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Excitation functions (#i ab = 235-320 MeV) for producing nuclei close to the compound system (evaporation residues) were measured for 58 » 64 Ni+ 112 " 124 Sn, corresponding to a 20% variation in the compound-nucleus neutron number. A dramatic, order-of-magnitude increase was observed in the maximum cross section as a function of the compoundnucleus neutron excess. The subbarrier energy dependence of the cross sections is well described by an optical-model calculation, but not by the frequently used parabolic approximation.PACS numbers: 25.70.Jj, 25.70.GhConsiderable theoretical and experimental effort has focused recently on fusion reactions in moderately heavy systems (compound nuclei with A <; 160). Points of particular interest include (i) the subbarrier energy dependence of cross sections, 1 " 5 (ii) the competition between the survival of the compund nucleus leaving an evaporation residue, a ER , and fission, 6 ' 7 and (iii) the possible requirement of an "extra-push" energy to achieve fusion in heavy systems. 8 ' 9 Identification of the evaporation residues, left after lightparticle evaporation from the compound nucleus, represents an unambiguous signature of fusion. This Letter reports measurments of cross sections for 58 » 64 Ni fusing with the even-mass Sn isotopes {A = 112-124) at energies ranging from below the classical fusion barrier to well above it. The choice of Sn as the target material allowed a systematic study of the target (and compound nucleus) neutron-number dependence of cr ER . The compund nuclei span the range from the very-proton-rich 170 Pt to 188 Pt, a change of 18 neutrons or ~20% in neutron number.For such heavy systems, direct measurements of a ER are complicated by the fact that the angular distributions are strongly forward peaked. Measurements must be made at angles well inside 5°, where the counting rate from elastic scattering is high. In addition, cr ER may be small, either because the bombarding energy is well below the classical fusion barrier, or because of fission. For these reasons, several laboratories have utilized recoil-mass separators or velocity filters to reduce the background. 10 " 12 In the present measurements, the nuclei emerging from the target at ~0° were separated from the beam (and small-angle elastic scattering) by an electrostatic deflector. The separated evaporation residues were then identified by their energy loss and residual energy in a AE-E detector assembly.The isotopically enriched targets (250-300 [ig/ cm 2 Sn evaporated on 20-/ig/cm 2 C backings) were positioned in front of the entrance to a 1.7-m-diam scattering chamber with the deflector (25 cm longx3 cm plate separation, V m3iX~4 5 kV) 25 cm further downstream. The entrance to the deflector was restricted by a vertical slit to particles scattered at the target by less than ±1° in the deflection plane. This enabled the detection of evaporation residues without requiring a substantial reduction in incident beam current (typical values were 1-2 particle nA).The AE-E detector consisted of up to...

The fully energy-damped cross sections of the Mg+ Mg reaction at E . . = 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+ Mg and Ne+ Si 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 in6uence 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.PACS number(s): 25.70. Jj, 25.70.Lm, 25.70.Gh, 24.60.Dr