重核强阻尼碰撞中巨复合体系的形成和衰变

The Formation and Decay of Giant Composite Systems in Strongly Damped Collisions for very Heavy Nuclei

用改进的量子分子动力学模型研究了重核强阻尼反应过程中巨复合体系的形成和衰变机制。计算了强阻尼反应238U+238U和232Th+250Cf的入射道动力学势、绝热势和密度冻结势并对它们进行了比较,我们发现动力学势与绝热势非常接近,相对于非常陡峭的密度冻结势来说,在弹靶接触位型阶段两者都变的比较平坦。正因为这种比较平坦的相互作用势使巨复合体系能够存活一定的时间。通过不同能量下反应体系的密度分布随时间的演化,从而得到每个反应事件形成巨复合体系的寿命,进一步给出复合体系的寿命分布和平均寿命的能量依赖关系。研究表明:选择合适的入射能量可以使强阻尼反应形成的复合体系有最长的寿命。在动量空间中,通过复合体系形变的各向异性作为入射能量的函数的研究,发现使形成的巨复合体系具有各向同性的动量分布所对应的能量是最佳入射能量的选择,它能使复合体系保持最长的寿命,这种情况最有利于正电子的产生。

The mechanism of giant composite systems （GCS） form and decay in strongly damped collisions is investigated. The dynamic, adiabatic and frozen density approximation entrance channel potentials in strongly damped reactions of ^238U＋^238U ^232Th＋^250Cf are calculated and compared. By means of the improved quantum molecular dynamics （ImQMD） model the time evolution of the density distribution of ^238U＋^238U ^232Th＋^250Cf at different incident energies are investigated, from which the lifetimes of the GCS at different energies are obtained. The lifetime distribution of the GCS, the average lifetime of the formed GCS and its dependence on incident energies are investigated. The longest average lifetime of the GCS is about 1200fm/c when the incident energy is suitably chosen. The shapes of giant composite systems are found to be largely deformed. The orientation of the deformed composite system changes from 0（180） to 90（270）, when energy increasing from 680 to 1880 MeV. The anisotropy in the momentum space of the GCS as function of incident energies is investigated and it is found that the longest lifetime of the GCS exists with isotropie momenturn distribution at the special incident energy, and the probability of spontaneous positron emission is in this case. largest