Single and multinucleon transfer yields for the {sup 136}Xe+{sup 64}Ni reaction at a scattering energy {approx}5{percent} above the Coulomb barrier energy are studied using particle{endash}{gamma}-ray coincidence data. Q-value and scattering-angle distributions are extracted for the stronger channels. A fast transfer mechanism dominates the yields to these channels over an extended Q-value range, leading to a concentration of the cross section near the grazing angle. Analysis of the angular distributions based on a semiclassical barrier penetration model suggests that the single-nucleon and two-neutron exchange channels are dominated, respectively, by direct and two-step sequential transfer from the ground or low-lying excited states of the participating nuclei. The multiproton transfer channels have angular distributions that indicate a more complex mechanism, although direct cluster transfer from an excited configuration cannot be fully discounted. In a separate analysis, the relative population of different mass channels is found to be in general agreement with the expectations of a {open_quotes}random walk{close_quotes} model of particle exchange. {copyright} {ital 1997} {ital The American Physical Society}

The fully energy-damped yields for the Ar-36 + C-12 and Ne-20 + Si-28 reactions at E(c.m.)=47.0 MeV and 45.5 MeV, respectively, are explored using particle-particle-gamma coincidence data. These reactions reach a similar excitation energy of E(CN)*=59.5 MeV in the Cr-48 compound nucleus as was obtained in an earlier particle-particle coincidence study of the Mg-24 + Mg-24 reaction. The overall mass and total kinetic energy distributions of the fission fragments are found to be well reproduced by statistical-model calculations. These calculations are also found to reproduce structure seen in the excitation-energy spectra for the Ne-20 + Si-28 and Mg-24 + Mg-24 exit channels for all three reactions. In previous excitation-function measurements, strong heavy-ion resonance behavior has been observed in elastic and inelastic cross sections for the Mg-24 + Mg-24 system. There has been speculation that peaks observed in the corresponding excitation-energy spectra at more negative Q values may also be a consequence of this resonance phenomenon. The observation of very similar behavior with the asymmetric-mass entrance channels makes it less likely, though, that the peaks arise from any special configuration of the compound system. Instead, an analysis of the gamma-ray data and the results of statistical-model calculations support the conclusion that most of the observed high-lying structure can be accounted for in terms of statistical fission from a fully energy- and shape-equilibrated compound nucleus. For the Mg-24 + Mg-24 entrance channel, however, comparisons with the statistical model indicate a reduction of high-angular-momentum partial cross sections, leading to the Mg-24 + Mg-24 fission channel. For the first time, we are able to deduce the nature of the competition between the resonance and statistical-fission mechanisms in this mass region.

The fully energy-damped cross sections of the Mg-24+Mg-24 reaction at E(c.m.) = 44.4 MeV have been measured for all of the major fission channels. High-resolution Q-value spectra have been obtained for the large-angle yields in the Mg-24+Mg-24 and Ne-20+Si-28 channels. Calculations based on the transition-state model are found to reproduce the fully damped cross sections in all of the observed mass channels. The pronounced structure that is observed in the excitation-energy spectra for the more symmetric mass channels, even for the strongly damped yields, is shown to be qualitatively reproduced by assuming a spin-weighted population of the fragment states. There is also evidence, however, that the structure of the nascent fission fragments at scission may influence the population of states in the fragments. These results, taken together with earlier measurements of the resonance behavior of this system, suggest the coexistence of fission from the normal, compact compound nucleus with that from the deformed configurations believed to be responsible for the resonance behavior.

Fusion evaporation-residue cross sections for the Si-28 + C-12 reaction have been measured in the energy range 18 less-than-or-equal-to E(c.m.) less-than-or-equal-to 136 MeV using time-of-flight techniques. Velocity distributions of mass-identified reaction products were used to identify evaporation residues and to determine the complete-fusion cross sections at high energies. The data are in agreement with previously established systematics which indicate an entrance-channel mass-asymmetry dependence of the incomplete-fusion evaporation-residue process. The complete-fusion evaporation-residue cross sections and the deduced critical angular momenta are compared with earlier measurements and the predictions of existing models.

The total fusion cross section for the system /sup 15/N+/sup 27/Al has been measured over an energy range 27 MeV< or =E/sub lab/< or =70 MeV by detection of the fusion-evaporation residues. In addition elastic scattering was measured at six energies and fitted by optical model calculations. The fusion cross section for the system saturates at 1150 +- 50 mb. The data can be well described by the model of Glas and Mosel, using a reasonable set of parameters. The model of Horn and Ferguson also describes the data well if an appropriate charge radius is used. Comparison is made between these results and the fusion cross sections for /sup 16/O+/sup 26/Mg and /sup 18/O+/sup 24/Mg, which lead to the same compound nucleus. The results for /sup 15/N+/sup 27/Al are quite similar to those for /sup 18/O+/sup 24/Mg, and the differences between the fusion cross sections for these two systems and those for /sup 16/O+/sup 26/Mg may be evidence for an entrance channel effect.

The ${\mathrm{Ca}}^{44}({\mathrm{He}}^{3},t){\mathrm{Sc}}^{44}$ reaction has been studied, at in incident energy of ${E}_{{\mathrm{He}}^{3++}}=25$ MeV, with a magnetic spectrograph. A total of 52 levels in ${\mathrm{Sc}}^{44}$ have been identified below an excitation energy of 3.35 MeV. 10 of the observed levels have not been previously reported. Angular distributions have been measured for most of the levels and analyzed by means of the distorted-wave Born approximation. Definite $L$-transfer values and spin assignments have been made to many levels. As a result, eight states predicted by a recent ${(\mathrm{fp})}^{4}$ calculation have been identified. In addition, four of the states predicted by a calculation with a single ${d}_{\frac{3}{2}}$ hole have been tentatively identified. The isobaric analog of the ${\mathrm{Ca}}^{44}$ ground state was located at an excitation energy of 2781 \ifmmode\pm\else\textpm\fi{} 5 keV, in good agreement with previous results.

Correlation measurements have resulted in an unambiguous $\frac{9}{2}$ spin assignment to the level in ${\mathrm{F}}^{19}$ at 2797 keV. The lifetime of this level has been measured with the Doppler shift attenuation method to be 246\ifmmode\pm\else\textpm\fi{}44 fsec, which establishes its positive parity assignment.