1、 激發能相關的能級密度參數和重核衰變性質激發能相關的能級密度參數和重核衰變性質 葉 巍 （東南大學物理系 南京 ） 內容 問題背景 理論模型 計算結果和結論 能級密度的重要性 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)2I12

2、Ua level-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 s

3、pectraa R.J.Charity, 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 mech

4、anism of heavy nuclei at 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 di

5、ssipation effects. Theoretical Model 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 inert

6、ia parameter and is friction strength (t) 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 param

7、eter a(q) = a1A + a2 A2/3Bs(q)where Bs(q) is the dimensionless 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, adju

8、sting af /an to fit residue cross section data based on a statistical 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

9、sections ER(L) W.Ye, NPA853 (2011) 61 prescission particle yields role of spin: 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 ar

10、e taken from L.G.Moretto et al., PRL75, 4186 (1995)and Th. Rubehn et al., PRC54, 3062 (1996)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) + Ecol

11、l + Eevap (tsc)Ecoll is the kinetic energy of the collective degreesof freedom, and Eevap(t) is the energy carried 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, et

12、c. BUU + SM, G.C.Yong, W.Zuo INCL + SM , Belgium main characteristics: the thermal excitation energy of the produced excited 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 (E*). Experimentally, to probe information of af /an (E*), populating heavy systems with spallation reactions can significantly lower side effects associated with ang