Study of structure of unstable nuclei with the new method of beta-delayed spectroscopy

一种新的β延迟衰变谱方法已被成功地应用于11Be的奇异结构的研究。本方法的基本特征是使用自旋极化的放射性核。β衰变的不对称性使我们能够指定β衰变子态的自旋宇称。对11Be中的七个态进行了严格的自旋宇称分配，并确定了它们的详细衰变路径。从这些证据中发现，各种类型的集团态共存的11 Be。在过去的两年中，我们的重点是低能中子数据分析。在11Be →10Be + n的β缓发中子衰变中，至少存在两条与甚低能中子辐射有关的衰变路径。我们使用了5个6Li掺杂的玻璃闪烁器来探测能量小于200 keV的中子。为了评估中子散射的影响，在检测之前，由实验装置的材料，基于中子输运程序MCNP的蒙特卡罗模拟已被执行。利用模拟的中子响应函数对实验中子能谱进行了详细的分析。首次对动能为16 keV和72 keV的两个中子峰进行了归属，确定了两个中子衰变路径和强度，并计划将该方法应用于富中子Mg同位素的结构研究。并对其可行性进行了详细的研究。在加拿大TRIUMF进行实验的建议已经完成。该方案被实验评审委员会推荐为优先实验。实验将于2007年进行。

A new method in beta-delayed decay spectroscopy has been successfully applied to investigate the exotic structure of 11Be. The essential feature of the present method is a use of spin-polarized radioactive nucleus. The beta-decay asymmetry enables us to assign the spin-parity of the beta-decay daughter state. Firm spin-parity assignments have been made for seven states in 11Be, and their detailed decay paths have also been determined. From these evidences it was found that various types of cluster states co exist in 11Be.In the last two years we have focused on the low energy neutron data analysis. In the beta-delayed neutron decay of 11Be →10Be + n there must be at least two decay paths associated with very low energy neutron emission. We used five 6Li-doped glass scintillators to detect the neutrons with energies of less than 200 keV. In order to assess the effects of the neutron scattering, prior to the detection, by material of the experimental apparatus, Monte Carlo simulations based on the neutron transport code MCNP have been performed. The experimental neutron spectra have been carefully analyzed by the help of the simulated neutron response function. Two neutron peaks with the kinetic energies of 16 keV and 72 keV were successfully assigned and the two neutron decay paths and intensities have been determined for the first time.It was planned to apply the presently established method to the structure studies of the very neutron-rich Mg isotopes. The feasibility has been investigated in detail. A proposal for the experiment at TRIUMF, Canada has been finished. The proposal was recommended as the experiment with high priority by the experiment evaluation committee. The experiment will be performed in 2007.