iron in the Universe are synthetized by the so-called slow neutron capture process (s-process). The $^{13}$C($\alpha$,n)$^{16}$O reaction is the main neutron sources in thermally pulsing low mass AGB stars at temperatures of about 90 MK. Due to the coulomb repulsion and the effect of level structure of the 17O compound nucleus, the uncertainty on the extrapolated $^{13}$C($\alpha$,n)$^{16}$O cross-section at astrophysically relevant energies, so-called Gamow window (150-230 keV) prevent to place reliable constraints on s-process nucleosynthesis. In the last three years the LUNA collaboration performed a measurement of the $^{13}$C($\alpha$,n)$^{16}$O cross-section in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS).
        A huge effort was made to analyze data in an energetic region where the signal to noise ratio approached the unity.
        The measurement indeed covers an energy range between 233 keV$<$E$_{cm}$$<$306 keV reaching the high energy edge of the s-process Gamow window, and obtaining an overall uncertainty lower than 20%.
        In this talk, the data analysis and the final results of the LUNA measurement will be illustrated, together with the astrophysical impact of new reaction rate.

        Speaker: Giovanni Francesco Ciani (University degli Studi di Bari & INFN Ba)
      • 12:50
        Discussion/Break in Gather.Town 30m

        https://gather.town/app/qEmfdSensNrdQW6t/Alphan%20Workshop

    • 13:20 → 16:50
      Astrophysics and Experiments
      • 13:20
        Astrophysical environments for nucleosynthesis involving (alpha, n) reactions 40m

        Nuclei heavier than the iron group can be produced during expansion of gas with initial temperature exceeding several billion degrees. Reactions involving free nucleons and alpha particles usually play important roles in such nucleosynthesis, whose details depend on whether the gas is neutron or proton rich and how its temperature and density evolve with time. Astrophysical environments relevant for this nucleosynthesis include neutrino-heated ejecta from core-collapse supernovae and neutron star mergers. These environments are discussed in the hope to guide the studies of (alpha, n) reactions under the relevant conditions.

        Speaker: Yongzhong Qian
        alpha-n.pdf
      • 14:00
        The Search for Weak R-process Signatures in Stardust 40m

        Stardust are condensates from stellar outflows and preserve the fingerprints of nucleosynthesis events in their parent stars. They are found in primitive meteorites and can be isolated and studied in the laboratory. This makes them an excellent source of data to place constraints on the astrophysical conditions of nucleosynthesis sites. Thus far, no smoking gun r-process signatures have been found in stardust grains for several reasons: (1) supernova grains are very rare and difficult to find; and (2) analysis of the trace elements in these micrometer-sized grains is technically challenging. I will discuss how stardust is useful to understanding supernova nucleosynthesis and present new results and methods critical for the laboratory analyses of these grains.

        Speaker: Wei Jia Ong (LLNL)
      • 14:40
        Observations of the weak r-process in metal-poor stars 20m

        Old, low-mass stars are excellent tracers of the most pristine neutron-capture nucleosynthesis that the Universe experienced. Through high-resolution spectra detailed information on the exact chemical composition locked up in old stars can be unveiled. Combining the chemical abundances of several elements provides some of the most useful observational insight into the formation processes that led to the observationally derived abundance patterns. In this talk I will present stellar abundance patterns focusing on the region Sr - Ag. By comparing observations and theory we learn about the enrichment processes that polluted some of the first stars. We found strong indications that the elements between Sr and Ag can be produced in a weak rapid n-capture (r-)process that could be associated with neutrino-driven winds from supernovae while the heavier elements tend to show a more robust pattern possibly stemming from neutron star mergers or rare supernovae. However, recently we have also seen that Sr can be produced by neutron star mergers. I will show how we can use observations of old stars, and discuss their applications and shortcomings, in the quest of mapping the r-process.

        Speaker: Camilla Hansen (Max Planck Institute for Astronomy)
      • 15:00
        Constraining the $^{17}$O$+\alpha$ reactions and their impact on the s-process in rotating massive stars 20m

        Low metalicity rotating massive stars are of great interest for the s-process and its impact on the early galactic chemical evolution of elements heavier than iron. The nuclear reaction rates that govern the s-process in those stars, however, remain poorly constrained. This is particularly true for the $^{17}$O($\alpha$,n)$^{20}$Ne and competing $^{17}$O($\alpha$,$\gamma$)$^{21}$Ne reactions that impact the available s-process neutron flux lost to neutron capture on $^{16}$O. The rates of these two reactions, which proceed through excited states in $^{21}$Ne, is of key importance to understanding nucleosynthesis during the s-process. We will present the results of two experimental studies on excited states in $^{21}$Ne that provide constraints on the $^{17}$O$+\alpha$ reaction rates. In the first, the particle transfer reaction, $^{20}$Ne(d,p)$^{21}$Ne, was used to populate the states. Outgoing protons were momentum-analyzed through the high-resolution split-pole spectrograph at the Triangle Universities Nuclear Laboratory, which yielded excitation energies, spin-parities, and neutron partial widths. To reveal states concealed by background in that experiment, a second experiment was performed at Argonne National Laboratory, where the same reaction was carried out in inverse kinematics using the HELIOS spectrometer. These excited state properties place constraints on the ($\alpha$,n)/($\alpha$,$\gamma$) reaction rate ratio, which was determined with realistic uncertainties for the first time. The new rates favour more neutron recycling, indicating that the weak s-process in rotating low-metalicity massive stars can produce elements up to lead.

        Speaker: Richard Longland (Triangle Universities Nuclear Laboratory)
      • 15:20
        Measurement of the $^{18}$O($\alpha$,n)$^{21}$Ne reaction 20m

        The isotopic composition of presolar grains, more specifically low-density graphites, are a rare window into explosive stellar nucleosynthesis. Analysis of presolar grains from the Orgueil meteorite found that $^{18}$O had an isotopic enrichment of up to 98,000% over terrestrial abundance, in addition to high spatial correlation with hotspots of $^{15}$N. A potential cause of this is that during explosive He-shell burning, the bulk of neutrons required for $^{15}$N production are sourced from the $^{18}$O($\alpha$,n)$^{21}$Ne reaction. To better understand the $^{18}$O($\alpha$,n)$^{21}$Ne reaction, and how the rate affects the abundance of $^{15}$N, a measurement of the $^{18}$O($\alpha$,n)$^{21}$Ne was conducted at the University of Notre Dame to extract partial and total cross sections over the energy range E$_{\alpha}$ = 2-8 MeV. Neutrons were detected using 10 ODeSA detectors via the spectrum unfolding method of neutron spectroscopy, and secondary gamma rays were detected with 2 HPGe detectors from the LANL GEANIE array. Preliminary partial cross sections extracted from secondary gamma ray data will be presented and compared to existing evaluations. In addition, preliminary neutron energy spectra and yield curves will also be shown.

        This work has been supported in part by the U.S. Department of Energy and the National Science Foundation.

        Speaker: Rebecca Toomey (Rutgers University)
      • 15:40
        Discussion/Break in Gather.Town 30m
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