Improved astrophysical reaction rates for[sup 116,120]Sn(n, [gamma]) are of interest because nucleosynthesis models have not been able to reproduce the observed abundances in this mass region. For example, previous s-process calculations have consistently underproduced the s-only isotope[sup 116]Sn. Also, these studies have resulted in residual reprocess abundances for the tin isotopes which are systematically larger than predicted by reprocess calculations. It has been suggested that these problems could be solved by reducing the solar tin abundance by 10-20%, but there is no experimental evidence to justify this renormalization. Instead, it is possible that the problem lies in the (n, T) cross sections used in the reaction network calculations or in the s-process models. One reason to suspect the (n, [gamma]) data is that previous measurements did not extend to low enough energies to determine accurately the Maxwellian-averaged capture cross sections at the low temperatures (kT=6-8 keV) favored by the most recent stellar models of the s process. Also, the two most recent high-precision measurements of the[sup 120]Sn(n, [gamma]) cross section are in serious disagreement. Because of its small size, this cross section could affect (via the s-process branching at[sup 121]Sn) the relative abundances of the three s-only isotopes of Te.

Improved astrophysical reaction rates for {sup 116,120}Sn(n, [gamma]) are of interest because nucleosynthesis models have not been able to reproduce the observed abundances in this mass region. For example, previous s-process calculations have consistently underproduced the s-only isotope 116Sn. Also, these studies have resulted in residual reprocess abundances for the tin isotopes which are systematically larger than predicted by reprocess calculations. It has been suggested that these problems could be solved by reducing the solar tin abundance by 10-20%, but there is no experimental evidence to justify this renormalization. Instead, it is possible that the problem lies in the (n, T) cross sections used in the reaction network calculations or in the s-process models. One reason to suspect the (n, [gamma]) data is that previous measurements did not extend to low enough energies to determine accurately the Maxwellian-averaged capture cross sections at the low temperatures (kT=6-8 keV) favored by the most recent stellar models of the s process. Also, the two most recent high-precision measurements of the 12°Sn(n, [gamma]) cross section are in serious disagreement. Because of its small size, this cross section could affect (via the s-process branching at 121Sn) the relative abundances of the three s-only isotopes of Te.

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 - 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT (almost equal to) 6 - 12 keV) favored by the most recent stellar models of the {ital s}-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical {ital s}-process temperature, kT = 30 keV, but show significant differences at lower 40 temperatures. We discuss the astrophysical implications of these differences.

In addition to the extensive list of detailed individual resonance parameters for each isotope, this book contains thermal cross sections and average resonance parameters, as well as a short survey of the physics of thermal and resonance neutrons with emphasis on evaluation methods.