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

KEV区的^&lt；18&gt；O(n，γ)^&lt；19&gt；O反应截面对于研究非标准非均匀大爆炸模型中的核合成和冷却巨星中的S过程具有重要意义。此外，轻核的中子俘获反应，如氧同位素，适合于测量共振反应和直接反应之间的干涉，因为这些能级密度很低。在本研究中，测量了30keV区的^&lt；18&gt；O的中子俘获截面，这对于评价S过程是重要的。测量是利用脉冲中子和探测中子俘获反应产生的瞬发射线进行的。使用136.5克95.1原子%富集度的D_2^&lt；18；O样品。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，还对~lt；197Au样品进行了测量，首次观察到~lt；19&gt；^O的三个基态和第二激发态和第一激发态的初级跃迁。最强的是向第二激发态的转变，其分支率约为50%。在42keV和65keV下获得的总俘获截面分别为8.5keV±1.7keV和11.9±1.8keVμb。目前的数据与Meissner等人的数据是一致的。在KT=25keV时：7.93±0.70μb。2000年在布拉格举行的“俘获伽马射线能谱及相关问题”国际研讨会上发表了上述结果。

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.