Dynamics of Coulomb fission

Abstract

A general formalism is described for the treatment of Coulomb fission, within the framework of the semiquantal theory. We develop a model for the fission probabilities of levels excited in Coulomb excitation. This model contains penetration of the double-humped fission barrier, competition from gamma and neutron emission, and the spreading of the collective states into noncollective compound states. For $_{74}^{184}\mathrm{W}$ + $_{92}^{238}\mathrm{U}$, the fission probability at ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}=180\ifmmode^\circ\else\textdegree\fi{}$ is increased by a factor of 3.9, 3.3, and 2.0 at $\frac{E}{{E}_{\mathrm{Coul}}}=0.77, 0.85, \mathrm{and} 0.935$, respectively, compared to the simplified sharp cutoff model used in earlier model calculations. The enhancement comes from barrier penetration. The damping of the fission probability due to spreading into noncollective compound states is small. Prompt Coulomb fission (near the distance of closest approach) is studied in a one-dimensional model. The results clearly imply that prompt fission is negligible. We have also studied the sudden approximation for collective rotational levels in connection with Coulomb fission. At high spins ($I\ensuremath{\approx}20$), it leads to significant errors. Contrary to the basic assumption of the sudden approximation that the nuclear symmetry axis remains fixed during the collision, it is shown that Coulomb excitation results in a strong alignment of the nuclear symmetry axis perpendicular to the beam axis at small internuclear distances.