STUDAY OF NUCLEOSYNTHESIS USING keV NEUTRONS

在恒星中子能量为10-250keV的情况下，测量^&lt；14&gt；N(n，p)^&lt；14&gt；C反应的截面对于研究STARA的核合成是非常重要的。为了测量截面，我们研制了一种新型的气体闪烁漂移室，工作在N_2气体中。在建造了暗室后，我们使用阿尔法源进行了离线测试。能量和时间的分辨率都达到了预期。在在线测试中，获得了平均40keV的1.3m谷仓的平均截面。虽然另一个获得更高统计量的实验是重要的，但上述初步结果表明，N很可能吸收用于S过程的中子，特别是在缺乏金属的恒星中。对C反应进行了分析，首次得到了10-250keV中子能的准确截面。结果表明，在大质量贫金属星中，碳是一种中子毒。这一结果可能会对恒星的核合成产生强烈的影响。此外，还首次证明了截面随中子能量的增加而增大。这是由于非直接p波俘获的贡献，非直接p波俘获是由俘获态截面的能量依赖关系和伽马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，所以使用了大量的样品。因此，对厚样品中的多次散射事件进行适当的校正是非常重要的。为了进行修正，我们编制了蒙特卡罗程序，并用不同厚度的石蜡样品对该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.