角动量和激发能相关的能级密度参数对热核退激机制影响的研究

Decay modes of hot nuclei are a hot topic in the research of modern nuclear physics，because it is extremely important for understanding experimental data provided in fusion, intermediate energy heavy-ion collisions and spallation reactions induced by medium and high energy protons. Moreover, it is of improtant value for many practical application fields such as the design of spallation neutron sources, nuclear data evaluation and space radiation protection. Current knowledge of nuclear decay modes is chiefly derived by comparing predictions from standard statistical models with measured data of particle emission and evaporation residue cross sections. In the model calculations the level-density paratmeter and the ratio of its value at saddle to that at ground state (af/an) are key input quantities. The present project studies the influence of the dependence of af/an on angular momentum L [i.e. af/an(L)] on the competition between different decay channels. We suggest a new approach based on a coupling of the intranuclear cascade model (INCL4) to stochastic model to describe proton-nucleus reactions and apply it to survey the effects caused by the change of af/an with excitation energy E* [i.e. af/an(E*)] on decay mechanisms. Furthermore, in our project the suggested (INCL4 + Langevin) approach will be applied to p + Fe, W, Pb, Bi, etc. spallation reactions that are encountered in the design of spallation neutron sources, nuclear data evaluation and space radiation protection. We will calculate A and Z distributions of fragments，double differential cross sections of neutrons, protons,etc. for these reactions. The obtained data could be useful for those scientists working in these practical fields.

热核的退激模式不仅是当今核物理基础研究领域中的热点问题，因为它对理解熔合反应、中能重离子碰撞以及中、高能质子诱导的散裂反应的实验数据极其重要，而且它在散裂靶设计、核数据评价、太空辐射防护等实际应用领域也具有重要的价值。目前关于热核衰变机制的知识主要是通过统计模型与轻粒子发射和蒸发剩余截面等观测量的比较获得的。能级密度参数及其在鞍点与基态平衡处的比值af/an是进行这类衰变模式计算的关键输入量。本申请基于随机模型系统研究af/an对角动量L的依赖性,即af/an(L)在热核不同衰变道之间竞争中的重要作用；发展一条基于(INCL4+Langevin)研究质子-核散裂反应的新途径并研究af/an随激发能E*的变化对热核衰变机制的影响；应用发展的新途径对实际应用领域关心的p+Fe、W、Pb、Bi等散裂反应进行计算，提供所需的碎片质量和电荷分布、中子发射双微分截面等量的数据，供相关领域参考使用。

核-核碰撞是研究原子核的基本实验途径。通过测量碰撞产生的热核衰变产物是获取原子核性质的基本方法。因此，热核的退激机制是当今核物理基础研究领域中的一个关键问题。该项目基于随机途径对热核衰变过程中的耗散机制进行了研究。这条途径完全考虑了能级密度参数随激发能和角动量的变化。我们发现（1）高能条件下断点激发能的测量能显著增强其对大形变处耗散强度的敏感性；（2）实验上测量高同位旋和低自旋复合核的裂变截面有利于精确确定鞍点前摩擦。通过比较理论计算和质子诱导的裂变激发函数实验数据，提取的鞍点前摩擦强度为（3-5）zs^-1；（3）平衡前发射显著影响了通过重裂变系统蒸发的中子多重性所提取的鞍点后摩擦强度；（4）轻系统的裂变激发函数对鞍点前摩擦有更大的敏感性；（5）时间相关的Hartree-Fock方法和统计衰变模型的结合能够很好地描述深度非弹性碰撞的产物特性；（6）重裂变系统首次裂变几率的测量能对鞍点前摩擦强度的精确确定提供一个敏感的和适当的实验途径；（7）测量由中能重离子反应产生的重裂变系统发射的粒子多重性能更强地约束鞍点后的耗散性质；（8）鞍点附近的回流效应对实验上通过粒子多重性来准确探测鞍点后摩擦强度有明显影响。因此，目前被广泛用来分析裂变数据的统计模型仍然需要进一步改进。