Memory effects on descent from the nuclear fission barrier - art. no. 054302

Non-Markovian transport equations for nuclear large amplitude motion are de rived from the collisional kinetic equation. The memory effects are caused by Fermi surface distortions and depend on the relaxation time. It is shown that nuclear collective motion and nuclear fission are influenced strongly by memory effects at the relaxation time tau greater than or equal to 5 x 10(-23) s. In particular, the descent of the nucleus from the fission barri er is accompanied by characteristic shape oscillations. The eigenfrequency and the damping of the shape oscillations depend an the contribution of the memory integral in the equations of motion. The shape oscillations disappe ar at the short relaxation time regime at tau-->0, which corresponds to the usual Markovian motion in the presence of friction forces. We show that th e elastic forces produced by the memory integral lead to a significant dela y for the descent of the nucleus from the barrier. Numerical calculations f or the nucleus U-236 show that due to the memory effect the saddle-to-sciss ion time grows by a factor of about 3 with respect to the corresponding sad dle-to-scission time obtained in liquid drop model calculations with fricti on forces.