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

一种新的延迟衰变光谱方法成功地研究了11Be的奇异结构。本方法的主要特点是利用了自旋极化的放射性原子核。衰变的不对称性使我们能够确定衰变子态的自旋宇称。确定的自旋宇称赋值已经在2011年对7个状态进行了赋值，它们的详细衰变路径也已经确定。从这些证据中可以发现11Be中存在多种类型的团簇态。在过去的两年中，我们一直致力于低能中子的数据分析。在11Be→10Be + n的β -延迟中子衰变中，必须至少有两条与极低能中子发射相关的衰变路径。我们使用5个掺6li的玻璃闪烁体来探测能量小于200kev的中子。为了评估探测前中子散射对实验装置材料的影响，基于中子输运程序MCNP进行了蒙特卡罗模拟。利用模拟中子响应函数对实验中子能谱进行了细致的分析。成功地分配了两个动能为16 keV和72 keV的中子峰，并首次确定了两个中子的衰变路径和衰变强度。计划将目前建立的方法应用于非常富中子的Mg同位素的结构研究。对其可行性进行了详细的研究。在加拿大的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高优先级实验。这项实验将于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.