Triangle Universities Nuclear Laboratory

Durham, NC, United States

Triangle Universities Nuclear Laboratory

Durham, NC, United States
Time filter
Source Type

Longland R.,North Carolina State University | Longland R.,Triangle Universities Nuclear Laboratory
Astronomy and Astrophysics | Year: 2017

Context. Monte Carlo methods have enabled nuclear reaction rates from uncertain inputs to be presented in a statistically meaningful manner. However, these uncertainties are currently computed assuming no correlations between the physical quantities that enter those calculations. This is not always an appropriate assumption. Astrophysically important reactions are often dominated by resonances, whose properties are normalized to a well-known reference resonance. This insight provides a basis from which to develop a flexible framework for including correlations in Monte Carlo reaction rate calculations. Aims. The aim of this work is to develop and test a method for including correlations in Monte Carlo reaction rate calculations when the input has been normalized to a common reference. Methods. A mathematical framework is developed for including correlations between input parameters in Monte Carlo reaction rate calculations. The magnitude of those correlations is calculated from the uncertainties typically reported in experimental papers, where full correlation information is not available. The method is applied to four illustrative examples: a fictional 3-resonance reaction, 27Al(p, γ)28Si, 23Na(p, α)20Ne, and 23Na(α, p)26Mg. Results. Reaction rates at low temperatures that are dominated by a few isolated resonances are found to minimally impacted by correlation effects. However, reaction rates determined from many overlapping resonances can be significantly affected. Uncertainties in the 23Na(α, p)26Mg reaction, for example, increase by up to a factor of 5. This highlights the need to take correlation effects into account in reaction rate calculations, and provides insight into which cases are expected to be most affected by them. The impact of correlation effects on nucleosynthesis is also investigated. © ESO, 2017.

Longland R.,Polytechnic University of Catalonia | Longland R.,University of North Carolina at Chapel Hill | Longland R.,Triangle Universities Nuclear Laboratory | Iliadis C.,University of North Carolina at Chapel Hill | Iliadis C.,Triangle Universities Nuclear Laboratory
Physical Review C - Nuclear Physics | Year: 2012

The 22Ne(α,n)25Mg reaction is an important source of neutrons for the s-process. In massive stars responsible for the weak component of the s-process, 22Ne(α,n)25Mg is the dominant source of neutrons, both during core helium burning and in carbon-shell burning. For the main s-process component produced in asymptotic giant branch (AGB) stars, the 13C(α,n)16O reaction is the dominant source of neutrons operating during the interpulse period, with the 22Ne+α source affecting mainly the s-process branchings during a thermal pulse. Rate uncertainties in the competing 22Ne(α,n)25Mg and 22Ne(α,γ)26Mg reactions result in large variations of s-process nucleosynthesis. Here, we present up-to-date and statistically rigorous 22Ne+α reaction rates using recent experimental results and Monte Carlo sampling. Our new rates are used in postprocessing nucleosynthesis calculations both for massive stars and AGB stars. We demonstrate that the nucleosynthesis uncertainties arising from the new rates are dramatically reduced in comparison to previously published results, but several ambiguities in the present data must still be addressed. Recommendations for further study to resolve these issues are provided. © 2012 American Physical Society.

Mitchell G.E.,North Carolina State University | Mitchell G.E.,Triangle Universities Nuclear Laboratory | Richter A.,TU Darmstadt | Weidenmuller H.A.,Max Planck Institute for Nuclear Physics
Reviews of Modern Physics | Year: 2010

The application of random-matrix theory (RMT) to compound-nucleus (CN) reactions is reviewed. An introduction into the basic concepts of nuclear scattering theory is followed by a survey of phenomenological approaches to CN scattering. The implementation of a random-matrix approach into scattering theory leads to a statistical theory of CN reactions. Since RMT applies generically to chaotic quantum systems, that theory is, at the same time, a generic theory of quantum chaotic scattering. It uses a minimum of input parameters (average S matrix and mean level spacing of the CN). Predictions of the theory are derived with the help of field-theoretical methods adapted from condensed-matter physics and compared with those of phenomenological approaches. Thorough tests of the theory are reviewed, as are applications in nuclear physics, with special attention given to violation of symmetries (isospin and parity) and time-reversal invariance. © 2010 The American Physical Society.

Jose J.,Polytechnic University of Catalonia | Jose J.,Institute Destudis Espacials Of Catalonia Ieec | Moreno F.,Polytechnic University of Catalonia | Parikh A.,TU Munich | And 2 more authors.
Astrophysical Journal, Supplement Series | Year: 2010

Type I X-ray bursts (XRBs) are thermonuclear stellar explosions driven by charged-particle reactions. In the regime for combined H/He-ignition, the main nuclear flow is dominated by the rp-process (rapid proton-captures and β+-decays), the 3α-reaction, and the αp-process (a suite of (α, p) and (p, γ) reactions). The main flow is expected to proceed away from the valley of stability, eventually reaching the proton drip line beyond A = 38. Detailed analysis of the relevant reactions along the main path has only been scarcely addressed, mainly in the context of parameterized one-zone models. In this paper, we present a detailed study of the nucleosynthesis and nuclear processes powering type I XRBs. The reported 11 bursts have been computed by means of a spherically symmetric (one-dimensional), Lagrangian, hydrodynamic code, linked to a nuclear reaction network that contains 325 isotopes (from 1H to 107Te), and 1392 nuclear processes. These evolutionary sequences, followed from the onset of accretion up to the explosion and expansion stages, have been performed for two different metallicities to explore the dependence between the extension of the main nuclear flow and the initial metal content. We carefully analyze the dominant reactions and the products of nucleosynthesis, together with the physical parameters that determine the light curve (including recurrence times, ratios between persistent and burst luminosities, or the extent of the envelope expansion). Results are in qualitative agreement with the observed properties of some well-studied bursting sources. Leakage from the predicted SbSnTe cycle cannot be discarded in some of our models. Production of 12C (and implications for the mechanism that powers superbursts), light p-nuclei, and the amount of H left over after the bursting episodes will also be discussed. © 2010. The American Astronomical Society.

Jose J.,Polytechnic University of Catalonia | Jose J.,Institute Destudis Espacials Of Catalonia | Iliadis C.,University of North Carolina at Chapel Hill | Iliadis C.,Triangle Universities Nuclear Laboratory
Reports on Progress in Physics | Year: 2011

Half a century has passed since the foundation of nuclear astrophysics. Since then, this discipline has reached its maturity. Today, nuclear astrophysics constitutes a multidisciplinary crucible of knowledge that combines the achievements in theoretical astrophysics, observational astronomy, cosmochemistry and nuclear physics. New tools and developments have revolutionized our understanding of the origin of the elements: supercomputers have provided astrophysicists with the required computational capabilities to study the evolution of stars in a multidimensional framework; the emergence of high-energy astrophysics with space-borne observatories has opened new windows to observe the Universe, from a novel panchromatic perspective; cosmochemists have isolated tiny pieces of stardust embedded in primitive meteorites, giving clues on the processes operating in stars as well as on the way matter condenses to form solids; and nuclear physicists have measured reactions near stellar energies, through the combined efforts using stable and radioactive-ion beam facilities. This review provides comprehensive insight into the nuclear history of the Universe and related topics: starting from the Big Bang, when the ashes from the primordial explosion were transformed to hydrogen, helium and a few trace elements, to the rich variety of nucleosynthesis mechanisms and sites in the Universe. Particular attention is paid to the hydrostatic processes governing the evolution of low-mass stars, red giants and asymptotic giant-branch stars, as well as to the explosive nucleosynthesis occurring in core-collapse and thermonuclear supernovae, γ-ray bursts, classical novae, x-ray bursts, superbursts and stellar mergers. © 2011 IOP Publishing Ltd.

Iliadis C.,University of North Carolina at Chapel Hill | Iliadis C.,Triangle Universities Nuclear Laboratory | Champagne A.,University of North Carolina at Chapel Hill | Champagne A.,Triangle Universities Nuclear Laboratory | And 2 more authors.
Astrophysical Journal, Supplement Series | Year: 2011

We investigate the effects of thermonuclear reaction rate variations on 26Al production in massive stars. The dominant production sites in such events were recently investigated by using stellar model calculations: explosive neon-carbon burning, convective shell carbon burning, and convective core hydrogen burning. Post-processing nucleosynthesis calculations are performed for each of these sites by adopting temperature-density-time profiles from recent stellar evolution models. For each profile, we individually multiplied the rates of all relevant reactions by factors of 10, 2, 0.5, and 0.1, and analyzed the resulting abundance changes of 26Al. In total, we performed 900 nuclear reaction network calculations. Our simulations are based on a next-generation nuclear physics library, called STARLIB, which contains a recent evaluation of Monte Carlo reaction rates. Particular attention is paid to quantifying the rate uncertainties of those reactions that most sensitively influence 26Al production. For stellar modelers our results indicate to what degree predictions of 26Al nucleosynthesis depend on currently uncertain nuclear physics input, while for nuclear experimentalists our results represent a guide for future measurements. We also investigate equilibration effects of 26Al. In all previous massive star investigations, either a single species or two species of 26Al were taken into account, depending on whether thermal equilibrium was achieved or not. These are two extreme assumptions, and in a hot stellar plasma the ground and isomeric states may communicate via γ-ray transitions involving higher-lying 26Al levels. We tabulate the results of our reaction rate sensitivity study for each of the three distinct massive star sites referred to above. It is found that several current reaction rate uncertainties influence the production of 26Al. Particularly important reactions are 26Al(n,p)26Mg, 25Mg(α,n) 28Si, 24Mg(n,γ)25Mg, and 23Na(α,p)26Mg. These reactions should be prime targets for future measurements. Overall, we estimate that the nuclear physics uncertainty of the 26Al yield predicted by the massive star models explored here amounts to about a factor of three. We also find that taking the equilibration of 26Al levels explicitly into account in any of the massive star sites investigated here has only minor effects on the predicted 26Al yields. Furthermore, we provide for the interested reader detailed comments regarding the current status of certain reactions, including 12C(12C,n)23Mg, 23Na(α,p) 26Mg, 25Mg(α,n)28Si, 26Al m(p,γ)27Si, 26Al(n,p)26Mg, and 26Al(n,α)23Na. © 2011. The American Astronomical Society. All rights reserved.

Gould C.R.,North Carolina State University | Gould C.R.,Triangle Universities Nuclear Laboratory | Sharapov E.I.,Joint Institute for Nuclear Research
Physical Review C - Nuclear Physics | Year: 2012

Background: Lutetium thermometry has been used to analyze Oklo natural nuclear reactor zones but leads to widely varying and puzzling predictions for the temperatures T O which in turn impacts bounds on time variation of the fine structure constant α. Purpose: We revisit results for reactor zone RZ10 in light of new measurements of the isomer branching ratio Bg in 175Lu neutron capture at 5 and 25 keV. Method: We recalculate predictions for T O as a function of Bg using realistic models of the Oklo neutron flux. Results: We find T O=100±30 -C using a new value of Bg, in contrast to 350

Bhike M.,Duke University | Bhike M.,Triangle Universities Nuclear Laboratory | Tornow W.,Duke University | Tornow W.,Triangle Universities Nuclear Laboratory
Physical Review C - Nuclear Physics | Year: 2014

Fast-neutron-capture cross-section data on Xe136 have been measured with the activation method between 0.4 and 14.8 MeV. The cross section was found to be of the order of 1 mb at the eleven energies investigated. This result is important to interpret potential neutron-induced backgrounds in the enriched xenon observatory and KamLAND-Zen neutrinoless double-β decay searches that use xenon as both source and detector. A high-pressure sphere filled with Xe136 was irradiated with monoenergetic neutrons produced by the reactions 3H(p,n)3He, 2H(d,n)3He, and 3H(d,n)4He. Indium and gold monitor foils were irradiated simultaneously with the Xe136 to determine the incident neutron flux. The activities of the reaction products were measured with high-resolution γ-ray spectroscopy. The present results are compared to predictions from ENDF/B-VII.1 and TENDL-2012. © 2014 American Physical Society.

Kidd M.F.,Duke University | Kidd M.F.,Triangle Universities Nuclear Laboratory | Esterline J.H.,Duke University | Finch S.W.,Duke University | Tornow W.,Duke University
Physical Review C - Nuclear Physics | Year: 2014

Conclusions: Our half-life measurement agrees within uncertainties with another recent measurement in which no coincidence was employed. Our nuclear matrix element calculation may have an impact on a recent neutrinoless double-β decay nuclear matrix element calculation which implies that the decay to the first excited state in Sm150 is favored over that to the ground state.Background: Double-β decay is a rare nuclear process in which two neutrons in the nucleus are converted to two protons with the emission of two electrons and two electron antineutrinos.Purpose: We measured the half-life of the two-neutrino double-β decay of Nd150 to excited final states of Sm150 by detecting the deexcitation γ rays of the daughter nucleus.Method: This study yields the first detection of the coincidence γ rays from the 01+ excited state of Sm150. These γ rays have energies of 333.97 and 406.52 keV and are emitted in coincidence through a 01+→21+→0gs+ transition.Results: The enriched Nd2O3 sample consisted of 40.13 g Nd150 and was observed for 642.8 days at the Kimballton Underground Research Facility, producing 21.6 net events in the region of interest. This count rate gives a half-life of T1/2=[1.07-0.25+0.45(stat)±0.07(syst)]×1020 yr. The effective nuclear matrix element was found to be 0.0465-0.0054+0.0098. Finally, lower limits were obtained for decays to higher excited final states. © 2014 American Physical Society.

Oginni B.M.,Triangle Universities Nuclear Laboratory | Iliadis C.,University of North Carolina at Chapel Hill | Champagne A.E.,University of North Carolina at Chapel Hill
Physical Review C - Nuclear Physics | Year: 2011

The reactions that destroy Al26 in massive stars have significance in a number of astrophysical contexts. We evaluate the reaction rates of Al26(n,p)26Mg and Al26(n,α)23Na using cross sections obtained from the codes empire and talys. These have been compared to the published rates obtained from the non-smoker code and to some experimental data. We show that the results obtained from empire and talys are comparable to those from non-smoker. We also show how the theoretical results vary with respect to changes in the input parameters. Finally, we present recommended rates for these reactions using the available experimental data and our new theoretical results. © 2011 American Physical Society.

Loading Triangle Universities Nuclear Laboratory collaborators
Loading Triangle Universities Nuclear Laboratory collaborators