Dynamical Effects in the Region of Heavy and Super-Heavy Nuclei

Abstract

The transfer-induced fission channel has been studied in the collision of 340 MeV 2 8 Si on 2 3 2 Th as a function of the atomic number of the projectile-like fragments (PLF) by using a 4π detector array. The average excitation energy of the target-like fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. The measured ratio of transfer-fission yield to PLF singles, Y f , first increase up to a net charge transfer ΔZ = 4, then shows a plateau around the values Y f = 0.4 - 0.6. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfer AZ <6, suggesting a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of the deexcitation, such effects are important in the population of nuclei in the heavy and superheavy mass region by transfer reactions. Moreover pre- and post-scission multiplicities of neutrons and alpha particles have been simultaneously measured for the fission-like channel of the same reaction. Dynamical model calculations using HICOL code predict that 90% of the observed events are of quasi-fission type while the remaining 10% are from compound nucleus fission decay. From a comparison of PACE2 Statistical Model predictions with the measured pre-scission neutron multiplicity, the fission delay is estimated to be of 5 + 7 - 3 ×10 - 2 0 s, which overlaps with the average duration of fission-like process from the contact to the scission point (2×10 - 2 0 s) as determined from HICOL-based dynamical calculations.