Presaddle and postsaddle dissipative effects in fission using complete kinematics measurements

A complete kinematics measurement of the two fission fragments was used for the first time to investigate fission dynamics at small and large deformations. Fissioning systems with high excitation energies, compact shapes, and low angular momenta were produced in inverse kinematics by using spallation reactions of lead projectiles. A new generation experimental setup allowed for the first full and unambiguous identification in mass and atomic number of both fission fragments. This measurement permitted us to accurately determine fission cross sections, the charge distribution, and the neutron excess of the fission fragments as a function of the atomic number of the fissioning system. These data are compared with different model calculations to extract information on the value of the dissipation parameter at small and large deformations. The present results do not show any sizable dependence of the nuclear dissipation parameter on temperature or deformation.

Figure 1

Top schematic view of the experimental setup. Sizes are not to scale.

Figure 2

Fission cross sections as a function of the atomic number of the fissioning nuclei. The lines represent incl4.6+abla07 calculations for different values of the reduced dissipation parameter ${\beta }_{gs}$ in units of ${10}^{21}{\mathrm{s}}^{-1}$.

Figure 3

Prescission neutron multiplicities (a) and average neutron excess of the final fission fragments (b) as a function of the atomic number of the fissioning nuclei. The lines represent model calculations for different values of the dissipation parameters in units of ${10}^{21}{\mathrm{s}}^{-1}$.