The generation of angular momentum in fission fragments and its energy dependence*

We present new experimental results that show the existence of an excitation energy dependence of fission fragment angular momenta. The setup used for this experiment is the FS-3 detection system, an array of forty trans-stilbene scintillators sensitive to neutrons and gamma rays, and a twin Frisch-gridded ionization chamber, capable of measuring fragment masses, kinetic energies, and polar emission angles relative to the source. We have measured mean gamma-ray and neutron multiplicities in coincidence with fragment masses and kinetic energies. We have determined that the gamma-ray multiplicity, the primary information carrier of fission fragment angular momenta, increases with decreasing total kinetic energy, eventually reaching a saturation plateau. Interestingly, the total number of gamma rays at high excitation energies is independent of the fragment mass split. However, the saturation energy, i.e., the excitation energy required to reach this plateau, changes significantly with mass split, being highest for very asymmetric and very symmetric fission events. In fact, the energies of the gamma rays correlating with excitation energy are characteristic of quadrupole band transitions and are thus strongly correlated with fragment angular momenta. The results are interpreted in terms of an angular momentum saturation model: in this model, the fragment angular momenta are primarily determined at scission and they strongly couple to their relative orbital angular momentum. Because of the lowering of the fission barrier with increasing orbital angular momentum, the saturation of fragment angular momenta is observed.