Binary and ternary quasifission induced by shell effects at near barrier energy in heavy nuclear systems

Abstract

The study of the true ternary fission (simultaneous decay in three fragments of similar mass) with formation of a heavy third fragment due to strong shell effects has received new interest among nuclear physicists. It is a very rare process which has been studied both theoretically and experimentally since several decades, but still it is quite far from proper understanding. This PhD thesis concerns with the experimental observation of true ternary decay in three reactions. Specifically, ^{37}Cl\,+\,^{208}Pb at 195 MeV, ^{40}Ar\,+\,^{205}Tl at 193 MeV and ^{40}Ar\,+\,^{208}Pb at 193 and 230 MeV reactions were studied at the K130 Cyclotron accelerator of the University of Jyv\ddot{\mbox {a}}skyl\ddot{\mbox {a}}, Finland. The first two reactions populating the same compound nucleus were investigated for possible entrance channel effects, whereas for the last system, ^{40}Ar\,+\,^{208}Pb, two different bombarding energies have been provided to investigate the effect of excitation energy of the compound system. Four time-of-flight (TOF) arms coupled with six $\Delta E-E$ telescopes of silicon detectors and a beam monitoring system constitute the complete experimental setup. Two dimensional velocity coincidence maps were built from TOF arms. The presence of cluster patterns in the two-velocity maps for all the reactions is the main observation of this work. Another outcome of the present investigation is the consistent overlap between the tripartitions that reproduce the data in the companion reactions ^{37}Cl\,+\,^{208}Pb and ^{40}Ar\,+\,^{205}Tl, and in the reactions ^{40}Ar\,+\,^{208}Pb at 193 and 230 MeV. These two findings give support to the occurrence of the simultaneous three-body decay. The comparison between the rate for binary and ternary decays has also been achieved by calculating the ratio of such events corrected for the efficiency. These ratios are quite striking because in spontaneous fission the simultaneous ternary decay is several orders of magnitude less abundant. In the case of ^{37}Cl\,+\,^{208}Pb and ^{40}Ar\,+\,^{205}Tl, true ternary fission is even from 2 to 4 times more abundant than binary fission