Gamma-ray multiplicity measurements for the determination of the initial angular momentum ranges in normal and fast fission processes

Abstract

Gamma-ray multiplicities (first and second moments) have been measured, in the 220 MeV 20Ne + natRe and 315 MeV 40Ar+ 165Ho reactions, as a function of fission fragment masses and centre-of-mass total kinetic energies. The two reactions lead to the same fusion nucleus, 205At, at the same excitation energy (167 MeV). The experimental critical angular momentum for the fission process in the Ne + Re system (91 ± 3) ℏ is close to l Bf=0 (~80ℏ) while in the Ar + Ho reaction this critical angular momentum (136 ± 4) ℏ is much larger than the l Bf=0 value, favoring the occurrence of the fast fission process. The observed widths of the fission fragment mass distribution: (42 ± 2) u in the Ne + Re system and (56 ± 4) u in the Ar + Ho reaction strengthen this hypothesis. For both compound nucleus fission and fast fission components in Ar + Ho, the total spin values obtained in absolute magnitude and in their dependence on the mass asymmetry are well described by assuming rigid rotation of the fissioning complex and statistical excitation of some collective rotational modes such as “Bending” and “Wriggling” according to the Schmitt-Pacheco model. These modes, however, are not all fully excited, their degrees of excitation are approximately the same for both fission components. From theoretical estimates of equilibration times, one anticipates the “Tilting” mode to be by far the last to be excited, and from its non-excitation in the present data together with the excitation of bending and wriggling, a time interval of about 10 −21 s to 2 × 10 −20 s can be derived for the reaction time of both normal fission and fast fission. The γ-ray multiplicity as a function of the c.m. total kinetic energy decreases in the Ne + Re system, while it increases in the Ar + Ho system even for symmetric splitting, which indicates experimentally for the first time that fast fission populates the whole mass range. This difference between the two reactions is in agreement with the normal-fission-fast-fission distinction in angular momentum space.