New measurement of neutron capture resonances in 209Bi
New measurement of neutron capture resonances in 209Bi
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DOI：
10.1103/physrevc.74.025807
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发表时间：
2006-08-01
影响因子：
3.1
通讯作者：
Wisshak, K.
中科院分区：
文献类型：
--
作者：
Domingo-Pardo, C.;Abbondanno, U.;Wisshak, K.
Bi-209 的中子俘获截面已在 CERN n_TOF 设施中采用脉冲高度加权技术进行了测量。与之前测量相比的改进主要是由于优化的检测系统，导致中子灵敏度几乎可以忽略不计。系统误差的其他实验来源，例如探测器中的电子阈值、伽马射线求和、内部电子转换和铋的异构态，都已被考虑在内。样品内部的伽马射线吸收效应已通过采用非多项式加权函数进行校正。由于 Bi-209 是恒星 s 过程反应路径中最后一个稳定同位素，因此麦克斯韦平均俘获截面对于 α 衰变反应流的再循环非常重要。在 kT=5 和 8 keV 之间的相关恒星热能范围内，我们的新捕获率比目前接受的用于核合成计算的值高出约 16%。在如此低的温度下，重 Pb-Bi 同位素的一个重要部分应该是通过低质量、热脉冲渐近巨分支星的 He 壳层中的 s 过程合成的。通过改进的横截面集，我们获得了 19 +/- 3% 的太阳铋丰度的 s 过程分数，从而导致 81 +/- 3% 的 r 过程残留。目前的 (n,gamma) 横截面测量也与基于液态金属 Pb/Bi 散裂靶的加速器驱动系统的设计相关。
The neutron capture cross section of Bi-209 has been measured at the CERN n_TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of gamma-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. gamma-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because Bi-209 is the last stable isotope in the reaction path of the stellar s-process, the Maxwellian averaged capture cross section is important for the recycling of the reaction flow by alpha decays. In the relevant stellar range of thermal energies between kT=5 and 8 keV our new capture rate is about 16% higher than the presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19 +/- 3% of the solar bismuth abundance, resulting in an r-process residual of 81 +/- 3%. The present (n,gamma) cross-section measurement is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target.