Angular Momentum Transfer to Medium Mass Evaporation Residues

In the present work, the high spin (gamma)-decay properties of evaporation residues from the reaction ('28)Si + ('54)Fe are being studied as a function of the bombarding energy (75 (LESSTHEQ) E(,L)(('28)Si) (LESSTHEQ) 145 MeV). The evaporation residues at zero degrees are identified according to their mass with Rochester's Recoil Mass Spectrometer (RMS). The RMS serves as a mass gate for a multidetector system consisting of an array of ten 3" x 3" and two 6" x 6" NaI detectors capable of determining the moments of the (gamma)-multiplicity distribution and continuum (gamma) -energy spectra. Limits for the continuum (gamma)-spectroscopy studies of masses 80 and 79 are found at bombarding energies above which the average (gamma)-multiplicity <M(,(gamma))> saturates. The effect, for mass 80, is also corroborated from the observed continuum (gamma) spectra. The bombarding energy dependence of the first three multiplicity moments, for masses 80, 79 and 78, is shown to be independent of the entrance channel from similar measurements performed on the reaction ('24)Mg + ('58)Ni. The study of the first reaction is complemented with a measurement of the evaporation residue cross sections in a wide energy range (72.5 (LESSTHEQ) E(,L)(('28)Si)(' )(LESSTHEQ) 130 MeV). The above experimental data are compared to the predictions of the statistical evaporation codes PACE2 and CASCADE. It is shown that the observed behaviour of <M(,(gamma))> is caused by alpha competition in the compound nucleus decay. However, a consistent description of the cross section and multiplicity data requires an enhanced alpha competition compared to that for unexcited spherical nuclei. It is suggested that angular momentum induced deformations in ('82)Zr cause the observed effects. An overall description of the data is given by a model taking into account the effect of deformations in the calculation of the optical model transmission coefficients.