The two-nucleon density distributions in states with isospin T=0, spin S=1, and projection ${\mathit{M}}_{\mathit{S}}$=0 and ±1 are studied in ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^{3,4}{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{6,7}{}_{}{}^{}\mathrm{Li}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. The equidensity surfaces for ${\mathit{M}}_{\mathit{S}}$=0 distributions are found to be toroidal in shape, while those of ${\mathit{M}}_{\mathit{S}}$=±1 have dumbbell shapes at large density. The dumbbell shapes are generated by rotating tori. The toroidal shapes indicate that the tensor correlations have near maximal strength at r<2 fm in all these nuclei. They provide new insights and simple explanations of the structure and electromagnetic form factors of the deuteron, the quasideuteron model, and the dp, dd, and αd L=2 (D-wave) components in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. The toroidal distribution has a maximum-density diameter of ∼1 fm and a half-maximum density thickness of ∼0.9 fm. Many realistic models of nuclear forces predict these values, which are supported by the observed electromagnetic form factors of the deuteron, and also predicted by classical Skyrme effective Lagrangians, related to QCD in the limit of infinite colors. Due to the rather small size of this structure, it could have a revealing relation to certain aspects of QCD. Experiments to probe this structure and its effects in nuclei are suggested. Pair distribution functions in other T,S channels are also discussed; those in T,S=1,1 have anisotropies expected from one-pion-exchange interactions. The tensor correlations in T,S=0,1 states are found to deplete the number of T,S=1,0 pairs in nuclei and cause a reduction in nuclear binding energies via many-body effects. © 1996 The American Physical Society.

Estimates for a weaker isovector potential than previously adopted are put forward. It is also strongly mass and energy dependent. The resulting isovector particle-hole interaction is weaker in lighter nuclei and the isovector quadrupole resonance should be located at lower energy than previously expected. The uncertainties arising from the separation of the potential asymmetry energy in nuclear matter calculations are discussed. © 1996 The American Physical Society.

The neutron and proton density distributions of the Na isotopes out to the proton and neutron drip lines are calculated in the spherical Hartree-Fock model with a wide variety of density-dependent Skyrme interactions. The results are compared to recent experimental data for the interaction cross sections and for the rms radii. We discuss the role of the deformed intruder states and the problems associated with predicting the location of the neutron drip line for this mass region. © 1996 The American Physical Society.

The experimental evidence presented recently by Casten and von Brentano, to support their suggestion that the lowest excited ${\mathit{K}}^{\mathrm{\pi }}$=$0^{+}$ states in deformed nuclei should be given a new interpretation as a phonon built on the γ vibration, is critically examined. It is argued that in the determination of B(E2,$0_{\mathrm{\beta }}^{+}$→$2_{\gamma }^{+}$) values from the higher-spin members of the β band to the γ band the rotational transitions caused by the mutual coupling of the β and γ bands have to be taken into account. An intensity limit is obtained for the $0_{\mathrm{\beta }}^{+}$→$2_{\gamma }^{+}$ γ transition in ${}_{}{}^{160}{}_{}{}^{}\mathrm{Dy}$ yielding B(E2,$0_{\mathrm{\beta }}^{+}$→$2_{\gamma }^{+}$)/B(E2,$0_{\mathrm{\beta }}^{+}$→$2_{\mathit{g}}^{+}$) s25. We conclude that there is no experimental evidence supporting the new interpretation of the β vibrations in the deformed rare-earth nuclei. © 1996 The American Physical Society.

Iterative techniques of the solution of the decoupling equation are used to construct various types of effective Hamiltonians in the framework of nonorthogonal transformations. A general and numerically convenient Hermitization procedure of the non-Hermitian Hamiltonians obtained by such techniques is proposed and the definition of effective operators with the related normalization problems is reviewed. © 1996 The American Physical Society.

We present a method to study spin magnetic dipole excitations in deformed nuclei within the quasiparticle random phase approximation based on self-consistent Hartree-Fock mean fields and residual interactions derived from the same effective two-body force. We perform a comprehensive study covering different Skyrme forces and various mass regions, and discussing the role of the mean field and of the residual interaction. An overall agreement with experimental data is obtained with the SG2 force. We study the systematics and the deformation dependence of the spin M1 strength distributions of ${\mathit{K}}^{\mathrm{\pi }}$=$1^{+}$ excitations. It is found for the first time that the summed spin M1 strength obeys a quadratic dependence on deformation in the two isotope chains studied, ${}_{}{}^{142,146,148,150}{}_{}{}^{}\mathrm{Nd}$ and ${}_{}{}^{144,148,150,152,154}{}_{}{}^{}\mathrm{Sm}$. © 1996 The American Physical Society.

The coupling between the nuclear shape dynamics and the internal degrees of freedom is investigated in the frame of a time-dependent Hartree-Fock-Bogolyubov Langevin formalism. The shape coordinate considered here is the quadrupole deformation, and the internal configuration space is restricted to the sd shell. The numerical results concern the effects of pairing, dissipation, and temperature on the giant quadrupole shape vibration of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. Time scales for decay, thermalization, and configuration transitions are obtained. © 1996 The American Physical Society.

According to the argument of excitation energy systematics, new spin values are assigned to the lowest observed states of the π${\mathit{h}}_{11/2}$⊗ν${\mathit{h}}_{11/2}$ bands for doubly odd nuclei ${}_{}{}^{124,126,130,132,134}{}_{}{}^{}\mathrm{La}$, ${}_{}{}^{130–134}{}_{}{}^{}\mathrm{Pr}$, ${}_{}{}^{136,138}{}_{}{}^{}\mathrm{Pm}$, and ${}_{}{}^{138}{}_{}{}^{}\mathrm{Eu}$, and possible new spin values are discussed for doubly odd nuclei ${}_{}{}^{120–130}{}_{}{}^{}\mathrm{Cs}$. Based on these new spin assignments, two systematic features of the π${\mathit{h}}_{11/2}$⊗ν${\mathit{h}}_{11/2}$ bands are revealed. First, the low spin signature of the π${\mathit{h}}_{11/2}$⊗ν${\mathit{h}}_{11/2}$ band is inverted for all doubly odd nuclei discussed in the present work. Second, the observed inversion spin increases with increasing neutron number for Cs and La isotopes. These features are in agreement with the recent systematic calculations based on the particle-triaxial-rotor model, with the inclusion of zero-range residual interaction between unpaired proton and neutron of Semmes and Ragnarsson, performed by Tajima. © 1996 The American Physical Society.

The energy-momentum tensor for isospin-symmetric semi-infinite nuclear matter (SINM) as well as for finite spherically symmetric nuclei is evaluated in the relativistic Thomas-Fermi (RTF) approximation of the σ-ω model. The ${\mathit{T}}_{33}$ stress tensor component of self-bound SINM vanishes identically. Our field theoretical RTF approach reflects the elementary relation p=2 σ/R between the bulk pressure p, the surface tension σ and the radius R of a nucleus. © 1996 The American Physical Society.

The Indiana Silicon Sphere 4π detector has been used to measure light-charged particles and intermediate-mass fragments (IMFs) emitted in the 18–4.8 GeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$${+}^{\mathrm{nat}}$Ag, ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ reactions. Ejectile multiplicity and total event kinetic energy distributions scale systematically with projectile energy and target mass, except for the ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ag}$ target at 3.6 and 4.8 GeV. For this system, a saturation in deposition energy is indicated by the data, suggesting the upper projectile energy for stopping has been reached. Maximum deposition energies of ∼950 MeV for the ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ag}$ target and ∼1600 MeV for the ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ target are inferred from the data. The results also demonstrate the importance of accounting for fast cascade processes in defining the excitation energy of the targetlike residue. Correlations between various observables and the average IMF multiplicity indicate that the total thermal energy and total observed charge provide useful gauges of the excitation energy of the fragmenting system. Comparison of the experimental distributions with intranuclear cascade predictions shows qualitative agreement. © 1996 The American Physical Society.

Multifragmentation of ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ag}$ and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ nuclei induced by 1.8–4.8 GeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ ions has been studied with the Indiana Silicon Sphere 4π detector array. Rapidity, moving source, and sphericity-coplanarity analyses are consistent with near-simultaneous emission from a source in approximate kinetic equilibrium. For the most dissipative collisions, the spectral peaks are broadened and shifted to very low energies, indicative of emission from an extended nuclear system with ρ/${\mathrm{\rho }}_0$∼1/3. Predictions of an intranuclear cascade/expanding, emitting source model compare well with experimental multiplicity distributions and the evolution of fragment spectral shapes. © 1996 The American Physical Society.

It is demonstrated experimentally that the reaction systems on the different side of Businaro-Gallone critical mass asymmetry (${\mathrm{\alpha }}_{\mathrm{BG}}$) have different characteristics in fusion-fission reactions. Fragment anisotropies resulting from the reactions with the entrance-channel mass asymmetry α≳${\mathrm{\alpha }}_{\mathrm{BG}}$ are well described by the saddle-point transition-state model. However, the measured fragment anisotropies for the systems with α<${\mathrm{\alpha }}_{\mathrm{BG}}$ are obviously greater than the predictions of this model at subbarrier and near-barrier energies and gradually tend to coincide with the theoretical expectations as the bombarding energy increasing over fusion barrier. These observations have led us to a suggestion of preequilibrium fission for low angular momentum. The predictions of such a preequilibrium fission model are compared with the available experimental data and it is shown that they can satisfactorily reproduce the observed trends as a function of the bombarding energy for the reaction systems studied. © 1996 The American Physical Society.

Results of the measurements of fusion cross sections and average angular momenta in the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ + ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ system, for center-of-mass energies ranging from 7 MeV below to 13 MeV above the Coulomb barrier are presented. The average angular momenta of the compound system were obtained using three independent methods, namely, the γ-ray multiplicity, the ratios of the evaporation residues, and the fusion excitation function. The transfer probabilities for various one- and two-particle channels have also been measured at a beam energy of 110 MeV, an energy much higher than the Coulomb barrier. The measured Q-integrated one- and two-proton transfer probabilities have been analyzed in terms of an earlier suggested semiclassical calculation taking into account the nuclear branch in addition to the usual Coulomb branch of the classical deflection function. It is seen that the ‘‘slope anomaly’’ in the measured one- and two-proton transfer probabilities as a function of the distance of closest approach can be explained within this framework. The strengths of the form factors, required in a coupled channel calculation of fusion, for the important transfer channels were obtained from the data within such a semiclassical analysis. The observed enhancement of the fusion cross sections and average angular momenta is discussed in terms of the coupled channel model using static deformation for the target, inelastic excitations in the projectile, and couplings to the transfer channels. The data are also compared with the neutron flow model suggested by Stelson. © 1996 The American Physical Society.

Measurements of inclusive (p,α) reactions on ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at incident energies between 120 and 200 MeV are compared with the trend expected from systematics and also with calculations based on a statistical multistep direct reaction theory. Although reasonable agreement is obtained between the experimental angular distributions and the predictions of the parametrized systematics at various emission energies, significant discrepancies are nevertheless observed. Calculations with the quantum-mechanical multistep formulation confirm that the higher-step contributions are more important at these incident energies than found in a previous study at lower projectile energies. © 1996 The American Physical Society.

A procedure to minimize ${\mathrm{\chi }}^2$ is described which explores the fact that the ${\mathrm{\chi }}^2$ distribution is of the fourth degree in the S-matrix elements. The fact that all three roots of the scale parameter for the minimum of ${\mathrm{\chi }}^2$ in its gradient direction are algebraically determined gives the present procedure some global features that previous methods did not contemplate. The automatic search procedure also preserves the unitary bound constraint of the S-matrix at every step. When the search in the gradient direction slows down, the procedure reverts to the traditional quadratic approximation with zero-order regularization. The method is applied to the elastic scattering of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${+}^{16}$O reaction near the Coulomb barrier. © 1996 The American Physical Society.

According to Stelson’s neutron flow model the threshold barrier is correlated to the onset of neutron flow. A systematic study of a large set of systems indicates that there exists a very good correlation between the threshold barrier and the intersurface distance after including the zero point motion of collective surface vibrations of the colliding nuclei in the prescription of Stelson. © 1996 The American Physical Society.

The perturbation of azimuthal α-α correlations by many-body Coulomb interactions with other emitted particles is investigated. Individual emissions are simulated by modeling instantaneous emission from the surface of a hot rotating gas. Upon emission, the particle trajectories are calculated by means of classical calculations which incorporate the many-body Coulomb interaction between the emitted particles and the emitting source. For high-multiplicity events, the initial azimuthal correlation between emitted α particles can be attenuated by final-state Coulomb interactions. The effect is most pronounced for α particles emitted close to the barrier, but it appears of minor importance for α particles emitted at large relative angles and at energies well above the Coulomb barrier. © 1996 The American Physical Society.

We investigate the accuracy of two methods used to approximate the exchange nonlocality for the case of neutrons scattering off ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$: the Taylor series expansion approach of Sinha and the zero range exchange approximation. These two approximations are evaluated analytically by methods previously employed in our microscopic folding calculations. The resulting local potential is compared to a phase-equivalent local potential that is derived from rigorously obtained S-matrix elements through the inversion method of Mackintosh, which, being very accurate, serves as our benchmark. The two approximations compare favorably to this inversion potential. The deviations in the potentials decrease with increasing energy, for both the real and imaginary parts. At 20 MeV the differences in the imaginary part are as large as 40% in the nuclear interior. At 50 MeV and 100 MeV the differences in the imaginary parts are comparable to those of the real parts: on the order of 10% far inside the nucleus and close to zero at the nuclear surface. The effects of these differences on the cross sections are far from negligible, as is also shown. © 1996 The American Physical Society.

The accuracy of the usual angle-averaging approximations involved in the solution of the nuclear matter Bethe-Goldstone equation is tested numerically in the case where self-energy effects are taken into account. It is found that the errors stemming from the additional angle averaging, which is needed to get rid of the ‘‘energy denominator’’ angular dependence, are much smaller than those already introduced by the Pauli operator angle averaging. These results apply to both the bound and scattering regimes. © 1996 The American Physical Society.

The entrance channel dynamics in a heavy-ion fusion reaction is followed by solving the Langevin equations of motion for the elongation and mass asymmetry degrees of freedom. Thermal diffusion over the conditional barrier in early stages of the dynamics is shown to result in a small number of fission-like reaction events having a memory of the entrance channel mass asymmetry. Evidence for such a component arising from the precompound nucleus phase of the reaction is shown to exist in the measured data on the yield of binary fragments in the reaction ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$${+}^{12}$C reported earlier. © 1996 The American Physical Society.