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We present data for small-angle particle-particle correlations from the reactions 80, 140, 215, and 250 MeV {sup 16}O+{sup 27}Al{r arrow}{ital p}-{ital p} or {ital p}-{ital d}. The main features of these data are anticorrelations for small relative momenta ({le}25 MeV/{ital c}) that strengthen with increasing bombarding energy. Statistical model calculations have been performed to predict the mean lifetimes for each step of evaporative decay, and then simulate the trajectories of the particle pairs and the resulting particle correlations. This simulation accounts very well for the trends of the data and can provide an important new test for the hypothesis of equilibration on which the model is built.

Light charged particles from the reactions 247 and 337 MeV $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag have been measured in singles and in coincidence with fission-like products. Most of the $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in coincidence with fusion-fission and quasifission can be accounted for by evaporation from the composite nucleus or by the fragments. Multiplicities are determined for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in association with evaporation residues, fusion-fission, and quasifission. The probability for composite nucleus emission decreases with increasing spin zone of the emitter and gives a view of the role of emitter lifetime. The ratio of composite-nucleus multiplicities for quasifission/fusion-fission is about 1/2 ((1/4) for 337 MeV $^{40}\mathrm{Ar}$ (247 MeV $^{40}\mathrm{Ar}$) which implies a similar ratio for the lifetimes of the respective composite nuclei between thermalization and scission. Energy spectra and angular distributions of particles (in coincidence with fusion-fission and quasifission) are consistent with emission from a strongly distorted nuclear system en route toward scission. Prethermalization emission of $^{4}\mathrm{He}$ is observed at 10\ifmmode^\circ\else\textdegree\fi{} in comparable amounts for peripheral collisions (quasifission and fusion-fission) and for central collisions.

Evaporation-like emission of light particles (n, H, He...) has been observed for many nuclear reactions that populate composite nuclei with temperatures of up to ≤ 5 MeV and spins as high as ≤ 150h. The measured average energies for the light charged particles are often significantly lower than calculations for evaporation from spherical nuclei. Detailed calculations have not been reported for the role of deformation in evaporative emission patterns, and thus the interpretation of such data has been severely limited. In this paper we describe a method for making evaporation calculations for deformed nuclei, and we present results for several systems. The trends of the calculated results are compared to those for spherical emitters in order to get a feeling for those effects peculiar to the deformations. The patterns of calculated spectra and angular distributions encourage the notion that such studies can give valuable clues to the shapes and surface properties of these very hot nuclei. Comparisons are made to experimental data for 1H-4He and 4He-4He correlations in the reaction 337 MeV 40Ar + natAg. These comparisons point out some novel features of the nuclear surface.

Angular and energy distributions are reported for 4 He in correlation with 1,2,3 H and 4 He in the reaction 337 MeV 40 Ar+ nat Ag. Comparisons are made to a statistical model that includes emitter. Energy spectra important barrier reductions compared to those for cold nuclei, ≈25%, 12%, 8% and 0 for 1 H, 4 He, 2 H and 3 H respectively. Emitter deformation alone cannot give an explanation. A transitory nuclear stratosphere is suggested with proton emission favored from its fringes of lowest density.

We have measured coincidences between $^{4}\mathrm{He}$ and $^{1}$,2,3H or a second $^{4}\mathrm{He}$ (large-angle correlations) in the reactions 247 and 337 MeV $^{40}$Ar${+}^{\mathrm{nat}}$Ag. These coincidences are dominated by reactions leading to evaporation residues with spins of about 0--70\ensuremath{\Elzxh}. Large values of the angle-integrated double and triple coincidence cross sections indicate that many particle evaporation chains involve 2 or 3 protons and/or 2 or 3 alphas. The angular and energy distributions are well described by an evaporation model with Hauser-Feshbach coupling. However, one must employ effective evaporation barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for $^{2}\mathrm{H}$ is rather small, and for $^{3}\mathrm{H}$ no barrier reduction is needed. Evaporation calculations that include emitter deformation do not give a satisfactory explanation for these results. We suggest a new aspect of composite nucleus excitation and evaporation-like decay. A hot, very diffuse nuclear surface may be formed that promotes evaporative decay prior to its relaxation. The low barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ reflect the radial extent of this transitory, diffuse surface.

Evaporation-like emission of light particles (n, H, He, . . .) has been observed for many nuclear reactions that populate composite nuclei with temperatures of up to roughly-equal5 MeV and spins as high as roughly-equal150h-dash-bar. The measured average energies for the light charged particles are often significantly lower than calculations for evaporation from spherical nuclei. Detailed calculations have not been reported for the role of deformation in evaporative emission patterns, and thus the interpretation of such data has been severely limited. In this paper we describe a method for making evaporation calculations for deformed nuclei, and we present results for several systems with temperatures of 2--5 MeV. The trends of the calculated results are compared to those for spherical emitters in order to get a feeling for those effects peculiar to the deformations. The patterns of calculated spectra and angular distributions encourage the notion that such studies can give valuable clues to the shapes of these very hot nuclei.

Particle-particle correlations (p-p and ..cap alpha..-..cap alpha..) vs relative momentum (P/sub rel/) are reported (theta/sub lab/approx. =19.5/sup 0/) for the reaction 140 MeV /sup 16/O+/sup 27/Al. A plot of the Galilean invariant cross section indicates that evaporative emission dominates the gross proton production even at this forward angle. The observed anticorrelation between two protons for small values of P/sub rel/ is analyzed to give an evaporation lifetime in the range of 1--5 x 10/sup -21/ s. The ..cap alpha..-..cap alpha.. correlation is completely different; it seems to arise from preformed /sup 8/Be.

An extensive study of the decay of the compound nuclei 58,60,62 Ni formed in the reactions ( 12 C + 46 Ti), ( 28 Si + 30 Si), ( 12 C + 48 Ti), ( 30 Si + 30 Si), ( 12 C + 50 Ti), ( 18 O + 44 Ca) was undertaken in the energy range ~ 0.7 to ~ 1.6 Coulomb barrier in about 20 steps. The fusion cross sections are fragmented in a large number of exit channels, with cross sections from 0.1 to 430 mb. By means of in-beam γ-ray measurements, the evaporation residues were identified both in A and Z . This large data base was compared with fusion-evaporation calculations through individual residue excitation functions and summed evaporation-residue cross sections (resulting from nucleons or α-emission). The model, with a standard set of parameters, does account for the general features of the experimental data, especially for the stronger exit channels. However, these data are not sensitive enough to the presence of other types of mechanisms and must be complemented by other observables to boost the predictive power of the model.

Excitation functions of the reactions 12, 13 C+ 48 Ti and 30 Si+ 30 Si were measured by in-beam γ-ray spectrometry in the energy ranges 20–60 MeV for the 12, 13 C induced reactions and 55–126 MeV for the 30 Si+ 30 Si reaction. Light-particle angular distributions were measured at 46 MeV and 47.5 MeV for the 13 C and 12 C induced reactions. Measurements of elastic scattering angular distribution and particle-γ coincidences were carried out for the system 13 C+ 48 Ti at 46 MeV. The limitation to complete fusion detected for the system 30 Si+ 30 Si appears to be related to entrance channel effects and is well reproduced by a barrier penetration calculation using the KNS potential. The angular distribution measurements carried out for the 12 C+ 48 Ti and 13 C+ 48 Ti systems allowed to identify an incomplete fusion mechanism with emission of direct α-particles before the formation of a fully equilibrated system.

Fusion excitation functions for the systems 12 C + 46, 48, 50 Ti, 28, 30 Si + 30 Si and 18 O + 44 Ca have been determined at energies near and below the Coulomb barrier. Inelastic excitation functions for the targets 48, 50 Ti and 44 Ca have been also measured in the same energy range. The absolute cross sections were obtained by in-beam γ-ray spectroscopy technique using a rotating target. Fusion cross sections ranging in magnitude from ∼ 0.3 mb to ∼ 1300 mb were determined with an accuracy of 10–20%. The fusion excitation functions are analysed in the frame of a semiclassical barrier-penetration model. From the analysis, the height, the radius and the curvature of the fusion barrier for the different systems are extracted. The fusion cross sections are compared with the calculations performed using different heavy-ion potentials. The enhancement of the cross sections at sub-Coulomb energies can be reproduced with a one-dimensional barrier-penetration model taking into account the zero-point motion of the surface of the reaction partners. The fusion cross section of the system 18 O + 44 Ca is well reproduced by quantum-mechanical calculations, introducing a new degree of freedom taking into account the formation of a neck during the fusion process.