Induced nuclear fission viewed as a diffusion process: Transients

Dynamical effects in fission investigated at high excitation energy

“…Most of the measurements are compatible with a value of the transient time around 1-5 10 −21 s. Those values are negligible when compared to typical statistical times for low-energy fission, therefore reactions inducing fission at high-excitation energies are better suited [17]. Indeed, Grangé and collaborators [6] established that the best conditions for the manifestation of transient effects are the use of reactions leading to fissioning systems not only with large values of excitation energy but also with low values of angular momentum and spherical shapes.…”

“…This delay of the fission time is understood as the transient time needed for the coupling between intrinsic and collective degrees of freedom until a stationary flow of the fission width over the barrier is reached. That time can be quantified in terms of a nuclear viscosity parameter in transport equations [6]. Most of the measurements are compatible with a value of the transient time around 1-5 10 −21 s. Those values are negligible when compared to typical statistical times for low-energy fission, therefore reactions inducing fission at high-excitation energies are better suited [17].…”

“…The first attempt to describe the dynamics of fission was put forward by Kramers [5] assuming that the evolution of the deformation degree of freedom can be viewed as a Brownian motion of this collective degree of freedom with a heat bath defined by the single-particle degrees of freedom. The timedependent formulation of this model using a Fokker-Planck equation with a dissipative term describing the coupling between the collective and singleparticle degrees of freedom was introduced by Grangé and collaborators [6]. An analogue formulation is the one based on the Langevin equation [2] or as a random walk, in the limit of strong dissipative coupling [7], on a multidimensional macroscopic-microscopic potential energy surface [8].…”

New experimental approaches to investigate the fission dynamics

“…A dissipation parameter determines how fast the fission coordinate reaches the saddle point, the so called transient time, and the saddle-toscission time. A time-dependent description of this dissipative process was provided by Grangé and Weidenmüller in the 80's by using transport calculations based on the Langevin equation [6] Those ideas were confirmed in the 90's by measuring multiplicities of neutrons [3], light-charged particles [4], and gamma rays [5] emitted until the scission point. It was found that those multiplicities were much larger than the ones predicted by the statistical model.…”

“…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].…”