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Published January 1, 1976 | public
Journal Article Open

Microscopic calculation of nuclear dissipation


By use of a time-dependent wave function of the BCS form, we compute microscopically the energy dissipated for a system with a monopole pairing force moving under the influence of a time-dependent single-particle potential. Quasiparticle generation and coupling of the two-quasiparticle modes of the system are included automatically and provide contact with the Landau-Zener formula. The single-particle potential is related to nuclear shapes generated by viscous hydrodynamical calculations of a fissioning 236U nucleus. We attempt to determine the energy dissipated between the saddle point and scission point by requiring that at the scission point the energy dissipated in the microscopic calculations equal that dissipated in the macroscopic hydrodynamical calculations. This procedure leads to 34 MeV of dissipated energy, which is almost twice the value of 18 MeV obtained from macroscopic hydrodynamical calculations that reproduce experimental fission-fragment kinetic energies. The corresponding value of the nuclear viscosity coefficient determined from the microscopic calculations is 0.04 TP, compared to 0.015 ± 0.005 TP obtained from the macroscopic hydrodynamical calculations. The viscosity coefficient determined from the microscopic calculations is even larger if the dissipated energies are compared at a finite scission neck radius. As a possible resolution of this discrepancy, we propose that level splittings arising from axially asymmetric and reflection-asymmetric deformations during the descent from the saddle point to scission reduce the energy dissipation and make the nuclei only moderately viscous.

Additional Information

©1976 The American Physical Society. Received 2 September 1975. This work was supported by the U.S. Energy Research and Development Administration under Contract Nos. AT(11-1)/3069 and W-7405-ENG-36. We are grateful to R. Babinet, F. Beck, G.F. Bertsch, A. Bohr, Z. Fraenkel, J.J. Griffith, A.K. Kerman, S.J. Krieger, U. Mosel, W.D. Myers, J.W. Negele, S.G. Nilsson, A.J. Sierk, W.K. Swiatecki, and L. Wilets for stimulating discussions concerning this work, to T.A. Tombrello for suggesting the scaling procedure for the single-particle energies, and to K.T.R. Davis, for providing us with the hydrodynamical paths. In addition, S.E. Koonin thanks the Los Alamos Scientific Laboratory for its support and hospitality, and J.R. Nix thanks the Aspen Center for Physics for its hospitality during the completion of this manuscript.


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