Orateur
Description
The synthesis of superheavy nuclei (SHN) in heavy-ion fusion-evaporation reactions is governed by a delicate interplay between two competing effects. On one hand, the capture and fusion cross section rises with the bombarding energy as the projectile overcomes the Coulomb barrier and the dinuclear system evolves toward a compound nucleus. On the other hand, the survival probability of the excited compound nucleus against fission decreases rapidly with increasing excitation energy, since each neutron evaporation step competes with the dominant fission channel. The convolution of these two opposing trends produces sharply peaked excitation functions and pushes the production cross sections for the heaviest elements down to the picobarn level, and approaching the femtobarn regime for the most exotic systems.
This contribution discusses what can be learned about the reaction mechanism from the available experimental data — including excitation functions, evaporation residue cross sections, and quasifission signatures — and how this understanding can be used to constrain theoretical models and guide predictions for as-yet unobserved reactions. Particular emphasis is placed on the synthesis of elements 119 and 120, where the unavailability of suitable actinide targets paired with 48Ca beams forces the use of heavier projectiles. In such asymmetric systems the entrance-channel dynamics and the choice of the optimal excitation energy window become decisive factors for the success of the experimental campaigns currently being prepared at major SHE factories worldwide.