Relaxation of angular momentum in fission and quasifission reactions

Abstract

Binary reaction products from reactions of a $^{238}\mathrm{U}$ beam at energies 5.4, 5.9, 6.7, and 7.5 MeV/nucleon with targets of $^{16}\mathrm{O}$, $^{26}\mathrm{Mg}$, $^{27}\mathrm{Al}$, $^{32}\mathrm{S}$, $^{35}\mathrm{Cl}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{\mathrm{nat}}\mathrm{Zn}$ have been measured with a kinematic coincidence method. The \ensuremath{\gamma}-ray multiplicities are measured at the same time with NaI detectors and presented as a function of fragment mass. The results indicate that all aligned and statistically excited spin modes of the fragments are fully populated in compound nucleus fission reactions. In quasifission reactions the relaxation processes seem to terminate before full statistical spin equilibrium is reached. The results are analyzed and discussed in terms of various dinuclear spin modes with different characteristic relaxation times. The influence of various terminal freeze-out shapes is also discussed.