Abstract
The reaction is thought to be the neutron source during the s process in massive and intermediate mass stars as well as a secondary neutron source during the s process in low-mass stars. Therefore, an accurate determination of this rate is important for a better understanding of the origin of nuclides heavier than iron as well as for improving s-process models. Also, the s process produces seed nuclides for a later p process in massive stars, so an accurate value for this rate is important for a better understanding of the p process. Because the lowest observed resonance in direct measurements is considerably above the most important energy range for s-process temperatures, the uncertainty in this rate is dominated by the poorly known properties of states in between this resonance and threshold. Neutron measurements can observe these states with much better sensitivity and determine their parameters (except much more accurately than direct measurements. I have analyzed previously reported total and cross sections to obtain a much improved set of resonance parameters for states in between threshold and the lowest observed resonance, and an improved estimate of the uncertainty in the reaction rate. For example, definitely two, and very likely at least four, of the states in this region have natural parity and hence can contribute to the reaction, but two others definitely have non-natural parity and so can be eliminated from consideration. As a result, a recent evaluation in which it was assumed that only one of these states has natural parity has underestimated the reaction rate uncertainty by at least a factor of 10, whereas evaluations that assumed all these states could contribute probably have overestimated the uncertainty.
- Received 19 July 2002
DOI:https://doi.org/10.1103/PhysRevC.66.055805
©2002 American Physical Society