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Fusion excitation functions for 1H and 4He have been compared to a one-dimensional, barrier-penetration model. In contrast to fusion for heavier nuclei this simple model is completely adequate, except for the statically deformed target 233U. Empirical barrier heights are obtained and compared to those from two theoretical nuclear potentials. These empirical barriers (from cold reactants) are used as input for calculating evaporation spectra that arise from hot nuclear emitters. The excess of observed low-energy 1H and 4He emission signals information content concerning distortions of the hot nuclei.

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.

Emission of 4 He and 1 H has been studied in reactions of 485 MeV 56 Fe + 197 Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the 4 He/ 1 H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed 4 He/ 1 H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for 4 He and 1 H are deduced for each source of emission, and are compared with results from similar systems. The experimental 4 He/ 1 H multiplicity ratios indicate roughly equal probability for 4 He and 1 H evaporation from a given excited source. The observation of pre-scission 4 He/ 1 H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable 4 He/ 1 H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

We have analyzed a large set of mean energies and angular anisotropies for evaporative4He emission to obtain barriers to evaporation,B. These exit channel barriers are often substantially smaller than the corresponding empirical s-wave fusion barriersE0. The differences (E0-B) are interpreted as indicators of the extent of distortion of the emitters. These distortions have in turn been characterized by the deformation parameter for a spheroid α20. ForZ=80 the dependence ofB or {α}20 on spin is somewhat suggestive of the superdeformed shapes predicted by the liquid drop model. ForZ>70 significant distortions are indicated for emitters of both large and small spin.

For the reaction /sup 16/O + /sup 208/Pb, experimental data are available for cross sections for complete fusion and all reactions from 80 --102 MeV and for elastic scattering from 80 to 313 MeV. Most of these data can be adequately described by an energy-independent spherical optical potential with the proximity formulation for the real part and a Woods-Saxon form for the imaginary part. Three parameters have been varied, radius and surface width in the proximity potential and radius in the imaginary potential. The sum of the equivalent sharp radii of the interacting nuclei must be enlarged significantly in order to fit the fusion and scattering data. As a consequence, the sum of the ''best fit central radii'' for /sup 16/O and /sup 208/Pb is approx. = 0.8 fm larger than the sum of the independently measured radii. If the proximity formulation is correct this implies dynamical shape or density adjustments during the collision.

Fusion excitation functions for1H and4He have been compared to a one-dimensional, barrier-penetration model. In contrast to fusion for heavier nuclei this simple model is completely adequate, except for the statically deformed targed233U. Empirical barrier heights are obtained and compared to those from two theoretical nuclear potentials. These empirical barriers (from cold reactants) are used as input for calculating evaporation spectra that arise from hot nuclear emitters. The excess of observed low-energy1H and4He emission signals information content concerning distortions of the hot nuclei.

Correspondences are shown between semiclassical and quantum mechanical calculations that employ the same real potential. Comparisons are made to experimental scattering, complete fusion, and total reaction cross sections for /sup 16/O + /sup 208/Pb. The separate roles of rainbow and absorptive effects are shown. Deductions of the nuclear potential from fits to data clearly depend on the adopted sequence of data fitting procedures and the freedom given at each stage to the parameters of the chosen potential. It is this sequence that plays a large role in the inference of weak vs intermediate vs strong surface absorption. The sequence we choose implies intermediate surface absorption for this case. Parameter adjustments are explored for data fits that employ the proximity potential.

Particle-particle coincidence measurements have been made between light charged particles (/sup 1,2,3/H and /sup 4/He) produced in the reaction (8.5 MeV/nucleon) /sup 56/Fe+/sup nat/Ag. This coincidence requirement emphasizes central collisions leading to evaporation residues. Average multiplicities are found to be approximately three alphas and approximately four protons per evaporation residue; these values imply that approximately (1/2 the initial excitation energy of about 211 MeV is evacuated from the evaporation residues by these particles. This fractional energy evacuation by charged particles is considerably larger than that found for the similar reaction (8.5 MeV/nucleon) /sup 40/Ar+/sup nat/Ag. In both reactions the effective emission barriers for /sup 1/H and /sup 4/He (as deduced from the energy spectra) are found to be significantly lower than corresponding empirical fusion barriers between cold nuclei. This is particularly surprising for emitter nuclei that survive fission.

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.

Fission angular distributions have been studied for years and have been treated as classic examples of trasitions-state theory. Early work involving composite nuclei of relatively low excitation energy E ∗ (⪅35 MeV ) and spin I (⪅25ħ) gave support to theory and delimited interesting properties of the transitions-state nuclei. More recent research on fusion fission and sequential fission after deeply inelastic reactions involves composite nuclei of much higher energies (⪅200 MeV) and spins (⪅100ħ). Extension of the basic ideas developed for low-spin nuclei requires detailed consideration of the role of these high spins and, in particular, the “spin window” for fussion. We have made empirical correlations of cross sections for evaporation residues and fission in order to get a description of this spin window. A systematic reanalysis has been made for fusion fission induced by H, He and heavier ions. Empirical correlations of K 2 0 (K 2 0 = I eff T h 2 ) are presented along with comparisons of I eff to moments of inertia for saddle-point nuclei from the rotating liquid drop model. This model gives an excellent guide for the intermidiate spin zone (30⪅ I ⪅65), while strong shell and/or pairing effects are evident for excitations less than ⪅35 MeV. Observations of strong anisotropies for very high-spin systems signal the demise of certain approximation commonly made in the theory, and suggestions are made toward this end.

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.

The emission of4He and1H has been measured in coincidence with fission for reactions of 469-MeV56Fe+238U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the4He/1H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for4He and1H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed4He/1H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For4He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For1H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of4He/1H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.

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.

There is quite a variety of statistical-model equations in the literature that have been derived with different assumptions and for various classes of nuclear reactions. Some have been obtained by consideration of the quantum mechanics and some by semiclassical approximations thereto. Often it is not clear whether or not the results of certain analyses are clouded by these semiclassical approximations. We consider three statistical models that have been heavily employed in recent years: (1) fission decay from a rigid rotor (2) particle evaporation from a spherical system and (3) fission decay from two uncoupled spheres. Comparisons are made of numerical results from semiclassical and from quantum mechanical equations for certain specific reactions. In addition, some general features are displayed and discussed for the differences between these formalisms. For average spins that are often encountered in heavy ion reactions, the results are very similar. We conclude that major differences in current analyses are not due to the methodology; instead they must center on assumptions concerning the physical nature of the transition-state (or decision-point) configurations and their associated level densities.

The reactions of 215 MeV16O with12C,181Ta,208Pb and238U have been studied. Inclusive measurements for4He emission are given from each target, and for fission and1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for4He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of16O with12C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.

A systematic semiempirical analysis is presented for excitation functions for complete fusion of complex nuclei. Barrier penetration is included as is the $l$ dependence of the fusion radius. The general form of the proximity potential is assumed and small but important parameter adjustments are made to achieve a fit to experimental data. A correction is presented of $s$-wave barrier heights and radii for complete fusion. For ${Z}_{t}{Z}_{p}l500$ these fusion radii are greater than the empirical reaction radii obtained at energies well above the barrier. This result implies weak surface absorption for ${Z}_{t}{Z}_{p}l500$ and the contrary for higher ${Z}_{t}{Z}_{p}$.NUCLEAR REACTIONS Semiempirical analysis of complete fusion excitation functions with a modified proximity potential; correlation between $s$-wave barrier heights and fusion radii; fusion radii exceed empirical interaction radii for ${Z}_{t}{Z}_{p}l500$ and the contrary.

Inclusive and exclusive measurements of light-charged particles (/sup 1,2,3/H,/sup 4/He) and heavy fragments have been made for the reaction 480 MeV /sup 56/Fe+/sup nat/Ag. The backward hemisphere emission of /sup 4/He and /sup 1/H in coincidence with a heavy fragment can be well described by evaporative emission from a combination of three sources: the detected fragment, the undetected fragment, and the composite system prior to scission. Multiplicities for each of these sources are determined for two coincident fragment groups: a fusion-fission-like group and a deeply-inelastic-reaction group. These multiplicities have also been studied at two angles for the trigger fragment (theta/sub TR/ = 26/sup 0/ and 50/sup 0/). For theta/sub TR/ = 26/sup 0/ the multiplicity of the light-charged particles (/sup 1/H or /sup 4/He) emitted from the composite nuclear system (i.e., prior to scission) is approx. =(2/3 for the fusion-fission-like fragments, compared to approx. =(1/3 for the deeply inelastic reactions. This decrease implies that the deeply inelastic reactions occur in approx. =(1/2 the time required for fission reactions. For the deeply inelastic reactions, these multiplicities change with trigger angle. This result also suggests that the lifetime of the composite system exerts some control on the extent of evaporative particle emission.more » Energy and angular distributions of the coincident light particles suggest extensive emission from a strongly deformed composite system, such as that of a system en route to scission. Low-energy /sup 4/He particles at forward angles, which cannot be attributed to evaporation, exhibit a dependence on the angle with respect to the scission axis. This result is attributed to emission that occurs very near to the instant of scission.« less

Semiempirical fusion barriers have been obtained by analysis of 87 excitation functions for complete fusion. These barriers are used as a test for several theoretical potentials. For most of the reactions at energies below the operational s-wave fusion barrier, the cross sections are greater than expected from considerations of simple barrier penetration alone. We ascribe this to dynamic effects, such as neck formation which may be expected as precursors of fusion. The effects of static deformation do not appear to be sufficient to explain the discrepancies.

Twelve reactions have been studied that produce the compound system $^{194}\mathrm{Hg}$* at excitation energies of 57 - 195 MeV and with ${l}_{\mathrm{crit}}$ values of $25\ensuremath{-}142\ensuremath{\hbar}$. Beams of $^{12}\mathrm{C}$, $^{19}\mathrm{F}$, $^{20}\mathrm{Ne}$, and $^{40}\mathrm{Ar}$ ions in conjunction with appropriate targets have been used to measure cross sections for evaporative H/He, fission, and evaporation residues. These results confirm that most $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ is evaporated prior to fission or instead of fission and very little if any from the fission fragments. The probability of H/He evaporation increases dramatically with excitation energy. The evaporation residue cross sections ($\frac{{\ensuremath{\sigma}}_{\mathrm{ER}}}{\ensuremath{\pi}{\ensuremath{\lambda}}^{2}}$) indicate fission survival for entrance channel $l$ up to $27\ensuremath{-}39\ensuremath{\hbar}$. Fission survival becomes stronger and corresponding fission competition becomes weaker for excitation \ensuremath{\ge} 100 MeV; a connection with charged particle emission is suggested. The dimensionless cross section for evaporation residue ($\frac{{\ensuremath{\sigma}}_{\mathrm{ER}}}{\ensuremath{\pi}{\ensuremath{\lambda}}^{2}}$) depends on both the entrance channel and on energy, indicating that nonequilibrium mechanisms must play an important role, even for $l\ensuremath{\lesssim}40$. Heretofore evaporation residue production has been usually thought to arise from lower partial waves while direct reactions have been thought to dominate only for the higher partial waves.NUCLEAR REACTIONS $^{12}\mathrm{C}$+ $^{182}\mathrm{W}$; $^{20}\mathrm{Ne}$+ $^{174}\mathrm{Yb}$; $^{40}\mathrm{Ar}$+ $^{154}\mathrm{Sm}$; $^{19}\mathrm{F}$+ $^{175}\mathrm{Lu}$; reactions forming $^{194}\mathrm{Hg}$* at excitation energies 57, 80, 100, 142, 195 MeV. Measured cross sections for fission and evaporation residues, evaporative H and He including some energy spectra. Discuss relationship to statistical evaporation model.

The trend of empirical interaction barriers and related parameters is explored for various heavy ion reaction systems for 100 1000. This behavior is shown to be consistent with predominantly refractive scattering for low Z/sub 1/Z/sub 2/ and dominance for surface absorption for Z/sub 1/Z/sub 2/ > 1000.

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.

Inclusive1H/4He emission has been studied for three reactions in “reversed kinematics”,40Ar/56Fe/86Kr (8.5MeV/u)+12C. The4He spectra include the heretofore unobserved region of low c.m. energies in the direction of the heavy reactant. A test of forward-backward symmetry is made for the hot-spot, moving source model; the data do not support the model. Effective emission barriers for4He evaporation are found to be within ∼10% of the corresponding fusion barriers, while for1H evaporation the emission barriers are more than 25% lower. It is possible that deformations predicted by the rotating liquid drop model can account for the4He evaporation spectra, but substantial change in the standard evaporation treatment is required to explain the spectra for1H.

From measured cross sections for evaporative1H and4He, we have unfolded the separate effects of excitation energy (E) and spin (J) on the decay of194Hg. The analysis depends only on Bohr's independence hypothesis and on certain general features of the transmission coefficients for fusion. Evaporation-like emission of1H and4He increases rapidly with energy but decreases rather mildly with initial spin. Significant emission occurs even for very high spins where the calculated fission barrier is essentially zero. The equilibrium model for fission-evaporation competition is brought into question forE*≳98 MeV and J≳65 ħ.

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

Multiplicities are compared for4He evaporation in reactions of40Ar and56Fe projectiles (8.5 MeV/u) with withnatAg and238U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like4He emission from the composite nuclei is shown to be large for mean spins ≦100ℏ, and to decrease dramatically with increasing spin. Coincidence measurements between two4He particles have also been studied for40Ar+natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the4He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.

It has often been stated that fission fragment angular anisotropy, as predicted by equilibrium statistical theory, should disappear with increasing spin of the composite nucleus. However, several recent experimental studies reveal strong anisotropies for fission fragments from high-spin nuclear systems. We discuss this apparent discrepancy and its relationship to the rigid-rotor approximation used in the standard theory. A systematic comparison is given for fission fragment anisotropies from many experiments via the empirical parametersK02 andℐeff. These systematics indicate a strong regularity, provided one allows for the perturbing effects of fission after transfer reactions. Many of the observed anisotropies exceed the predictions of the standard theory, but, as these predictions are based on a rigid rotor model, this does not seem particularly noteworthy.

The influence of isospin on statistical nuclear decay at approximately 20 MeV is investigated for two composite systems $^{66}\mathrm{Zn}$ and $^{64}\mathrm{Zn}$ by measurement of the cross sections and the energy spectra of protons and \ensuremath{\alpha} particles emitted from the two composite systems. The composite system $^{66}\mathrm{Zn}$ was formed at an excitation energy of 19.7 MeV in the $^{65}\mathrm{Cu}+p$ and $^{62}\mathrm{Ni}+\ensuremath{\alpha}$ reactions, while the composite system $^{64}\mathrm{Zn}$ was formed at an excitation energy of 19.5 MeV in the $^{63}\mathrm{Cu}+p$ and $^{60}\mathrm{Ni}+\ensuremath{\alpha}$ reactions. In both systems the experimental cross-section ratio $\frac{\ensuremath{\sigma}(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}})\ensuremath{\sigma}(p,{p}^{\ensuremath{'}})}{\ensuremath{\sigma}(\ensuremath{\alpha},p)\ensuremath{\sigma}(p,\ensuremath{\alpha})}$ exceeded the value of approximately unity predicted by the Bohr independence hypothesis and the statistical theory of compound-nuclear decay including angular momentum. The cross-section ratios were 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 and 1.3 \ifmmode\pm\else\textpm\fi{} 0.1 for the $^{66}\mathrm{Zn}$ and $^{64}\mathrm{Zn}$ composite systems, respectively. The major part of the deviation of these ratios from unity in both the composite systems is ascribed to an isospin selection rule acting in the ${T}_{g}$ states of both composite systems which causes these states to decay preferentially by proton emission. Other factors which might enhance these ratios, such as direct reactions and reduced moments of inertia, are considered and are found to change the above ratios by a relatively small amount.

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.

Angular and energy distributions have been measured for products of 3≦Z≦9 from reactions of 336MeV40Ar+107,9Ag/154Sm/197Au. The c.m. differential cross sections and mean energies increase as one moves from ≈100deg. to ≈170deg. similar to those for fission or evaporation-like emission. Most characteristics mimic those observed for4He, but the cross sections are smaller by a factor of as ≈1/300 for each reaction studied. A semiclassical analysis has been used to estimate the effective barriers to emission and root-mean-square spins of the emitters.

We have investigated the emission of 4 He in coincidence with mass-selected fragments from fission for 334 MeV 40 Ar + 238 U. Upper-limit multiplicities are assigned for fragment evaporation and composite-nucleus evaporation, but additional mechanisms are also required. A new low-energy component is found at backward angles; it is sensitive to the fragment masses but does not reflect their full kinematic shifts. Thus it must be due to near-scission emission (prior to full fragment acceleration).

The fusion-fission contributions to /sup 16/0+/sup 209/Bi and /sup 16/0+/sup 238/U are estimated. (AIP)