Fission of targetlike fragments populated in the multinucleon transfer reactions of 340 MeV ²⁸Si on ²³²Th

Abstract

The transfer-induced fission channel has been studied in the collision of 340 MeV ²⁸Si on ²³²Th as a function of the atomic number of the projectilelike fragments (PLF's) by using a 4π detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer ΔZ from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with Z_TLF=90-96 (ΔZ=0-6) have average excitation energies up to about 100 MeV and angular momenta up to about 40h. The measured ratio of transfer-fission yield to PLF singles, Y_f, first increases with increasing net charge transfer up to ΔZ=4 and then shows a plateau around the values Y_f=0.4-0.6 followed by a decrease for higher ΔZ transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to ΔZ<~6, where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests 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 deexcitation, as also indicated by the large fission times (t_f=10-100 zs) derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.