Forces between heavy nuclei in the independent-particle model

We use the linear response theory to study the force on a heavy nucleus moving near the symmetry axis of ²³⁸U, in an independent-particle model. We present the first detailed microscopic computation showing relaxation of the single-particle motion due to incoherence, and find a relaxation time of about 0.3 MeV ≈ 2 × 10^-22 sec. We consider three stages of the reaction. While the nuclei approach each other, the induced force is very weak, and is computed as a function of the position, velocity and acceleration of the other nucleus; it involves excitons of 10 to 20 MeV. The computed force agrees well with phenomenological values. The second stage of the reaction is the formation of a neck, which we argue will begin already when the nuclear half-density surfaces are more than 4 fm from each other. For slow collective motion, thought to occur after the neck forms, we show how to represent the incoherent excitation of low-frequency excitons by a friction force, and present a method suitable for numerical computations of the friction coefficient.