20Na的β-γ-α奇异衰变模式研究

20Na decays with a half life of 0.448s and a decay energy of 13.9 MeV, to the daughter nucleus 20Ne of which the α threshold energy is 4.73 MeV. The spin and parity of the ground state of 20Na is 2+ and the isospin T=1, its decay mainly proceeds through the transitions to the 1+，2+ and 3+ states in 20Ne with T=0 or 1. According to the available data, the total branch ratio of 20Na β-delayed α emission is about 20.5%. The high-lying 1+ and 3+ states of 20Ne cannot directly decay to the ground state of 16O via α emission because of the violation of the conservation of angular momentum. Therefore, exotic β-γ-α decay mode may exist in 20Na, i.e. the high-lying 1+ and 3+ states in 20Ne de-excite to 0+ and 2+ states by γ rays, and then emit α to the ground state of 16O..A general purpose decay station was constructed under the support of specific fund for scientific instrument from NSFC, based on the Beijing Radioacitve Ion-beam Facility(BRIF). The motivation of this project is to combine the intense 20Na ISOL beam and the high-efficiency measurement of β、γ and α to study the decay properties of 20Na, with the emphasis on the γ and α branching ratios of the 20Ne resonant states above the α threshold and the exotic β-γ-α decay mode in 20Na. Such kind of study is also promising to extend to other nuclei, for example, 16N and 18N isotopes.

20Na的半衰期0.448秒，衰变能为13.9 MeV, 基态的自旋宇称为2+，同位旋T=1，主要衰变到子核20Ne的T=0或1的1+，2+和3+态。 根据现有的数据，20Na衰变到20Ne阈上能级缓发α的总分支比约为20.5%。因为不满足角动量守恒，子核20Ne的1+和3+态无法直接发射α到16O的基态，因此20Na极可能存在β-γ-α奇异衰变模式，即子核20Ne先发射γ到0+和2+态然后发射α到16O的基态。.在已结题的基金委科学仪器专项的支持下，基于北京放射性离子束装置建立了衰变谱学终端。本项目的目的是利用北京放射性离子束装置产生的高流强20Na ISOL束，在新建的衰变终端上通过对20Na衰变发射的β、γ和α实现高效率同时测量，细致研究20Ne α分离阈上共振能级的性质，确定发射γ和α的分支比，寻找20Na极稀有的β-γ-α奇异衰变模式，进而扩展到其它核如16N和18N等。

在北京放射性核束装置上，利用100MeV质子束轰击MgO厚靶，通过24Mg(p, αn)20Na反应产生了平均流强约20000粒子/秒的20Na低能ISOL束，完成了20Na的β-γ-α奇异衰变的实验研究。这是北京放射性核束装置建成以来的首个在束物理实验，通过高效率的β、γ和α同时测量，实现了β-γ-α三重符合，国际上首次发现了20Na的3个β-γ-α奇异衰变序列，拓展了奇异核已知的衰变模式。此外还发现了20Na β-α衰变到16O的6130 keV 3- 态的一个衰变序列，借此确定了20Ne的12367 keV 1+ 激发能级。实验导出的B(F) and B(GT)值与壳模型预言进行了比较，与SD壳空间的理论结果比较一致。