Angular distributions in quasi-fission reactions: Evidence for incomplete relaxation of the tilting mode

Abstract

Angular distributions of fission-like fragments have been measured for⁵⁰Ti⁺²⁰⁸Pb and⁵⁶Fe⁺²⁰⁸Pb collisions. Z-dependent asymmetries around Θincm= 90° preclude their interpretation in terms of compound nucleus fission with the transition state theory. Fits of the data with a simple ansatz for statistical angular momentum fluctuations (tilting) give evidence for an incomplete relaxation of the tilting mode in quasi fission reactions.

Recently, there has been much focus on “fusion-fission” angular distributions in heavy systems with unusually large “anisotropies” /1–5/ if compared to expectations based on the transition state theory (TST) and on rotating liquid drop model (RLDM) saddle point shapes. The validity of these expectations for very heavy systems is being questioned /6/ and modifications of the TST /7,8/ are being vividly discussed /9,10/. Deviations from the RLDM expectations are being associated with the onset of quasi-fission reactions /1,3,5,11,12/ occuring whenever complete fusion is suppressed by dynamical entrance channel limitations /13/.

We have measured differential cross sections d² σ/dωdZ as a function of scattering angle Θ_cm for quasi-fission fragments from⁵⁰Ti +²⁰⁸Pb collisions at 5.0 and 5.5 MeV/A and⁵⁶Fe +²⁰⁸Pb collisions at 5.7, 6.1, 6.8, 8.3 MeV/A. The measurements involved an off-line X-ray activation technique /14/ where up to 15 catcher foils representing slices of lab. angles of 5° were measured simultaneously by 15 absolutely calibrated Ge low-energy photon spectrometers. Transformation of the cross sections into the centre of mass system was performed by assuming full momentum transfer and by using the experimental total kinetic energies of Ref./11/. Examples for distributions of d²σ/dΩdZ are given in Fig 1. Qualitatively similar results are obtained at the other bombarding energies and with the⁵⁶Fe projectile.