Tuesday, December 8, 2009

Friction in nuclear collisions

Heavy nuclei are complex quantum many-body systems with many degrees of freedom. The observation of deep inelastic collisions (DHIC) in the 1970's led to the notion of friction in nuclear physics. This concept can be used to describe the transfer of energy from the relative motion of the nuclei to the internal degrees of freedom.

I am eager to quantify this friction because it will also enable us to say something about the role of decoherence in nuclear physics.
Today at ANU, Cedric Simenel brought to my attention a recent paper that is relevant.
For several nuclear collisions the position dependent friction was recently evaluated from the
Dissipative Dynamics version of TDHF (Time-dependent Hartee-Fock) theory.

It was found the friction monotonically increases with decreasing inter-nuclear separation, with a value of about 10^{21} s^{-1} at the barrier. For the 40Ca + 40Ca reaction at a centre of mass energy of 100 MeV, about 10 MeV of energy is converted to internal excitation energy as the nuclei move beyond the barrier.
These numbers show that decoherence/friction may be important in tunneling because the dissipation rate is comparable to the barrier frequency.


  1. Dissipative Dynamics TDHF sounds like what R. Balian was on about (and Zwanzinger, and Prigogine). Same stuff?

  2. The dissipation transforms kinetic energy (of the relative motion) to excitation energy of the fragments. For completeness, we should also mention the recent paper of Umar et al., Phys. Rev. C 80, 041601 (2009). They compute the excitationenergy during the collision. They use the Density-Constrained TDHF technique.
    Their DC-TDHF and the DD-TDHF of Washiyama et al. are different techniques to compute the excitation energy (and the dissipatin) but they both assume a TDHF evolution of the fragments,while th work of Balian et al. mostly deals with extensions of TDHF.