Study of Giant resonances in Hot Nuclei produced in Heavy Ion Fusion Reactions

1.用RIKEN环回旋加速器研究了E/A=21 MeV和26 MeV的^&lt；40&gt；Ar+Ni，^&lt；92&gt；Mo和^&lt；122&gt；锡反应的热核性质。选择聚变反应过程研究热核能级密度和巨偶极子共振。结果表明，与聚变残基符合测量的中子能谱以能级密度参数a=A/(9&lt；正负&gt；1)重现。然而，GDR衰变伽马射线的产额在E_x-250 MeV以上不会增加。对于80&lt；A&lt；150，GDR伽马射线衰变的极限温度为T_c-17A。非常有必要继续进行系统的研究，以回答这样一个问题：是核的集体运动在高温下消失，还是热核在发出伽马射线之前被粒子蒸发冷却。为了研究巨偶极子共振对激发能和角动量的依赖关系，用JAERI串列加速器测量了E=150、180和210 MeV的S反应的高能伽马射线。结果表明，GDR宽度强烈地依赖于Ex&gt；100 MeV的热激发能，并且在日本理工学院前期工作中得到的GAMMA=4.5+0.035Ex+1.6x10^&lt；-8&gt；Ex^4能很好地解释这一数据。

1. We have studied properties of hot nuclei by using ^<40>Ar+Ni, ^<92>Mo, and ^<122>Sn reactions with E/A=21 and 26 MeV by using the RIKEN Ring Cyclotron. Fusion reaction process was selected to study level densities and giant dipole resonances in hot nuclei. It was found that neutron energy spectra measured in coincidence with fusion residues were reproduced with the level density parameter a=A/(9<plus-minus>1). However, yields of the GDR decay gamma rays do not increase above E_x-250 MeV. The limiting temperature for the GDR gamma -ray decay were deduced as T_c-17A^<-1/3> for 80 < A < 150. It is highly desirable to continue the systematic study in order to answer the question whether the collective motion of the nucleus disappears at the high temperature or the hot nucleus is cooled down by particle evaporations before it emitts gamma rays.2. In order to investigate the dependence of giant dipole resonances on the excitation energy and the angular momentum, high energy gamma rays were measured in ^<100>Mo+^<32>S reaction with E=150, 180 and 210 MeV using the tandem accelerator at JAERI. The result shows that the GDR width depends strongly on the thermal excitation energy for E_x > 100 MeV and an empirical formula GAMMA =4.5+0.035E_x+1.6x10^<-8>E_x^4 obtained in the previous work at RIKEN can explain the data very well.