Study Of Dissipative Dynamics In Fission Based On Langevin Equations

Dissipation is the fundamental feature of large-scale nuclear collective motions.It thus plays a crucial role in the entrance channel dynamics of low-and intermediate-energy nuclear reactions as well as in decay mechanisms of the produced nuclear systems.Dissipation proper-ties are not only important to understand deep-inelastic scattering,fusion and the synthesis of superheavy elements,but also the key to quantitatively explaining the recently measured fis-sion data of hot nuclei.In particular,the strengths of pre-and post-saddle dissipation are the current focus of investigations in the field of nuclear physics.The thesis is devoted to studying dissipative characteristics of excited nuclei in fission processes in the developed framework of the stochastic Langevin model that is coupled with light particle emissions,and the main results obtained are as follows:1.First-chance fission probability(P_{f 0})of heavy nuclei is proposed as a sensitive observ-able of the presaddle friction strength（β）.By calculating P_{f 0}of²⁴⁰Cf at differentβ,we find that P_{f 0}depends sensitively onβ,and the sensitivity is significantly larger than that of the total fission probability.In order to further guide the experimental exploration,we give typical reaction systems that can be carried out under conditions of current experiments.2.Light particle multiplicities of heavy fissioning systems produced in intermediate-energyheavy-ion collisions are shown to put a more strict constraint on the friction strength at large deformations.Langevin calculations reveal that the sensitivity of postsaddle neu-tron and light charged particle multiplicities of heavy²⁴⁰Am to the postsaddle friction strength is significantly enhanced at a high excitation energy.Furthermore,the calculations show that,compared with fusion mechanisms,the typical excitation energy and an-gular momentum conditions provided via intermediate-energy nuclear reactions are more preferable in pinning down the postsaddle friction with light particle emissions.3.Backstreaming effects（BEs）around the saddle point are shown to be important for ac- curately probing the postsaddle friction strength.The apparent deficiencies in analyzing fission data based on the approach of revised statistical models are pointed out.The cal-culated results for²⁴⁵Cf system show that the BEs increase presaddle particle multiplicity,but decrease postsaddle emission and that the effects become greater with increasing ex-citation energy.Furthermore,by fitting the prescission neutron multiplicity measured in the reaction¹⁶O(E_lab=104 Me V)+²³²Th-→²⁴⁸Cf,we find that,due to the BEs,the postsaddle friction strength required to satisfactorily describe the experimental data changes from 9.6×10²¹s^-1to 6.0×10²¹s^-1.It is clearly demonstrates that,to accu-rately extract the postsaddle friction strength,the BEs in fission process should be fully considered in theoretical calculations.4.Proposing to measure fission probabilities of heavy fissioning systems populated by ra-dioactive nuclear beams as a new experimental avenue to investigate the nuclear dissipa-tion properties.Langevin calculations for²²⁶U,²³⁴U and²⁴²U illustrate that the sensitivity of fission cross sections to the presaddle friction strength is substantially enhanced with increasing the isospin of the U system.Moreover,compared with the light ²⁰⁰Tl system,the fission cross section of the high-isospin heavy²⁴⁰U nucleus is found to exhibit a larger sensitivity to the friction strength.The traditional heavy-ion fusion reaction mechanisms based on stable beams can only produce neutron deficient（low-isospin）heavy compound system.Thus,our results suggest that in experiments,to more tightly limit the presad-dle friction strength with fission cross sections,it is optimal to yield those high-isospin heavy fissioning systems with the high-isospin projectile nucleus provided by radioactive nuclear beams.