1、沈彩万,重离子反应中质量碎片分布的计算,2011.9.28,Y. Abe, D. Boilley, E.G. Zhao,For Tea Time Activity,Outlook,Introduction Two-step model in the fusion stage Fragment mass distribution Conclusion,Commonly used model:,Compound Nucleus Theory,Sketch map of the process,Theories to describe the fusion state,Fluctuation-Dis
2、sipation theory (Abe,Shen,Bao,)DNS (di-nuclear system) model (Schide, Li, Zhou, )imQMD model,Parameters for the description of formation,A1,A2,q1 = R/R0 q2 = ap1 = pR/R0 p2 = pa,R,a: asymmetric parameter，R0: spherical radius of the compound nucleus,7/38,Average value of the neck parameter,Criteria f
3、or fusion hindrancein radial evolution,If system evolves to spherical case: without fusion hindrance.,If system evolves to two fragments: with fusion hindrance.,(F.H),(no F.H.),对称反应中液滴能随 z 的变化,Fusion hindrance area: (radial evolution),Two-step Model,(1) Coulomb barrier; (2) Liquid drop barrier,48Ca+
4、238URCB = 14.14fmRC = 11.86fmRLB = 9.5fm,R,V,CoulombEnergy,Liquid-dropEnergy,RLB,RC,RCB,including two-consecutive steps overcoming the barriers,Pfusion = Psticking* Pform,Sticking probability:,(1) Surface friction model,(2) Empirical formula by Swiatecki,Swiatecki et al., PRC 71, 014602(2005),Format
5、ion Probability,Formation probability: Pform (Using LD model),R,V,VB,Rc,Contact Point = Rp + Rt,Ec.m.,Coulomb Potential,Liquid Drop Potential,PSticking,Pform,Equation of motion for R and a,Langevin equaiton:,Tracks of motion with random force,Formation and mass-fragment distribution,formation,quasi-
6、fission,initial point with pk,Formation and mass-distribution probability,According to the friction model，the relative momentums are distributed in Gaussian form：,For the fusion of heavy systems, 0,Probability for mass-fragment distribution,在质量分布中，通常以单位质量数的相关量表示：,Mass-distribution probability in the
7、 formation stage,238U + 26Mg,Exp: W.Q. Shen, PRC (1987),238U+16O 238U+26Mg 238U+32S,238U+35Cl 238U+40Ca 238U+65Zn,1、理论计算较好符合实验。2、中心偏移: 理论仅包含了两分裂情况，三分裂等多碎片分裂会分布中心左移。3、两端差异：理论仅含准裂变，实验应含有散射等其他成分。4、仅在复合阶段仅考虑液滴能时无法给出细致结果。,结果分析,进一步改进,1、考虑壳修正影响； 2、自洽考虑颈部自由度；3、要与实验结果更好复合，还需 考虑其它形式引起的质量分布。,小结1、两步模型不但能较好地计算重离子反应的熔合截面，还能给出准裂变的质量碎片分布。2、若要更好地计算准裂变碎片分布，还需要将两步扩展至更多的自由度。,Thanks !,
