E. Fuschini

Investigation of shell effects for heavy fissionable nuclei by the blocking technique

Abstract The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the 28 Si + nat Pt reaction. Experimental results range from 10 −17 to 10 −18 s at bombarding energies from 140 to 170 MeV. It is shown that fission barriers keep their double-humped structure for temperature up to 1.7–1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to the observed delay times in the fission channel.

Axial blocking in a master equation approach

Abstract Axial blocking in crystals is studied by means of a proper extension of a method based on the master equation tor the transverse energy distribution and already developed for dechanneling. In contrast to diffusion models, it gives a consistent account of both multiple and single scattering processes. 〈110〉 blocking of ∼ 2 MeV α-particles in A1 for a depth of 0.2 μm is considered. The transition probability per unit depth between transverse energy groups is calculated by high statistics Monte Carlo simulation of the ion-atom-scattering. The master equation is solved by means of the eigenvalue method for the corresponding dechanneling matrix. A satisfactory agreement with the result of a full Monte Carlo simulation is found. The interplay of the assumption of statistical equilibrium, for such a small depth, and of the choice of the string potential is discussed. Exploiting Monte Carlo calculations at smaller depths the effects of deviations from statistical equilibrium are found to be of only slight importance provided that the thermally averaged potential is used.

Axial blocking dips from the master equation for transverse energy distribution

Abstract The basic features of a recently developed method for the study of axial blocking are outlined. For small depths the master equation is solved by the eigenvalue method for a depth-independent “dechanneling matrix” Q in a restricted transverse energy range. This is shown to be possible also in presence of energy loss effects, through a scaling transformation allowing for a factorizable energy dependence of Q and the inclusion of a source term describing the shift towards lower transverse energies due to energy loss. The results for 〈110〉 blocking of 2 MeV α-particles in Al for depths from 0.2 μm to 1 μm are in generally good agreement with those of a Monte Carlo simulation. The effects of deviations from statistical equilibrium and of the choice of the string potential are studied. The dependence of the results for large depths on electronic diffusion is shown.