Reaction Rate

14,000,000 Leading Edge Experts on the ideXlab platform

Scan Science and Technology

Contact Leading Edge Experts & Companies

Scan Science and Technology

Contact Leading Edge Experts & Companies

The Experts below are selected from a list of 318 Experts worldwide ranked by ideXlab platform

Christian G Iliadis - One of the best experts on this subject based on the ideXlab platform.

  • starlib a next generation Reaction Rate library for nuclear astrophysics
    Astrophysical Journal Supplement Series, 2013
    Co-Authors: A L Sallaska, Christian G Iliadis, S Starrfield, A E Champange, Stephane Goriely, Francis Timmes
    Abstract:

    STARLIB is a next-generation, all-purpose nuclear Reaction-Rate library. For the first time, this library provides the Rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended Rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other Rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the Rate and its uncertainty but also its distribution. Because a majority of Rates are lognormally distributed, this allows the construction of Rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate Reaction Rates, where uncertainties are rigorously defined. In STARLIB, experimental Rates are supplemented with: (1) theoretical TALYS Rates for Reactions for which no experimental input is available, and (2) laboratory and theoretical weak Rates. STARLIB includes all types of Reactions of astrophysical interest to Z = 83, such as (p, γ), (p, α), (α, n), and corresponding reverse Rates. Strong Rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting Rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based Rates.

  • the effects of thermonuclear Reaction Rate variations on 26al production in massive stars a sensitivity study
    Astrophysical Journal Supplement Series, 2011
    Co-Authors: Christian G Iliadis, Arthur E Champagne, Alessandro Chieffi, Marco Limongi
    Abstract:

    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.

  • new Reaction Rate for o 16 p gamma f 17 and its influence on the oxygen isotopic ratios in massive agb stars
    Physical Review C, 2008
    Co-Authors: Christian G Iliadis, Maria Lugaro, C Angulo, Pierre Descouvemont, Peter Mohr
    Abstract:

    The O-16(p, gamma)F-17 Reaction Rate is revisited with special emphasis on the stellar temperature range of T=60-100 MK, important for hot bottom burning in asymptotic giant branch (AGB) stars. We evaluate existing cross-section data that were obtained since 1958 and, if appropriate, correct published data for systematic errors that were not noticed previously, including the effects of coincidence summing and updated effective stopping powers. The data are interpreted by using two different models of nuclear Reactions, that is, a potential model and R-matrix theory. A new astrophysical S factor and recommended thermonuclear Reaction Rates are presented. As a result of our work, the O-16(p, gamma)F-17 Reaction has now the most precisely known Rate involving any target nucleus in the mass A >= 12 range, with Reaction Rate errors of about 7% over the entire temperature region of astrophysical interest (T=0.01-2.5 GK). The impact of the present improved Reaction Rate with its significantly reduced uncertainties on the hot bottom burning in AGB stars is discussed. In contrast to earlier results we find now that there is not clear evidence to date for any stellar grain origin from massive AGB stars.

  • Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars
    arXiv: Astrophysics, 2006
    Co-Authors: Robert G. Izzard, Maria Lugaro, Christian G Iliadis, Amanda I. Karakas
    Abstract:

    We study the effect of uncertainties in the proton-capture Reaction Rates of the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB nucleosynthesis is associated with the production of sodium, radioactive Al26 and the heavy magnesium isotopes, and it is possibly responsible for the O, Na, Mg and Al abundance anomalies observed in globular cluster stars. We model HBB with an analytic code based on full stellar evolution models so we can quickly cover a large parameter space. The Reaction Rates are varied first individually, then all together. This creates a knock-on effect, where an increase of one Reaction Rate affects production of an isotope further down the Reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most susceptible to current nuclear Reaction Rate uncertainties.

  • the effects of thermonuclear Reaction Rate variations on nova nucleosynthesis a sensitivity study
    Astrophysical Journal Supplement Series, 2002
    Co-Authors: Christian G Iliadis, Arthur E Champagne, J Jose, S Starrfield, Paul Tupper
    Abstract:

    We investigate the effects of thermonuclear Reaction-Rate uncertainties on nova nucleosynthesis. One-zone nucleosynthesis calculations have been performed by adopting temperature-density-time profiles of the hottest hydrogen-burning zone (i.e., the region in which most of the nucleosynthesis takes place). We obtain our profiles from seven different, recently published, hydrodynamic nova simulations covering peak temperatures in the range from Tpeak = 0.145 to 0.418 GK. For each of these profiles, we individually varied the Rates of 175 Reactions within their associated errors and analyzed the resulting abundance changes of 142 isotopes in the mass range below A = 40. In total, we performed ≈7350 nuclear Reaction network calculations. We use the most recent thermonuclear Reaction-Rate evaluations for the mass ranges A = 1-20 and 20-40. For the theoretical astrophysicist, our results indicate the extent to which nova nucleosynthesis calculations depend on currently uncertain nuclear physics input, while for the experimental nuclear physicist, our results represent at least a qualitative guide for future measurements at stable and radioactive ion beam facilities. We find that present Reaction-Rate estimates are reliable for predictions of Li, Be, C, and N abundances in nova nucleosynthesis. However, Rate uncertainties of several Reactions have to be reduced significantly in order to predict more reliable O, F, Ne, Na, Mg, Al, Si, S, Cl, and Ar abundances. Results are presented in tabular form for each adopted nova simulation.

Maria Lugaro - One of the best experts on this subject based on the ideXlab platform.

  • new Reaction Rate for o 16 p gamma f 17 and its influence on the oxygen isotopic ratios in massive agb stars
    Physical Review C, 2008
    Co-Authors: Christian G Iliadis, Maria Lugaro, C Angulo, Pierre Descouvemont, Peter Mohr
    Abstract:

    The O-16(p, gamma)F-17 Reaction Rate is revisited with special emphasis on the stellar temperature range of T=60-100 MK, important for hot bottom burning in asymptotic giant branch (AGB) stars. We evaluate existing cross-section data that were obtained since 1958 and, if appropriate, correct published data for systematic errors that were not noticed previously, including the effects of coincidence summing and updated effective stopping powers. The data are interpreted by using two different models of nuclear Reactions, that is, a potential model and R-matrix theory. A new astrophysical S factor and recommended thermonuclear Reaction Rates are presented. As a result of our work, the O-16(p, gamma)F-17 Reaction has now the most precisely known Rate involving any target nucleus in the mass A >= 12 range, with Reaction Rate errors of about 7% over the entire temperature region of astrophysical interest (T=0.01-2.5 GK). The impact of the present improved Reaction Rate with its significantly reduced uncertainties on the hot bottom burning in AGB stars is discussed. In contrast to earlier results we find now that there is not clear evidence to date for any stellar grain origin from massive AGB stars.

  • Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars
    arXiv: Astrophysics, 2006
    Co-Authors: Robert G. Izzard, Maria Lugaro, Christian G Iliadis, Amanda I. Karakas
    Abstract:

    We study the effect of uncertainties in the proton-capture Reaction Rates of the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB nucleosynthesis is associated with the production of sodium, radioactive Al26 and the heavy magnesium isotopes, and it is possibly responsible for the O, Na, Mg and Al abundance anomalies observed in globular cluster stars. We model HBB with an analytic code based on full stellar evolution models so we can quickly cover a large parameter space. The Reaction Rates are varied first individually, then all together. This creates a knock-on effect, where an increase of one Reaction Rate affects production of an isotope further down the Reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most susceptible to current nuclear Reaction Rate uncertainties.

  • Reaction Rate uncertainties and the production of 19f in asymptotic giant branch stars
    The Astrophysical Journal, 2004
    Co-Authors: Maria Lugaro, Amanda I. Karakas, C Ugalde, J Gorres, M Wiescher, John C Lattanzio, Robert C Cannon
    Abstract:

    We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the Reaction Rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(α, γ)18O and 19F(α, p)22Ne Reaction Rates. For these two Reactions we present new estimates of the Rates and their uncertainties. In both cases the revised Rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(α, γ)18O Reaction Rate. The importance of the partial mixing zone is reduced when using our estimate for this Rate. Overall, Rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass 3 M☉ and of about a factor of 7 in stellar models of mass 5 M☉. This larger effect at high masses is due to the high uncertainties of the 19F(α, p)22Ne Reaction Rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear Reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(α, γ)18O Reaction Rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Arthur E Champagne - One of the best experts on this subject based on the ideXlab platform.

  • the effects of thermonuclear Reaction Rate variations on 26al production in massive stars a sensitivity study
    Astrophysical Journal Supplement Series, 2011
    Co-Authors: Christian G Iliadis, Arthur E Champagne, Alessandro Chieffi, Marco Limongi
    Abstract:

    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.

  • the effects of thermonuclear Reaction Rate variations on nova nucleosynthesis a sensitivity study
    Astrophysical Journal Supplement Series, 2002
    Co-Authors: Christian G Iliadis, Arthur E Champagne, J Jose, S Starrfield, Paul Tupper
    Abstract:

    We investigate the effects of thermonuclear Reaction-Rate uncertainties on nova nucleosynthesis. One-zone nucleosynthesis calculations have been performed by adopting temperature-density-time profiles of the hottest hydrogen-burning zone (i.e., the region in which most of the nucleosynthesis takes place). We obtain our profiles from seven different, recently published, hydrodynamic nova simulations covering peak temperatures in the range from Tpeak = 0.145 to 0.418 GK. For each of these profiles, we individually varied the Rates of 175 Reactions within their associated errors and analyzed the resulting abundance changes of 142 isotopes in the mass range below A = 40. In total, we performed ≈7350 nuclear Reaction network calculations. We use the most recent thermonuclear Reaction-Rate evaluations for the mass ranges A = 1-20 and 20-40. For the theoretical astrophysicist, our results indicate the extent to which nova nucleosynthesis calculations depend on currently uncertain nuclear physics input, while for the experimental nuclear physicist, our results represent at least a qualitative guide for future measurements at stable and radioactive ion beam facilities. We find that present Reaction-Rate estimates are reliable for predictions of Li, Be, C, and N abundances in nova nucleosynthesis. However, Rate uncertainties of several Reactions have to be reduced significantly in order to predict more reliable O, F, Ne, Na, Mg, Al, Si, S, Cl, and Ar abundances. Results are presented in tabular form for each adopted nova simulation.

Hiroshi Sakugawa - One of the best experts on this subject based on the ideXlab platform.

  • Hydroxyl radical generation with a high power ultraviolet light emitting diode (UV-LED) and application for determination of hydroxyl radical Reaction Rate constants
    Journal of Photochemistry and Photobiology A: Chemistry, 2017
    Co-Authors: Kazuhiko Takeda, Katsunari Fujisawa, Hitoshi Nojima, Ryota Kato, Ryuta Ueki, Hiroshi Sakugawa
    Abstract:

    We propose a simple, efficient and selective hydroxyl radical generation system based on the photolysis of submicromolar concentrations of nitrite using a high-power ultraviolet light emitting diode (UV-LED). Hydroxyl radical formation by the 6.75-W UV-LED was at least 10 times greater than that by a 300-W Xe lamp. In the UV-LED system, the hydroxyl radical formation Rate from nitrite was about four orders of magnitude larger than that from nitRate and two orders of magnitude larger than that from hydrogen peroxide. Such efficient and selective hydroxyl radical formation can be attributed to the overlap of the emission spectrum of the UV-LED and absorption of nitrite. The system was used to determine the Reaction Rate constants between hydroxyl radicals and chemicals based on the competition method with terephthalate as the hydroxyl radical probe. The Reaction Rate constants between hydroxyl radicals and some low-molecular-weight organic compounds and inorganic halide salts with various Reaction Rate constants were determined. The values obtained ranged from 105 to 1010 M−1 s−1, and agreed well with previously reported values. The potential of the developed method to determine the Reaction Rate constants of hydroxyl radicals is discussed.

Zeger Hens - One of the best experts on this subject based on the ideXlab platform.

  • tuning the postfocused size of colloidal nanocrystals by the Reaction Rate from theory to application
    ACS Nano, 2012
    Co-Authors: Richard Capek, Bram De Geyter, Zeger Hens
    Abstract:

    We show that adjusting the Reaction Rate in a hot injection synthesis is a viable stRategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the Rate of monomer formation. We implement this size-tuning stRategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the Reaction Rate depends in first order on the Cd and Se precursor concentration, the proposed stRategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distr...

  • tuning the postfocused size of colloidal nanocrystals by the Reaction Rate from theory to application
    5th Conference on Nanoscience with Nanocrystals (NaNax 5), 2012
    Co-Authors: Richard Capek, Bram De Geyter, Zeger Hens
    Abstract:

    The synthesis of colloidal semiconductor nanocrystals or quantum dots (QDs) with sharp size distributions at a desired mean size is of fundamental importance for the characterization of their materials properties and their use in applications. Size control by adjusting the Reaction time is frequently used and successful, but implies a reduced Reaction yield for sizes reached before the end of the size distribution focusing. Here, we show that adjusting the Reaction Rate in a hot injection synthesis is a viable stRategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. Our approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations, which yield experimentally meaningful quantities such as diameter, time and concentration, indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the Rate of monomer formation. We implement this size-tuning stRategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the Reaction Rate depends in first order on the Cd and Se precursor concentration, the proposed stRategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions with diameters ranging from 2.8 to 4.1 nm. In addition, we demonstRate that the same tuning stRategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of Reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions. reference: Abe, S.; Capek, R. K.; De Geyter, B.; Hens, Z., Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application, ACS Nano, 2012, 6 (1), 42–53