The Interference between the Resonance Reaction and the Direct Reaction in the keV-Neutron Capture by ^<18>O and Nucleosynthesis

keV区^<18>O(n,γ)^<19>O反应截面对于非标准非均匀大爆炸模型核合成和冷巨星s过程的研究具有重要意义。此外，轻核的中子捕获反应，如氧同位素，由于其能级密度低，适合于测量共振反应和直接反应之间的干涉。在本研究中，测量了^<18>O在30keV区域的中子俘获截面，这对s过程的评价很重要。测量是通过使用脉冲中子和探测中子捕获反应的提示射线来进行的。采用136.5 g浓度为95.1原子%的D_2^<18>样品。从光谱仪获得的中子飞行时间和γ射线脉冲高度的二维数据以列表方式存储在UNIX工作站中。作为参考，还进行了^<197Au样品的测量。结果首次观测到向第二激发态和第一激发态的三个主跃迁，以及<19>^O的基态。最强的是向第二激发态的跃迁，其分支率约为50%。在42和65 keV下获得的总俘获截面分别为8.5±1.7和11.9±1.8 μb。本文的数据与Meissner等人在kT=25keV时的数据一致：7.93±0.70 μb。获得的结果在2000年在布拉格举行的“捕获伽马射线光谱学和相关主题”国际研讨会上发表。

The ^<18>O(n,γ)^<19>O reaction cross sections in the keV region are important for studies on the nucleosyntheses in the non-standard inhomogeneous big-bang model and the s process in cool giant stars. Moreover, neutron capture reactions by light nuclei, such as oxygen isotopes are suitable for the measurement of the interference between the resonance reaction and the direct reaction, because these level densities are low.In this study, neutron capture cross sections of ^<18>O in the 30keV region, which are important for the evaluation of the s-process, were measured. The measurement was carried out by using pulsed neutrons and detecting the prompt rays from the neutron capture reaction. A 136.5 g of 95.1 atom% enriched D_2^<18>O sample was used. Two-dimensional data on neutron time of flight and γ-ray pulse height from the spectrometer were stored into a UNIX workstation in a list mode. As a reference, the measurement of ^<197Au sample was also performed.As a result, three primary transitions to the second and the first excited states and the ground state of <19>^O were observed for the first time. The strongest one was the transition to the second exited state, whose branching ratio was about 50%. The obtained total capture cross sections were obtained as 8.5±1.7 and 11.9±1.8 μb at 42 and 65 keV, respectively. The present data are consistent with the data by Meissner et al. : 7.93±0.70 μb at kT=25keV.The obtained result was presented at the international symposium : "Capture Gamma-ray Spectroscopy and Related Topics" held at Prague in 2000.