scholarly journals Bias Effects on g- and s-Factors in Westcott Convention

2021 ◽  
Vol 11 (14) ◽  
pp. 6558
Author(s):  
Hideo Harada
Keyword(s):  
Cross Sections ◽  
Reaction Rate ◽  
Epithermal Neutron ◽  
Reaction Rates ◽  
Sample Temperature ◽  
Neutron Temperature ◽  
S Factor ◽  
Function Shape ◽  
Capture Cross Sections ◽  
Westcott Convention

For accuracy improvement of neutron activation analysis and neutron capture cross sections, bias effects are investigated on g- and s-factors in the Westcott convention. As origins of biases, a joining function shape, neutron temperature, and sample temperature have been investigated. Biases are quantitatively deduced for two 1/v isotopes (197Au, 59Co) and six non-1/v isotopes (241Am, 151Eu, 103Rh, 115In, 177Hf, 226Ra). The s-factor calculated with a joining function deduced recently by a detailed Monte Carlo simulation is compared to s-factors calculated with traditional joining functions by Westcott. The results show the bias induced by the sample temperature is small, in the order of 0.1% for the g-factor and in the order of 1% for the s-factor. On the other hand, the bias size induced by a joining function shape for the s-factor depends significantly on both isotopes and neutron temperature. As a result, the reaction rates are also affected significantly. The bias size for the reaction rate is given in the case of an epithermal neutron index r = 0.1, for the eight isotopes.

scholarly journals Tuning the Re/Os Clock: Stellar-Neutron Cross Sections

2009 ◽  
Vol 26 (3) ◽  
pp. 250-254 ◽  
Author(s):  
A. Mengoni ◽  
M. Mosconi ◽  
K. Fujii ◽  
F. Käppeler ◽  
Keyword(s):  
Cross Sections ◽  
Reaction Rates ◽  
Experimental Information ◽  
Nuclear Model ◽  
Time Duration ◽  
Model Calculations ◽  
Neutron Time ◽  
Neutron Cross Sections ◽  
Capture Cross Sections

AbstractThe neutron-capture cross sections of 186,187Os have been recently measured at the CERN neutron time-of-flight facility n_TOF for an improved evaluation of the Re/Os cosmo-chronometer. This experimental information was complemented by nuclear model calculations for obtaining the proper astrophysical reaction rates at s-process temperatures. The calculated results and their implications for the determination of the time-duration of nucleosynthesis during galactic chemical evolution is discussed.

scholarly journals An increase in the 12C+12C fusion rate from resonances at astrophysical energies

2019 ◽  
Vol 52 (382) ◽  
pp. MISC6-MISC8
Author(s):  
Aurora Tumino
Keyword(s):  
Neutron Stars ◽  
Cross Sections ◽  
Coulomb Barrier ◽  
Reaction Rate ◽  
Nuclear Reactions ◽  
Massive Stars ◽  
Reaction Rates ◽  
Fusion Reactions ◽  
Trojan Horse Method ◽  
Carbon Burning

Carbon burning powers pivotal scenarios that influence the fate of stars, such as the late evolutionary stages of massive stars (exceeding eight solar masses), superbursts from accreting neutron stars and progenitors of Type Ia supernovae. It proceeds through the 12C+12C fusion reactions that produce an \( \alpha \) particle and neon-20 or a proton and sodium-23 —that is, 12C(12C, \( \alpha \) )20Ne and 12C(12C, \( p \))23Na— at temperatures greater than \( 0.4 \cdot 10^9 \) K, corresponding to astrophysical energies exceeding a megaelectronvolt (MeV), at which such nuclear reactions are more likely to occur in stars. The cross-sections for those carbon fusion reactions (probabilities that are required to calculate the rate of the reactions) have never been measured below 2 MeV because of exponential suppression arising from the Coulomb barrier (the Coulomb barrier is around 6 MeV). The reference rate at temperatures below \( 1.2\cdot 10^9 \) K relies on extrapolations that ignore the effects of possible low-lying resonances. In Tumino et al. (2018), we report the measurement of the 12C(12C, \( \alpha_{0,1} \)) 20Ne and 12C(12C, \( p_{0,1} \)) 23Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of 20Ne and 23Na, respectively) at centre-of-mass energies from 2.7 to 0.8 MeV using the Trojan Horse method and the deuteron in 14N. This is an indirect technique aiming at measuring low-energy nuclear reactions unhindered by the Coulomb barrier and free of electron screening. The deduced cross-sections exhibit several resonances that are responsible for a very large increase of the reaction rate at the relevant temperatures. In particular, around \( 5\cdot 10^8 \) K, the reaction rate is more than 25 times larger than the reference value. This finding may have significant implications such as lowering the temperatures and densities required for the ignition of carbon burning in massive stars and decreasing the superburst ignition depth in accreting neutron stars in the direction to reconcile observations with theoretical models.

scholarly journals Effect of Excited States on Thermonuclear Reaction Rates

1983 ◽  
Vol 36 (4) ◽  
pp. 583 ◽  
Author(s):  
DG Sargood
Keyword(s):  
Statistical Model ◽  
Excited States ◽  
Cross Sections ◽  
Reaction Rate ◽  
Ground States ◽  
Reaction Rates ◽  
Thermonuclear Reaction ◽  
Thermal Distribution ◽  
Naturally Occurring ◽  
Thermonuclear Reaction Rate

Values of the ratio of the thermonuclear reaction rate of a reaction, with target nuclei in a thermal distribution of energy states, to the reaction rate with all target nuclei in their ground states are tabulated for neutron, proton and (X-particle induced reactions on the naturally occurring nuclei from 2�Ne to 70Zn, at temperatures of 1, 2, 3�5 and 5 x 109 K. The ratios are determined from reaction rates based on statistical model cross sections.

Correlation of Sodium Atom Reaction Rates with Electron Capture Cross-sections

Nature ◽  
1965 ◽  
Vol 208 (5006) ◽  
pp. 182-183 ◽  
Author(s):  
K. R. WILSON ◽  
D. R. HERSCHBACH
Keyword(s):  
Electron Capture ◽  
Cross Sections ◽  
Sodium Atom ◽  
Reaction Rates ◽  
Capture Cross ◽  
Capture Cross Sections

scholarly journals s-Processing in Asymptotic Giant Branch Stars in the Light of Revised Neutron-Capture Cross Sections

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 239
Author(s):  
Diego Vescovi ◽  
René Reifarth
Keyword(s):  
Cross Sections ◽  
Neutron Capture ◽  
Reaction Rates ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Capture Cross ◽  
Magnetic Buoyancy ◽  
Stellar Models ◽  
Pocket Formation ◽  
Capture Cross Sections

Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the 13C pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a best set of neutron-capture cross sections was recently re-evaluated. Using stellar models prescribing that the 13C pocket is a by-product of magnetic-buoyancy-induced mixing phenomena, s-process calculations were carried out with this database. Significant effects are found for a few s-only and branching point isotopes, pointing out the need for improved neutron-capture cross section measurements at low energy.

Measurements of effective actinides neutron capture cross sections in a cadmium-filtered epithermal neutron spectrum

2021 ◽  
Vol 159 ◽  
pp. 108348
Author(s):  
G.J. Youinou ◽  
G. Palmiotti ◽  
M. Salvatores ◽  
J.K. Nimmagadda ◽  
G. Imel ◽  
...  
Keyword(s):  
Neutron Spectrum ◽  
Cross Sections ◽  
Neutron Capture ◽  
Epithermal Neutron ◽  
Capture Cross ◽  
Capture Cross Sections

Investigation of Sub-Coulomb barrier fusion reaction of α +40Ca in different models

2017 ◽  
Vol 26 (12) ◽  
pp. 1750086 ◽  
Author(s):  
F. Koyuncu ◽  
A. Soylu
Keyword(s):  
Cross Sections ◽  
Coulomb Barrier ◽  
Reaction Rate ◽  
Fusion Reaction ◽  
Reaction Rates ◽  
Nucleon Nucleon ◽  
Talys Code ◽  
Comparative Results ◽  
Double Folding ◽  
Energy Dependent

In this study, microscopic nucleon–nucleon Double Folding (DF) and phenomenological potentials have been used to investigate [Formula: see text]Ca reaction observables at sub-barrier energies. In the calculations, semi-classical Wentzel–Kramers–Brillouin (WKB) approach has been used in order to obtain the cross-sections and reaction rates of [Formula: see text]Ca. Besides WKB approximation, we have also utilized Talys code in order to get the comparative results and find out the method differences. To estimate the reaction rates, energy-dependent cross-sections and astrophysical S-factors of [Formula: see text]+[Formula: see text]Ca have been used. Herewith, differences between models and potentials have been demonstrated using the reaction rate estimates.

Location-thinking, value-thinking, and graphical forms: combining analytical frameworks to analyze inferences made by students when interpreting the points and trends on a reaction coordinate diagram

2021 ◽  
Author(s):  
Alexander P. Parobek ◽  
Patrick M. Chaffin ◽  
Marcy H. Towns
Keyword(s):  
Reaction Rate ◽  
Research Study ◽  
Reaction Rates ◽  
Reaction Coordinate ◽  
Structured Interviews ◽  
Chemistry Students ◽  
Qualitative Research Study ◽  
The Impact ◽  
Analytical Frameworks ◽  
Chemical Representations

Reaction coordinate diagrams (RCDs) are chemical representations widely employed to visualize the thermodynamic and kinetic parameters associated with reactions. Previous research has demonstrated a host of misconceptions students adopt when interpreting the perceived information encoded in RCDs. This qualitative research study explores how general chemistry students interpret points and trends on a RCD and how these interpretations impact their inferences regarding the rate of a chemical reaction. Sixteen students participated in semi-structured interviews in which participants were asked to interpret the points and trends along provided RCDs and to compare relative reaction rates between RCDs. Findings derived from this study demonstrate the diversity of graphical reasoning adopted by students, the impact of students’ interpretations of the x-axis of a RCD on the graphical reasoning employed, and the influence of these ideas on inferences made about reaction rate. Informed by analytical frameworks grounded in the resources framework and the actor-oriented model of transfer, implications for instruction are provided with suggestions for how RCDs may be presented to assist students in recognizing the critical information encoded in these diagrams.

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