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Methods for Describing Astrophysics Reaction Rates in Low and High Mass Nuclei

This document discusses methodologies used for describing reaction rates in astrophysics, focusing on both low and high mass nuclei. For low mass nuclei, the R-matrix method is utilized to fit models based on available experimental data and extrapolate cross-sections to astrophysical energies. For higher mass nuclei near the iron mass region, a statistical model is required, incorporating averaged optical transmission coefficients and γ-strength functions. The study also emphasizes the need for experimental investigations into input parameters and level densities, especially in relation to neutron capture processes and reactions off the stability line.

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Methods for Describing Astrophysics Reaction Rates in Low and High Mass Nuclei

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  1. Methods used fordescriptionof astrophysics reaction rates: Carl Brune, Steve Grimes, Alexander Voinov (Ohio University) For low mass nuclei, modelswhich treat individual resonances, such as the R-matrixmethod,are fitted to the available experimental data. The cross-sectionscan beextrapolated down to astrophysical energies if needed. For higher mass nuclei (at and beyond the iron mass region), when the level density near threshold is large, the statistical model, using averaged optical transmission coefficients, level densities and γ-strength functions is required, especially to model the rapid neutron capture process.

  2. Statistical model of nuclear reactions Level densities Optical transmission coefficients Gamma strength functions Most uncertain input parameters, leading to factor of 2 of uncertainty in reaction rate calculations. The model is known. Efforts should be directed to experimental study of input parameters.

  3. Importance of low-energy enhancement of γ-strength function for (n,γ) reaction rates off the stability line A.C. Larsen et al, Acta. Phys. Pol. , B44, 563(2013).

  4. Level density issues • Widely used back shifted Fermi-gas level density systematics based on neutron resonance parameters are not accurate because of unknown spin cutoff parameter and parity distribution • More accurate level density information can be obtained from experimental study of particle evaporation spectra from compound nuclear reactions. Also spin cutoff parameter can be obtained from angular distribution of outgoing particles. • Experimental efforts are suggested to be directed to systematic study of alpha-, lithium- ,and heavy-ion-induced compound reactions. • Special efforts should be made to study of residual nuclei off of the stability line

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