EXPERIMENTAL STUDY ON THE HEAVY ELEMENT SYNTHESIS AT THE SUPERNOVA EXPLOSION

在这个项目中，我们研究了在II型超新星爆炸时可能的快速中子捕获之前的种子核创建过程，他的过程被认为是从质子和中子发生的，这在种子区域产生了由铁和镍同位素组成的重元素，通过核反应，如（α，n）和（n，γ）。到目前为止，在这一过程中的相关元素丰度的时间依赖性已估计的大型核反应网络程序的基础上预测的反应率的统计模型。然而，由于低能级密度的存在，在dais轻核区的统计预测对其可预测性的可信度很低。因此，直接测量天体物理反应速率成为进一步定量研究上述综合过程的问题，我们利用轻丰中子放射性核束和高效的探测器系统，系统地测量了（α，n）反应截面，从而使在精细宽能区测量激发函数成为可能。在这个项目中，我们观察到了8^Li、^<12>B和^<16>N（α，n）的反应截面。得到的^8Li实验的激发函数比以前的实验结果高10倍，反应速率比以前的实验结果低一半。这表明^8Li（α，n）反应速率以及由此产生的中子数密度在元素合成中应被抑制，以改变r-过程的质量丰度模式。作为这个项目的一部分，我们还着手研究激光离子源和一种新的自旋极化放射性核的核谱学的技术问题，这是在JAERI建造的新放射性核束设施上进行进一步天体物理研究的重要进展。

In this project, we studied seed-nuclei creation process prior to the possible rapid neutron capture at the moment of the type II supernova explosion his process has been believed to occur from proton and neutron, which create heavy elements in the seed region consisting of iron and nickel isotopes thorough nuclear reactions like (alpha, n) and (n, gamma). So far, time dependence of relevant element abundance in this process has been estimated by a large nuclear reaction network code based on the predicted reaction rates of the statistical model. However, the statistical prediction in dais light nuclear region keeps little confidence for its predictability due to the small level densities. Therefore a direct measurement of astrophysical reaction rate becomes issue to further quantitative investigation of the above synthesis senarios.We have measured systematically the (alpha, n) reaction cross sections by using light neutron rich radioactive nuclear beams and efficient detector system, which makes it possible to measure excitation functions exclusively in fine broad energy region. In this project, we observed 8^Li,^<12>B, and ^<16>N (alpha, n) reaction cross sections. The obtained excitation function of the ^8Li experiment has 10 times higher statistics and deduced reaction rates are about a half values compared to the previous ones. This indicates that the ^8Li(alpha, n) reaction rate as well as resultant neutron number density should be suppressed in the element synthesis senarios so that mass abundance pattern of the r-process will be changed.As a part of this project, we have also proceeded to study technical aspects of a laser ion-source and a new nuclear-spectroscopy with spun-polarized radioactive nuclei, which are important developments for further astrophysical studies at a new radioactive nuclear beam facility constructing at JAERI.