激发能相关的能级密度参数和重核衰变性质课件.ppt

1、 激发能相关的能级密度参数和重核衰变性质激发能相关的能级密度参数和重核衰变性质 叶 巍 （东南大学物理系 南京）内容 问题背景 理论模型 计算结果和结论 能级密度的重要性 controlling the statistical decay of excited nuclei an crucial input for nucleosynthesis calculations (r-process),reactor science,etc.Fermi-gas level-density expression3/22*FGrot2exp 2 aUa(E,J)2J1,UEE(J)2I12Ua leve

2、l-density parameter It is employed in most statistical-model calculationsThe level density parameter a is parametrized as:d()1)W/Ua Ua（1-exp(-U/EdU:W:a smoothed level-density parameter thermal energy shell correction to the liquid-drop mass 18-20 MeVE1.Evaporation process:evaporation spectraa R.J.Ch

3、arity,PRC82,014610(2010),and many other works To fit energy spectra of evaporated particles,is large at low E*and small at higher E*,suggesting that must be dependent on E*aa2.Fission process:cross sections,particle yieldsCritical factors that strongly influence the decay mechanism of heavy nuclei a

4、t high energyinclude:A,E*,J,a(U)af/an,etc.Experimental observation of enhanced emission of light particles prior to fission(with respect to predictions from standard statistical models)with increasing excitation energy in fusion-fission reactions.This is due to dissipation effects.Theoretical Model

5、dq1dS(q,E*)T(t)dtMdqMThe Langevin equation reads q is the dimensionless fission coordinate and is definedas half of the distance between the center of mass of the future fission fragments divided by the radius of the compound nucleus.T is temperature,M is inertia parameter and is friction strength(t

6、)is a time-dependent stochastic variable which satisfies =0 and =2(t-t)The driving force of the Langevin equation is calculated from the entropy:E*is the total internal energy of the system,V(q)is potential energy.deformation-dependent level density parameter a(q)=a1A+a2 A2/3Bs(q)where Bs(q)is the d

7、imensionless surface area(for a sphereBs=1).It is used to calculate af/an.s(q,E*)2 a(q)E*V(q)Evaporation residue cross section ER Previous works on the role of the parameter af/an in the decay modes of thermal nucleiB.Lott,et al.PRC 01,adjusting af/an to fit residue cross section data based on a sta

8、tistical model af/an changes with fissilityW.Ye,PRC81(2010)011603(R)relativistic heavy-ion collisions vs.fusion reactions CN:(high E*,low L)vs.(low E*,high L)W.Ye,PRC83(2011)044611 spin distribution of evaporation residue cross sections ER(L)W.Ye,NPA853(2011)61 prescission particle yields role of sp

9、in:af/an(L)Reactions systems 16O+181Ta 197Tl vs.3,4He+197Au 200,201TlScaling analysis of fission probability of systems 200Tl(right figure)and 201Tl(left figure)based on the standard statistical modelThese figures are taken from L.G.Moretto et al.,PRL75,4186(1995)and Th.Rubehn et al.,PRC54,3062(1996

10、)16O+181Ta 197Tl W.Ye,PRC84(2011)034617Recent work on excitation-energy dependent af/an(E*)and its effects on the decay of hot nuclei suggested probes:excitation energy at scissionE*=E*sc+V(q)+Ecoll+Eevap(tsc)Ecoll is the kinetic energy of the collective degreesof freedom,and Eevap(t)is the energy c

11、arried away by all evaporated particles by the scission time tscpicture of fission process Choose spallation reactions induced by high energy protons Models:QMD+SM,L.Ou,Z.X.Li,X.Z.Wu,etc.BUU+SM,G.C.Yong,W.Zuo INCL+SM,Belgium main characteristics:the thermal excitation energy of the produced excited

12、nuclei in spallation can reach 1 GeV significantly reduce side effects from compression,deformation and high spins.These distortions complicate the description of de-excitation process of excited nuclear systemsW.Ye,PRC85(2012)011601(R)The sensitivity of E*sc to nuclear friction depends on the af/an

13、(E*).Experimentally,to probe information of af/an(E*),populating heavy systems with spallation reactions can significantly lower side effects associated with angular momentum,deformation,etc.Applications to spallation-induced reactions and the decay of superheavy nuclei.Conclusions Thanks for your attentionsW.Ye,High Energy Phys.Nucl.Phys.26(2000)52转动自由度