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 expression It is employed in most statistical-model calculations

The level density parameter a is parametrized as:

1. Evaporation process: evaporation spectra 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*

2. Fission process: cross sections, particle yields Critical factors that strongly influence the decay mechanism of heavy nuclei at high energy include: A, E*, J, a(U) [a f /a n ],, 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

The Langevin equation reads q is the dimensionless fission coordinate and is defined as 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) is a time-dependent stochastic variable which satisfies =0 and = 2δ(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) = a 1 A + a 2 A 2/3 B s (q) where B s (q) is the dimensionless surface area (for a sphere B s = 1). It is used to calculate a f /a n.

● Evaporation residue cross section  ER Previous works on the role of the parameter a f /a n in the decay modes of thermal nuclei B.Lott, et al. PRC 01, adjusting af /an to fit residue cross section data based on a statistical model

a f /a n changes with fissility W.Ye, PRC81 (2010) (R)

relativistic heavy-ion collisions vs. fusion reactions CN: (high E*,low L) vs. (low E*,high L) W.Ye, PRC83 (2011)

● spin distribution of evaporation residue cross sections  ER (L)

W.Ye, NPA853 (2011) 61

● prescission particle yields

role of spin: a f /a n (L) Reactions systems 16 O+ 181 Ta  197 Tl vs. 3,4 He+ 197 Au  200,201 Tl

Scaling analysis of fission probability of systems 200 Tl (right figure) and 201 Tl (left figure) based on the standard statistical model These figures are taken from L.G.Moretto et al., PRL75, 4186 (1995) and Th. Rubehn et al., PRC54, 3062 (1996)

16 O+ 181 Ta  197 Tl W.Ye, PRC84 (2011)

Recent work on excitation-energy dependent a f /a n (E*) and its effects on the decay of hot nuclei suggested probes: excitation energy at scission E* = E* sc + V(q) + E coll + E evap (t sc ) E coll is the kinetic energy of the collective degrees of freedom, and E evap (t) is the energy carried away by all evaporated particles by the scission time t sc

picture 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 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 systems

W.Ye, PRC85 (2012) (R)

● The sensitivity of E* sc to nuclear friction depends on the a f /a n (E*). ● Experimentally, to probe information of a f /a n (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 attentions

W.Ye, High Energy Phys. Nucl. Phys. 26 (2000) 52 转动自由度