STUDAY OF NUCLEOSYNTHESIS USING keV NEUTRONS

在<14><14>恒星中子能量10 - 250 keV范围内，测量^ N（n，p）^ C反应截面对研究恒星核合成具有重要意义。为了测量截面，我们研制了一种新型的工作在N_2气体中的气体闪烁漂移室。在构建了腔室之后，我们使用α源进行了离线测试。如预期的那样，获得了良好的能量和时间分辨率。在线测试中，在40 keV平均能量下，获得了1.3mbarn的平均截面。虽然另一个获得更高统计的实验是很重要的，但上述初步结果表明，在s-过程中，特别是在缺金属星中，N很可能会吸收中子。对^<12>C（n，γ）^<13>C反应的实验数据进行了分析，首次获得了准确的截面。中子能量在10 - 250 keV之间。从目前的结果看来，碳在大质量缺金属星中起着中子毒物的作用。这一结果可能对恒星核合成的过程产生重要影响。此外，它是第一次表明，随着中子能量的增加，截面增加。这是由于非直接p波俘获的贡献，这是从截面的能量依赖性和俘获态的γ射线分支比推导出来的。在中子俘获反应中，由于截面通常很小，所以需要使用大量的样品。因此，正确地校正厚样品中的多次散射事件是重要的。为了进行修正，我们编制了一个Monte Garlo程序，并用不同厚度的石蜡样品对程序的有效性进行了检验。

It is quite important to measure the cross section of the ^<14>N(n, p) ^<14>C reaction at the stellar neutron energy, 10 - 250 keV, to investigate the nucleosynthesis in stara. In order to measure the cross section we have developed the new type of a gas scintillation drift chamber, operating in the N_2 gas. After constructing the chamber we did the off-line test using the alpha source. Good resolutions of energy and time were obtained as expected. In the on-line test, the average cross section of 1.3 mbarn was obtained at the average of 40 keV.Although another ezperiment to get higher statistics is certainly important, the above preliminary result indicates that the N is likely to absord neutrons for the s-process especially in metal deficient stars.The experimental data of the ^<12>C(n, gamma) ^<13>C reaction have been analyzed and for the first time we obtained the accurate cross section in the neutron energies between 10 - 250 keV.From the present resunt it turned out that the carbon acts as a neutron poison in massive metal-deficient stars. The result may affect strongly the scinario of the nucleosynthesis in stars. Furthermore, it was shown for the first time that the cross section increases as the neutron energy increases. This is due to the contribution of the non-direct p-wave capture, which was deduced from the energy dependence of the cross section and the gamma -ray branching ratio from a capturing state.In the neutron capture reaction large amounts of samples are used, as the cross section is usually very small. Therefore it is important to correct properly the multiple scattering events in the thick sample. For the correction wedeveloped a Monte Garlo code and the validity of the code was tested by using paraffine samples with various thickness.