The elastic angular distributions for Ni-58+Al-27 have been measured at five energies from E(c.m.)=48.8 to 69.5 MeV. The optical model analysis assumes both phenomenological and folding model potentials. Regions of sensitivity, where the potential is well determined, are found to be different for the real and the imaginary parts, and to vary with energy. The values of the real potential in the surface are weakly dependent on energy.

By integrating the classical formula for fusion of two charged spheres with a flat distribution of barriers extending down to a threshold barrier the fusion cross sections of heavy nuclei at energies near and below the Coulomb barrier have been successfully parametrized by P. H. Stelson. The systematics of the threshold barriers derived from the data are examined in terms of a simple picture in which the onset of free neutron flow between the collision partners promotes fusion at a large distance. A consistent correlation between observed barrier shifts and the conditions necessary to maintain free neutron flow has emerged.

Target and target-like ejectiles emitted forward from Nb-93-beam bombardment of thin Ti-50 targets were investigated at sub- and near-barrier energies using a magnetic spectrograph. Reaction products from three single-nucleon-transfer and seven multinucleon-transfer channels were observed. Multinucleon-transfer products were observed with a sharp onset for those collisions with apsidal distances smaller than about 12.6 fm, while single-nucleon-transfer products were observed throughout the apsidal distance range studied. Average Q values of the multinucleon-transfer-reaction products are much more negative than the respective optimum values. These results suggest the formation of a neck at subbarrier energies.

The significance of the interpretation of heavy-ion fusion cross sections in terms of a distribution of potential barriers is discussed. The smoothing due to the quantal barrier penetration is shown to replace a set of discrete barriers by an effective continuous distribution. It is shown how this smoothed distribution may be obtained rather directly from the measured cross sections at near- and sub-barrier energies.

A significant yield of multinucleon-transfer products is observed at back angles in quasielastic reactions between Ti-50 and Nb-93 at sub- and near-barrier energies. A variety of nuclidic species that require transfers of up to four nucleons are observed for E(c.m.) greater-than-or-equal-to 103.9 MeV. The internuclear separation distance where the multinucleon-transfer products first emerge nearly coincides with the closest approach distance of the experimentally established fusion threshold energy, indicating a common origin for the fusion enhancement and multinucleon-transfer reactions.

Since the observed fusion cross sections for collisions between heavy ions at subbarrier energies are orders of magnitude larger than would be expected for barrier tunnelling, one is faced with the task of identifying the basic force which is strong enough to overcome the strong Coulomb force and bring about fusion. The two possibilities seem to be (1) excursions of the nuclear surface (and strong nuclear force) due to collective motions of the colliding nuclei and (2) formation of a neck of nuclear matter. The first possibility has received the most attention. However, the systematics of fusion cross sections suggest neck formation is playing an important role. Neck formation can also result in a reseparation of the composite system and we review the experimental information on these reactions at barrier and subbarrier energies.

Measurements of the fusion cross sections in the barrier and subbarrier regions are presented for the four heavy-ion systems $^{46,50}\mathrm{Ti}$${+}^{90}$Zr${,}^{93}$Nb. The measured cross sections varied from 0.04 to 400 mb. The evaporation residues were detected by the use of a velocity filter. Procedures are described for extracting fusion cross sections from such measurements. The observed differences in the subbarrier fusion cross sections for the $^{90}\mathrm{Zr}$ and $^{93}\mathrm{Nb}$ nuclei, both of which have small collectivity, have led us to question the view that excursions of the strong nuclear force due to collective motions of the colliding nuclei are the primary enhancement mechanisms for the observed heavy-ion subbarrier fusion cross sections.A simple formula is presented, based on a flat distribution of barriers, which is applied to the near-barrier region (10--200 mb). This formula states that the quantity (\ensuremath{\sigma}E${)}^{1/2}$ vs E is linear in the near-barrier region with a zero intercept defining a threshold energy for fusion. These threshold values reflect variations in the binding energies of the valence neutrons. We also point out that the far-subbarrier region (10 mb) shows large variations in fusion cross sections and that these variations reflect differences in the collectivities of the colliding nuclei (especially noticeable differences are seen between permanently deformed nuclei and ``vibrational'' nuclei). These systematics of heavy-ion subbarier fusion suggest that neck formation is playing an important role. The barrier for neutron transfer vanishes at distances typically 1.5 fm beyond the typical barrier distance and this distance could vary with the binding energy of the valence neutrons. The presence of neutrons in the region between the nuclei could promote neck formation which provides a force strong enough to overcome the Coulomb force. The collective properties of the colliding nuclei are then interpreted as a modulation of the threshold for neck formation and thereby reflect the large observed differences in the far-subbarrier cross sections. Formulas are presented for ${\mathrm{\ensuremath{\sigma}}}_{\mathit{L}}$ which result from a flat distribution of barriers. These formulas predict a broad bell-shaped spin distribution and are compared to the measured distribution in the subbarrier region for the system $^{64}\mathrm{Ni}$${+}^{100}$Mo.

Coulomb excitation of $_{60}^{144,146,148,16050}\mathrm{Nd}$ by 10.5 and 11 MeV alpha particles was studied by magnetic analysis of particles scattered into 150\ifmmode^\circ\else\textdegree\fi{}. Values of B(E${2;0}^{+}$\ensuremath{\rightarrow}${2}^{+}$) for the ${2}^{+}$ states at 696, 454, 302, and 130 keV are 0.58(1), 0.78(1), 1.390(20), and 2.816(35) ${e}^{2}$${\mathrm{b}}^{2}$, respectively. For $^{148}$,150Nd, values of B(E${3;0}^{+}$\ensuremath{\rightarrow}${3}^{\mathrm{\ensuremath{-}}}$) for ${3}^{\mathrm{\ensuremath{-}}}$ states at 999 and 932 keV are 0.40(8) and 0.18(3) ${e}^{2}$${\mathrm{b}}^{3}$, respectively. For $^{148}$,150Nd the hexadecapole transition matrix elements were deduced to be ${0.36}_{\mathrm{\ensuremath{-}}12}^{+10}$ and 0.25(12) e ${\mathrm{b}}^{2}$, respectively. Our measurements are compared to others and to interacting boson model predictions.

Cross sections for the fusion of $^{28}\mathrm{Si}$ + $^{12}\mathrm{C}$ and $^{30}\mathrm{Si}$ + $^{12}\mathrm{C}$ have been measured at energies between 6.4 and 9.4 MeV/nucleon and 5.2 and 8.3 MeV/nucleon, respectively. Comparison with existing data for compound systems with $A=40 \mathrm{and} 42$ shows that the extracted critical angular momentum at saturation (${l}_{\mathrm{cr}}^{max}$) depends strongly on the entrance channel mass asymmetry. This effect is not accounted for by the standard entrance channel or compound nucleus models. A calculation of the conditional saddle-point shapes for touching spheroidal nuclei indicates that this mass asymmetry effect is a consequence of the dynamical fusion thresholds at high angular momentum. Quantitative agreement is obtained with the experimentally measured ${l}_{\mathrm{cr}}^{max}$ using the finite range model of Sierk for diffuse-surface nuclei.

We have studied the distributions of barriers required to fit experimental heavy-ion fusion cross sections at energies in the barrier region. Best fits were generally obtained with distributions which were flat, broad, and characterized by a sharp cutoff value at the low-energy end. The cutoff barrier is correlated with the separation energies of the valence neutrons and can be associated with the distance at which merged potentials just allow neutrons to flow between the nuclei. These features suggest that fusion initiated by neutron flow is the principal enhancement mechanism and that coupling to collective states plays a secondary role.

The charge and mass distribution of orbiting yields from the $^{28}$Si${+\mathrm{}}^{14}$N interaction have been studied at center of mass energies between 30 and 56.7 MeV. The data indicate that the orbiting system lives sufficiently long for the exit channel yields to be determined by phase space considerations. The data are compared with compound nucleus and orbiting calculations, and clearly favor the latter. Further comparisons between the data and the orbiting calculations indicate that full phase space equilibration of the orbiting yields is hindered at low excitation energies by the inaccessibility of single nucleon transfer channels. Also evident in the data is a correlation in the behaviors of the charge width, mass width, and final product kinetic energies, which is interpretable as arising from a maximum angular momentum that the $^{28}$Si${+\mathrm{}}^{14}$N orbiting composite can sustain.

The Coulomb excitation of states in 229Th has been observed using 17.0-MeV 4He2+ ions. For the 5/2+[633] ground-state rotational band, we deduced an intrinsic quadrupole moment, Q20, of 8.816 ± 0.090 eb and an intrinsic hexadecapole moment, Q40, of 3.69 ± 0.72 eb2. This Q40 value for 229Th is 43% larger than that for the even-even neighbor, 230Th, and is the largest thus far observed in the actinide region. Large E3 matrix elements are extracted for states at 512, 562, and 611 keV, which supports the assignment of these states as members of a Kπ = 0− octupole-vibrational band built on the 5/2+[633] ground state. This 0− octupole excitation occurs at much higher energy than the one inferred from earlier decay studies that is built on the 3/2+[631] state. This suggests state-dependent octupole correlations consistent with octupole shape transitional models of 229Th.

The analysis of quasielastic cross section data for the $^{90}\mathrm{Zr}$ projectile plus $^{50}\mathrm{Ti}$ target system shows that the probability for $^{50}\mathrm{Ti}$${(}^{90}$Zr, $^{49}\mathrm{Ti}$${)}^{91}$Zr, 1n-transfer reaction near the barrier is much larger than estimates based on semiclassical theory. The probability for $^{50}\mathrm{Ti}$${(}^{90}$Zr${,}^{51}$V${)}^{89}$Y, 1p-transfer reaction, on the other hand, agrees with the same theory. The internuclear distance where the 1n-transfer probability first deviates from tunneling predictions coincides with the threshold of the fusion barrier distribution deduced from the experimental fusion cross sections of the $^{50}$Ti${+}^{90}$Zr system, suggesting a common mechanism for the large enhancement of 1n-transfer and fusion cross sections.

Angular correlations between ..cap alpha.. particles and evaporation residues have been measured to study the equilibrium versus nonequilibrium effects on the fusion process. Measurements with ..cap alpha..-particle detectors were carried out to extreme forward angles (4/sup 0/) to maximize the sensitivity to nonequilibrium components. The results are consistent with complete fusion and equilibrium decay, in contrast with recent systematics that predict large amounts of incomplete fusion at this energy.

Adopted values for the reduced electric quadrupole transition probability, B(E2)↑, from the ground state to the first-excited 2+ state of even-even nuclides are given in Table I. Values of τ, the mean life of the 2+ state, E, the energy, and β2, the quadrupole deformation parameter, are also listed there. The ratio of β2 to the value expected from the single-particle model presented. The intrinsic quadrupole moment, Q0, is deduced from the B(E2)↑ value. The product E × B(E2)↑ is expressed as a percentage of the energy-weighted total and isoscalar E2 sum-rule strengths.

Angular correlations between \ensuremath{\alpha} particles and evaporation residues have been measured to study the equilibrium versus nonequilibrium effects on the fusion process. Measurements with \ensuremath{\alpha}-particle detectors were carried out to extreme forward angles (4\ifmmode^\circ\else\textdegree\fi{}) to maximize the sensitivity to nonequilibrium components. The results are consistent with complete fusion and equilibrium decay, in contrast with recent systematics that predict large amounts of incomplete fusion at this energy.