Nuclear Transport at Low Excitations

Numerical computations of transport coefficients at low temperatures are presented for shapes typically encountered in nuclear fission. The influence of quantum effects of the nucleonic degrees of freedom is examined, with pair correlations included. Consequences for global collective motion are studied for the case of the decay rate. The range of temperatures is specified above which this motion may be described as a quantal diffusion process. PACS numbers: 05.60.Gg, 24.10.Pa, 24.75.+i, 25.70.Ji Phys. Rev. Lett. 82 (1999) 4603 In the past decade much progress has been made in the understanding of nuclear transport phenomena in the regime of not too low temperatures, say between 1 and 5 MeV (with kB = 1). Such a situation is reached if two heavy ions collide at an energy above the Coulomb barrier, but where the excess energy per particle still is small compared to the Fermi energy. In this regime the dynamics of the composite system may be parameterized in terms of shape variables. Of particular interest is the outgoing channel which is dominated by fission and the emission of light particles and γ’s. It has been possible experimentally to deduce valid information on the time scale of collective motion [1], and, hence, on the size of nuclear dissipation. These experiments suggest collective motion to be over-damped, possibly providing an answer to the question raised by Kramers as early as 1940 in his seminal paper [2], namely whether nuclear friction is ”abnormally small or abnormally large”. Nowadays such processes are described theoretically in terms of the Langevin equation [3], which is understood to be equivalent to Kramers’ original equation (of Fokker Planck type) for the density in collective phase space. e-mail:[email protected] e-mail: [email protected]