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

1.我们<40><92><122>利用RIKEN环形回旋加速器研究了E/A=21和26 MeV的^ Ar+Ni、^ Mo和^ Sn反应的热核性质。选择聚变反应过程研究热核中的能级密度和巨偶极共振。结果表明，在能级密度参数a=A/（91）下，与聚变残留物相符合的中子能谱得到了再现<plus-minus>。然而，GDR衰变γ射线的产额在E_x-250 MeV以上并不增加。在80 &lt; A &lt; 150时，GDR γ射线衰变的极限温度为T_c ~（-17）A ~（-1/3）。为了回答核的集体运动是否在高温下消失或热核在发射γ射线之前被粒子蒸发冷却的问题，非常需要继续进行系统的研究。为了研究激发能和角动量对巨偶极共振的影响，<100><32>利用日本原子力科学研究院串列加速器，在E=150、180和210 MeV的条件下，测量了^ Mo+^ S反应中的高能γ射线.结果表明，当E_x &gt; 100 MeV时，GDR宽度强烈地依赖于热激发能，RIKEN在以前的工作中得到的经验公式&lt;$MA =4.5+ 0.035E_x +1.6x10^<-8>E_x ^4可以很好地解释这些数据。

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.