Conditions for the manifestation of transient effects in fission

Abstract

Abstract:We examine the manifestation of transient effects in fission by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Available total nuclear fission cross sections of 238 U at 1⋅A GeV on gold and uranium targets are compared with a nuclear-reaction code, where transient effects in fission are modelled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker-Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when transient effects are considered. The deduced value of the dissipation strength β depends strongly on the approximation applied for the time-dependent fission-decay width and is estimated to be of the order of 2·10 21 s -1. A careful analysis sheds severe doubts on the use of the exponential-like in-growth function largely used in the past. Finally, we discuss which should be the characteristics of experimental observables to be most sensitive to transient effects in fission.

New experimental approaches to investigate the fission dynamics

“…The fission probability is computed according to a time-dependent fission width following the analytical description of the solutions of the corresponding Fokker-Plank equation proposed in Refs. [24,11,25]. The code also allows to evaluate the fission width according to the transition-state model of Bohr and Wheeler [1] or the time-independent formulation of Kramers [2].…”

“…The boosting of the fission fragments induced by the inverse kinematics made it possible to fully identify in atomic, and mass number one of the final fission fragments using a high resolving power magnetic spectrometer [9], or identify in atomic number both fragments using large acceptance ionization chambers [10]. Moreover, fragmentation or spallation reactions produce fissioning systems with high excitation energy, moderate angular momentum, and small shape distortion, fulfilling the optimal conditions for the investigation of dissipative effects in fission [6,11].…”

Dynamical effects in fission investigated at high excitation energy

“…Moreover, spallation and fragmentation reactions introduce small shape distortions and low angular momenta. Therefore, these reactions are considered as an optimum tool for the investigation of the ground-to-saddle dynamics [30,31].…”

“…However, those works provided rather unclear conclusions concerning the absolute value of the dissipation parameter and its possible dependences on temperature and deformation. Some reasons could be the strong influence of angular momentum and deformation in the fission probability, and the limited values of excitation energies reached in these reactions [30,31,39].…”

“…An interesting alternative to fusion-fission is the use of spallation reactions induced by energetic protons [40,41] or anti-protons [42], and fragmentation reactions [30,43]. These reactions are expected to fulfill the conditions for the manifestation of dissipative effects in fission [6] because they can produced fissioning nuclei with large values of excitation energy but low angular momenta.…”