Relevant Reaction Rates Research Articles

Improved predictions of nuclear reaction rates with the TALYS reaction code for astrophysical applications

Nuclear reaction rates of astrophysical applications are traditionally determined on the basis of Hauser-Feshbach reaction codes. These codes adopt a number of approximations that have never been tested, such as a simplified width fluctuation correction, the neglect of delayed or multiple-particle emission during the electromagnetic decay cascade, or the absence of the pre-equilibrium contribution at increasing incident energies. The reaction code TALYS has been recently updated to estimate the Maxwellian-averaged reaction rates that are of astrophysical relevance. These new developments enable the reaction rates to be calculated with increased accuracy and reliability and the approximations of previous codes to be investigated. The TALYS predictions for the thermonuclear rates of relevance to astrophysics are detailed and compared with those derived by widely-used codes for the same nuclear ingredients. It is shown that TALYS predictions may differ significantly from those of previous codes, in particular for nuclei for which no or little nuclear data is available. The pre-equilibrium process is shown to influence the astrophysics rates of exotic neutron-rich nuclei significantly. For the first time, the Maxwellian-averaged (n,2n) reaction rate is calculated for all nuclei and its competition with the radiative capture rate is discussed. The TALYS code provides a new tool to estimate all nuclear reaction rates of relevance to astrophysics with improved accuracy and reliability.

Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters

Aims. We propose the Wind of Fast Rotating Massive Stars scenario to explain the origin of the abundance anomalies observed in globular clusters. Methods. We compute and present models of fast rotating stars with initial masses between 20 and 120 M ⊙ for an initial metallicity Z = 0.0005 ([Fe/H] ≃ -1.5). We discuss the nucleosynthesis in the H-burning core of these objects and present the chemical composition of their ejecta. We consider the impact of uncertainties in the relevant nuclear reaction rates. Results. Fast rotating stars reach critical velocity at the beginning of their evolution and remain near the critical limit during the rest of the main sequence and part of the He-burning phase. As a consequence they lose large amounts of material through a mechanical wind which probably leads to the formation of a slow outflowing disk. The material in this slow wind is enriched in H-buming products and presents abundance patterns similar to the chemical anomalies observed in globular cluster stars. In particular, the C, N, O, Na and Li variations are well reproduced by our model. However the rate of the 24 Mg(p, y) has to be increased by a factor 1000 around 50 x 10 6 K in order to reproduce the amplitude of the observed Mg-Al anticorrelation. We discuss how the long-lived low-mass stars currently observed in globular clusters could have formed out of the slow wind material ejected by massive stars.

Big bang nucleosynthesis constraints on universal extra dimensions and varying fundamental constants

The successful prediction of light element abundances from big bang nucleosynthesis (BBN) has been a pillar of the standard model of cosmology. Because many of the relevant reaction rates are sensitive to the values of fundamental constants, such as the fine structure constant and the strong coupling constant, BBN is a useful tool to probe and to put constraints on possible cosmological variations of these constants, which arise naturally from many versions of extra-dimensional theories. In this paper, we study the dependences of fundamental constants on the radion field of the universal extra-dimension model, and calculate the effects of such varying constants on BBN. We also discuss the possibility that the discrepancy between BBN and the Wilkinson Microwave Anisotropy Probe (WMAP) data on the baryon-to-photon ratio can be reduced if the volume of the extra dimensions was slightly larger---by $\mathcal{O}({10}^{\ensuremath{-}3})$---at the BBN era compared to its present value, which would result in smaller gauge couplings at BBN by the same factor.

The Chemistry of Fluorine‐bearing Molecules in Diffuse and Dense Interstellar Gas Clouds

We present a theoretical investigation of the chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds, combining recent estimates for the rates of relevant chemical reactions with a self-consistent model for the physical and chemical conditions within gas clouds that are exposed to the interstellar ultraviolet radiation field. The chemistry of interstellar fluorine is qualitatively different from that of any other element, because unlike the neutral atoms of any other element found in diffuse or dense molecular clouds, atomic fluorine undergoes an exothermic reaction with molecular hydrogen. Over a wide range of conditions attained within interstellar gas clouds, the product of that reaction, hydrogen fluoride, is predicted to be the dominant gas-phase reservoir of interstellar fluorine nuclei. Fluorine is the heavy element that shows the greatest tendency toward molecule formation; in diffuse clouds of small extinction, the predicted HF abundance can even exceed that of CO, even though the gas-phase fluorine abundance is 4 orders of magnitude smaller than that of carbon. Our model predicts HF column densities of ~1013 cm-2 in dark clouds and column densities as large as 1011 cm-2 in diffuse interstellar gas clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds will be detectable by means of absorption-line spectroscopy of the J = 1-0 transition at 243.2 ?m using the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is predicted to be the second most abundant fluorine-bearing molecule, with typical column densities a factor of ~102 below those of HF; with its lowest two rotational transitions in the millimeter-wave spectral region, CF+ may be detectable from ground-based observatories. HF absorption in quasar spectra is a potential probe of molecular gas at high redshift, providing a possible bridge between the UV/optical observations capable of probing H2 in low column density systems and the radio/millimeter-wavelength observations that probe intervening molecular clouds of high extinction and large molecular fraction; at redshifts beyond ~0.3, HF is potentially detectable from ground-based submillimeter observatories in several atmospheric transmission windows.

26Al g, m + p resonances studied via proton decays of 27Si

We used the Al(He,t)Si*(p)Al reaction at the Wright Nuclear Structure Laboratory to probe resonances of Al + p and Al + p for excitation energies of astrophysical interest. We used a beam of 25-MeV He ions produced by the ESTU Tandem at WNSL, and a target of 125 g/cm Al. Ejected tritons were detected at 0◦ in the gas ionization detector at the focal plane of the Enge Split-pole Spectrometer. Decay protons were detected in two position-sensitive silicon-strip detectors, located in the target chamber very close to the target, spanning an angular range of 55 − 125◦. We detected protons from the decay of states in Si as little as 600 keV above the proton threshold, populating the ground and first (metastable) and second excited states of Al. The excitation energies of states in Si that are resonances for Al + p and Al + p were determined. Work is ongoing to determine branching ratios for these decays, as well as resonance strengths, reaction rates, and consequences for stellar nucleosynthesis. The observation of Al in the galaxy [1] provided the first direct evidence for ongoing stellar nucleosynthesis. The gamma ray from the decay of Al is an observable that can be used to test astrophysical models of such nucleosynthesis sites as novae. These models use nuclear reaction rates as input. In order for the predictions made by the models to be reliable, the nuclear physics input also needs to be reliable: relevant reaction rates must be known to at least 20%. The Al(p,γ)Si reaction is important to the total amount of Al present in a star, since it is the only destruction mechanism of Al that competes with beta decay, thus preventing emission of the signature 1.8-MeV gamma ray. The rate is currently known to only about a factor of four. In addition to proton capture on the ground state, there could be proton captures on the low-energy metastable first excited state of Al. (Because this state has a very different spin from the ground state (0+ vs 5+), there is no direct communication between the two states, although at very high temperatures they can communicate via higher-lying states of intermediate spin; the states must therefore be treated as separate entities in most calculations.) No experimental information currently exists on Al(p,γ)Si. Any resonant contributions to this reaction rate could have a significant effect on the amount of Al synthesized in high-temperature astrophysical events.

Light-Mass Cluster Reactions in Supernova R-Process

In explosive circumstances of the neutrino-driven winds in gravitational core-collapse Type II supernovae (SNe), not only heavy neutron-rich nuclei but also light unstable nuclei play the significant roles. We here study the efficiency and sensitivity of the SN r-process nucleosynthesis to many relevant nuclear reaction rates, and discuss quantitatively that several specific nuclear reactions on light neutron-rich nuclei take very critical keys to the final nucleosynthesis yields of r-process elements [Sasaqui, Kajino, T., T., Otsuki, K., Mathews, G. J. and Nakamura, T., Astrophys. J.(2004), submitted].

TheLi8(d,p)Li9reaction and the astrophysicalLi8(n,γ)Li9reaction rate

The $^{8}\mathrm{Li}(n,\ensuremath{\gamma})^{9}\mathrm{Li}$ reaction plays an important role in both the r-process nucleosynthesis and the inhomogeneous big bang models. Its direct capture rates can be extracted from the $^{8}\mathrm{Li}(d,p)^{9}\mathrm{Li}$ reaction, indirectly. We have measured the angular distribution of the $^{8}\mathrm{Li}(d,p)^{9}\mathrm{Li}{}_{g.s.}$ reaction at ${E}_{c.m.} = 7.8$ in inverse kinematics using coincidence detection of $^{9}\mathrm{Li}$ and the recoil proton, for the first time. Based on distorted wave Born approximation (DWBA) analysis, the $^{8}\mathrm{Li}(d,p)^{9}\mathrm{Li}{}_{g.s.}$ cross section was determined to be 7.9 \ifmmode\pm\else\textpm\fi{} 2.0 mb. The single particle spectroscopic factor ${S}_{1,3/2}$ for the ground state of $^{9}\mathrm{Li}=^{8}\mathrm{Li}\ensuremath{\bigotimes}n$ was derived to be $0.68\ifmmode\pm\else\textpm\fi{}0.14$, and then used to calculate the direct capture cross sections for the $^{8}\mathrm{Li}(n,\ensuremath{\gamma})^{9}\mathrm{Li}{}_{g.s.}$ reaction at energies of astrophysical interest. The astrophysical $^{8}\mathrm{Li}(n,\ensuremath{\gamma})^{9}\mathrm{Li}{}_{g.s.}$ reaction rate for the direct capture was found to be $3970\ifmmode\pm\else\textpm\fi{}950\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{mole}}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{s}^{\ensuremath{-}1}$ at ${T}_{9}=1.$ This presents the first experimental constraint for the $^{8}\mathrm{Li}(n,\ensuremath{\gamma})^{9}\mathrm{Li}$ reaction rates of astrophysical relevance.

Element synthesis calculations for stellar explosions: robust uncertainties, sensitivities, and radioactive ion beam measurements

We utilize multiple-zone, post-processing element synthesis calculations to determine the impact of recent ORNL radioactive ion beam measurements on predictions of novae and X-ray burst simulations. We also assess the correlations between all relevant reaction rates and all synthesized isotopes, and translate nuclear reaction rate uncertainties into abundance prediction uncertainties, via a unique Monte Carlo technique.

An Insight into the Reaction Rates of the Ne‐Na and Mg‐Al Chains from the Abundance Anomalies in Globular Cluster Red Giants

The Na, Mg, and Al abundance anomalies that have been observed in globular cluster red giants are hard to understand in terms of standard evolutionary scenarios, especially with regard to the depletion of 24Mg. In previous papers, we have proposed a flash-assisted deep-mixing model that can burn 24Mg and help explain the observed abundance anomalies. Relevant reaction rates have considerable uncertainties at low energies. Using NACRE reaction rates, we recompute the nucleosynthesis predicted by our model and, by adjusting reaction rates within current uncertainties, find that agreement with the observations is considerably improved for almost all isotopes except for 25Mg, which remains overabundant. A resolution of the latter discrepancy by nuclear physics alone would require that the 25Mg(p,γ)26Al reaction rate be substantially larger than that given by the experiments. This suggests that our model may require burning of 25Mg (coupled with some extra mixing) during quiescent hydrogen-burning phases.

Understanding the tropospheric transport and fate of agricultural pesticides.

Many field monitoring studies have indicated the substantial role of the troposphere as both a sink and transport medium for pesticides. At the same time, this is the least studied and understood environmental compartment in regards to pesticide fate. Although the fundamental principles behind volatilization and tropospheric reactivity are well understood, it is becoming increasingly apparent that the ability to quantitatively measure flux and reaction rates in the air will continue to pose problems for researchers. To date, most deterministic models that try to simulate real world conditions generally fail to provide tropospherically relevant flux and reaction rates because it is virtually impossible to scale down all possible interactions occurring in a near-infinite reservoir. Better field methods for determining flux are emerging and more ambient air monitoring studies are being conducted. This growing database of information, together with an understanding of physicochemical properties and use of expert systems, has increased the predictive capability for estimating volatilization flux of pesticides. Unfortunately, there are very limited environmental data on the tropospheric reaction rates of pesticides, and more experimental studies using semivolatile to low-volatility pesticides or their more volatile homologues are required to validate existing structure-activity relationship (SAR) model predictions. The development of new analytical strategies using elevated temperatures for assessing semivolatile to low-volatility pesticide reaction rates and products may provide an alternative approach to the need for controlled environmental temperature data. Recent international workshops organized by the Health Council of The Netherlands for developing uniform approaches for assessing exposure risks to pesticides in air exemplify efforts to synchronize flux with environmental fate information for determining human health and ecological risks. When more detailed pesticide information is desired, especially in high-use agricultural areas, where exposure to humans and nontarget ecological communities is a major concern, field flux measurements and downwind monitoring, together with experimental fate studies, should be considered. The integration of models, empirical testing, and real world monitoring will provide the ultimate safety net needed for assessing exposure risks to airborne pesticides.

Applicability and limitation of OSARs for the toxicity of electrophilic chemicals.

The appropriate selection and application of quantitative structure-activity relationships (QSARs) for the prediction of toxicity is based on the prior assignment of a chemical to its mode of toxic action. This classification is often derived from structural characteristics with the underlying assumption that chemically similar compounds have similar mechanisms of action, which is often but not necessarily the case. Instead of using structural characteristics for classification toward a mode of toxic action, we used Escherichia coli based bioanalytical assays to classify electrophilic chemicals. Analyzing a series of reactive organochlorines, epoxides, and compounds with an activated double bond, three subclasses of reactive toxicity were distinguished: "glutathione depletion-related toxicity", "DNA damage", and "unspecific reactivity". For both subsets of specifically reacting compounds a direct correlation between effects and chemical reactivity was found. Reaction rate constants with either glutathione or 2'-deoxyguanosine, which was used as a model for complex DNA, served well to set up preliminary QSARs for either glutathione depletion-related toxicity or toxicity based on DNA damage in the model organism E. coli. The applicability of QSARs for electrophilic chemicals based on mechanistically relevant reaction rate constants is a priori limited to a small subset of compounds with strictly identical mechanism of toxic action and similar metabolic rates. In contrast, the proposed bioanalytical assays not only allowed the experimental identification of molecular mechanisms underlying the observable toxicity but also their toxicity values are applicable to quantitatively predict toxic effects in higher organisms by linear correlation models, independent of the assigned mode of toxic action.

Nuclear astrophysics with radioactive beams: a TRIUMF perspective

Explosive nuclear burning in stellar environments involves reactions with a wide range of isotopes. For isotopes that are unstable, information on relevant reaction rates can only generally be obtained at radioactive beam facilities. The ISAC facility at TRIUMF is purpose built to provide a wide range of radioactive beams for nuclear astrophysics purposes as well as a range of other science.

Impact of nuclear reaction rate uncertainties on Nova models

We have, for the first time, determined the effect of nuclear reactions rate uncertainties on nova model predictions by simultaneously considering uncertainties in all relevant reaction rates. Our unique Monte Carlo approach enables a robust determination of uncertainties of nuclear origin in predictions of synthetized abundances, including radioisotopes which may be observable. This technique also enables us to identify which reactions most influence the production of each isotope, thereby guiding future measurements.

Nucleosynthesis of heavy elements in massive stars

We present nucleosynthesis results form calculations that follow the evolution of massive stars from their birth on the main sequence through their explosion as supernovae. Our simulations include all relevant isotopes and reaction rates up to bismuth. These are the first calculations to follow self-consistently in a complete massive star model the heavy s-process, p-process (γ-process), and v-process. We discuss where in the star the heavy p-process and v-process are made. We show the first calculations of the charged and neutral current v-process including a two particle cascade for the production of 138La and 180Ta in complete stellar models.

The Chemical Composition of White Dwarfs as a Test of Convective Efficiency during Core Helium Burning

Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate 12C(α,γ)16O reaction rate are able to account for a high central oxygen abundance. The rate of the 12C(α,γ)16O used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the 12C(α,γ)16O reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase.

Modelling finite cylinder electronegative discharges

A model for a finite length cylindrical electronegative (EN) plasmais constructed by obtaining an approximate solution when negative ions existover the entire plasma, but with ion flux to the walls dominating internalrecombination flux. The profiles obtained in this manner are useful for makingcalculations even when the recombination flux competes with the diffusion fluxfor ion loss. The result is that there is an approximately parabolic profilein the smaller dimension (radius or axial length) with a flat central regionand a similar edge profile in the larger dimension. The results are extendedto include plasmas in which the electronegative region does not occupy theentire cylinder by introducing a single length scale that defines a smaller ENcylinder embedded in the larger plasma. Single types of positively andnegatively charged ions and electrons are assumed to be the principal chargedspecies. The plasma thus formed is embedded in a full model of an attachingfeedstock gas with all relevant species and reaction rates included. Thecomplete model can be considered as a generalized global model which includesone additional spatial parameter and one additional equation. Theresulting equations are readily solved as in previous global models. Theresults are compared with experiments in an oxygen discharge, obtainingreasonably good agreement for the measurable parameters.

Multi-Scale Simulations of Silicon Etching by Halides: Effects of Surface Reaction Rates.

ABSTRACTWe investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.

Evaluations of thermonuclear proton capture reaction rates for [formula omitted] H, [formula omitted] Li, [formula omitted] Be, and [formula omitted] B

Recent measurements and analyses of proton induced radiative-capture reactions in the vicinity of E p =100 keV performed at the Triangle Universities Nuclear Laboratory are used to calculate the thermonuclear reaction rates of relevance in astrophysical plasmas for the reactions: D(p, γ ) 3 He, 7 Li(p, γ 0,1 ) 8 Be, 9 Be(p, γ 0,1,2,3 ) 10 B, and 11 B(p, γ 0,1 ) 12 C. Higher energy data from previous measurements were included when possible. The Maxwellian-averaged reaction rates, obtained using numerical integration techniques, are presented in tabular form as a function of temperature. Capture to the ground and excited states are treated separately, thus allowing the determination of inverse reaction rates for specified target states. The high temperature cut-offs of the reaction rate calculations are discussed. The results are compared to those presented in the recent NACRE compilation, where appropriate, and new analytical approximations for the total reaction rates are presented in a standard form.

Modeling of SiGe deposition using quantum chemistry techniques for detailed kinetic analysis

Modeling of chemical vapor deposition (CVD) processes requires knowledge of the gas-flow dynamics and chemical reactions occurring in the reactor. In order to go beyond simple growth rate predictions based on lumped kinetics from incomplete experimental kinetic data, quantum chemistry techniques are used to investigate kinetics of chemical reactions involved in the deposition of SiGe heterolayers. Gas-phase reaction pathways for GeH4 decomposition are proposed and the relevant reaction rates are evaluated. Cluster calculations are presented to determine binding energies of H, Cl on Si, Ge and SiGe surfaces and surface migration of H and Cl adatoms on Si and SiGe surfaces is investigated. The reactions are summarized into a detailed reaction mechanism to predict growth of Si1−xGex heterolayers within the realistic thermal-fluid environment present in a CVD reactor.

A compilation of charged-particle induced thermonuclear reaction rates

Low-energy cross section data for 86 charged-particle induced reactions involving light (1 ⩽ Z ⩽ 14), mostly stable, nuclei are compiled. The corresponding Maxwellian-averaged thermonuclear reaction rates of relevance in astrophysical plasmas at temperatures in the range from 10 6 K to 10 10 K are calculated. These evaluations assume either that the target nuclei are in their ground state, or that the target states are thermally populated following a Maxwell-Boltzmann distribution, except in some cases involving isomeric states. Adopted values complemented with lower and upper limits of the rates are presented in tabular form. Analytical approximations to the adopted rates, as well as to the inverse/direct rate ratios, are provided.