Core polarization charges of quadrupole transitions in light neutron drip line nuclei are studied by using a particle-vibration coupling model with Hartree-Fock single-particle wave functions and random phase approximation response functions in ${}_{}{}^{28}{}_{}{}^{}\mathrm{O}$. We obtain very small core polarization charges, ${\mathit{e}}_{\mathrm{pol}}$(IS)=0.14 and ${\mathit{e}}_{\mathrm{pol}}$(IV)=0.06. In addition to the smallness expected in general for light very-neutron-rich nuclei, a considerable amount of further reduction on the ${\mathit{e}}_{\mathrm{pol}}$ values arises from the cancellation between the contributions of low-energy threshold strength and those of giant resonances. It depends crucially on both the property of the one-particle multipole operator and the shell structure around the Fermi level whether the very-low-lying threshold strength contributes to the polarization charges of a given multipole constructively or destructively. © 1996 The American Physical Society.

For algebraic models the coherent states are appropriate trial wave functions to study the energy surfaces of the system. The equilibrium configurations of these functions are classified by means of the separatrix of the catastrophe formalism, which is defined by the bifurcation and Maxwell sets. The bifurcation sets correspond to curves in the parameter space associated to degenerate critical points while the Maxwell sets constitute the locus of points for which the energy surface takes the same value in two or more critical points. As an example we study the energy surfaces associated to the dynamical symmetries of the interacting boson model in the essential parameter space. © 1996 The American Physical Society.

We extend the mean-field approximation to relativistic quantum hadrodynamics to include pairing in nuclear matter. This approach permits a simultaneous description of the energy gap in the single-particle spectrum and of the saturation point of nuclear matter. The mean field associated with the pairing correlations has large scalar and vector components that cancel each other resulting in a gap parameter that agrees with the results of nonrelativistic calculations. © 1996 The American Physical Society.

We apply the generator coordinate method in a fully microscopic calculation of the E2 decay lifetime of the shape isomeric states of ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, and ${}_{}{}^{240}{}_{}{}^{}\mathrm{Pu}$. The basis states have been generated by constrained Hartree-Fock Bardeen-Cooper-Schrieffer calculations with the ${\mathrm{SkM}}^{\mathrm{*}}$ force. Triaxial quadrupole deformations have been included. Implications upon the decay lifetime of the quasidegeneracy of the isomeric state and the one-phonon excitation in the first well are examined. © 1996 The American Physical Society.

The even neutron (40≤N≤60) isotopes of Rb are studied in the relativistic mean field theory. The two most widely used sets of Lagrangian parameters NL1 and NL-SH are employed in the calculation. For a given odd-A nucleus the deformed single particle orbital consistent with its observed ground state spin is blocked in the calculation (both for prolate and oblate solutions). The calculated ground state total binding energies, two neutron separation energies, and deformations impressively agree with the experiment. The calculated isotopic shifts reproduce the observed anomalous trend in the neutron deficient Rb isotopes for both the sets NL1 and NL-SH. © 1996 The American Physical Society.

High-accuracy measurements of ${\mathit{A}}_{\mathit{y}}$(θ) data for elastic scattering for n${+}^{209}$Bi have been performed at 6 and 9 MeV. The data are incorporated into a large database of σ(θ), ${\mathit{A}}_{\mathit{y}}$(θ), and ${\mathrm{\sigma }}_{\mathit{T}}$ for n${+}^{209}$Bi covering the energy range 1.0–80 MeV. A complementary database is constructed for n${+}^{208}$Pb and a dispersive optical-model analysis is performed for both scattering systems while constraining many of the parameters to be identical for both systems. A good representation of both databases is obtained with conventional geometry and spin-orbit parameters. The ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ model predicts quite well the measured energies of valence single-particle and single-hole bound states. Occupation probabilities and spectroscopic factors for the same bound states are also calculated. Finally, a fully constrained model is presented in which the only differences between the n${+}^{208}$Pb and the n${+}^{209}$Bi systems are the Fermi energy and the isospin dependence in the real volume potential. © 1996 The American Physical Society.

High-energy γ rays from targets of Be, C, and Al bombarded with 65 MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ ions have been measured by the use of a γ-ray detector system consisting of seven ${\mathrm{BaF}}_2$ scintillators. The energy spectra were obtained up to the maximum energy kinematically permitted in each collision at detection angles of 35°–144°. The experimental cross sections are compared with calculations of the potential bremsstrahlung on which the theory has been developed by Nakayama and Bertsch. It is shown that the prediction of potential bremsstrahlung can well reproduce the production cross sections of γ rays of energy near the kinematical maximum energy in collisions, while this result is contrary to the previous one of Tam et al. in α and d bombardments. © 1996 The American Physical Society.

Within the framework of classical molecular dynamics, we study the collision Au+Au at an incident energy of 35 MeV/nucleon. It is found that the system shows a critical behavior at peripheral impact parameters, revealed through the analysis of conditional moments of charge distributions, Campi scatter plot, and the occurrence of large fluctuations in the region of the Campi plot where this critical behavior is expected. When applying the experimental filters of the MULTICS-MINIBALL apparatus, it is found that criticality signals can be hidden due to the inefficiency of the experimental apparatus. The signals are recovered by identifying semiperipheral and peripheral collisions looking to the velocity distribution of the largest fragment, and selecting the most complete events. © 1996 The American Physical Society.

The neutron total cross section of ${}_{}{}^{124}{}_{}{}^{}\mathrm{Sn}$ has been measured over the energy range 0.014–0.315 MeV. An R-matrix analysis has been performed to obtain resonance and average parameters which provide for a complete representation of the neutron entrance channels for the ${\mathit{s}}_{1/2}$, ${\mathit{p}}_{1/2}$, and ${\mathit{p}}_{3/2}$ contributions. The s- and p-wave neutron strength functions have been determined (for a channel radius of 7.23 fm) to be 0.12±0.03 and 1.8±0.2, respectively (in units of ${10}^{\mathrm{-}4}$). Limits are placed on the average level spacings and strength functions for the individual partial-wave components. The number of definite ${\mathit{p}}_{1/2}$ levels exceeds the ${\mathit{p}}_{3/2}$ levels by the factor 2 and their average strengths differ by the factor 4. s- and p-wave potential scattering radii have been determined to be 6.4±0.2 fm and 10.5±0.3 fm, respectively. Average scattering functions, deduced from the average parameters, have been used to determine the real well depth of an optical-model potential which reproduces these functions. There is evidence of an angular momentum dependence of the real-well potential depth and the level spacings. © 1996 The American Physical Society.

The energy at which the collective transverse flow in the reaction plane disappears, the balance energy ${\mathit{E}}_{\mathrm{bal}}$, is found to increase linearly as a function of impact parameter for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$${+}^{45}$Sc reactions. Comparison of our measured values of ${\mathit{E}}_{\mathrm{bal}}$(b) with predictions from quantum molecular dynamics (QMD) model calculations agrees better with an approach incorporating momentum dependence in the nuclear mean field. © 1996 The American Physical Society.

Angular correlation techniques have been used to determine the spin of a strong resonance observed in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${+}^{12}$C($3^{\mathrm{-}}$;9.64 MeV) inelastic scattering channel, at a bombarding energy of 33.5 MeV in the center-of-mass system. The alpha particles produced in the sequential decay ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($3^{\mathrm{-}}$)${\to }^8$Be(g.s.) + ${\mathrm{\alpha }}_0$ were detected using four double-sided silicon strip detectors. The data are consistent with a spin assignment of ${\mathit{J}}^{\mathrm{\pi }}$=${18}^{+}$ for this resonance. The current results are compared to calculations of resonance behavior in this system from the band crossing model. © 1996 The American Physical Society.

Antiproton-induced fission has been investigated with a novel double-arm fission fragment spectrometer. The correlations in mass, energy, and velocity between two fragments were measured. The dependence of total kinetic energy, velocity, momentum of the fissioning nucleus, and momentum of the fission fragments on the mass loss was deduced and analyzed in the framework of the dynamical statistical model. This model takes into account all stages before, during and after p¯-induced fission (atomic cascade, intranuclear cascade, evaporation cascade, fission of the compound nucleus, evaporation from the fission fragments). The mass loss was used as a measure of the excitation energy to classify the fission events according to the corresponding excitation energies. Some discrepancies between the model calculation and the experiment show the important role of dissipative effects in the p¯-induced fission process. © 1996 The American Physical Society.

Elastic ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ + ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ cross sections are measured at center-of-mass energies of 8.15, 10.0, and 14.75 MeV, using a radioactive ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ beam and position-sensitive solid-state detectors. The ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ + ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ angular distributions display oscillations beyond 80° and even a significant rise at large angles for the two higher energies. These data are analyzed in the framework of the optical model including a parity-dependent term necessary to explain the backward behavior. This parity term is among the strongest encountered in heavy-ion collisions. Its properties are close to those of the parity term reproducing the ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ elastic scattering in the same energy range. Its long-range nature indicates that the parity dependence arises from a single rather than from a double exchange. One-pion charge exchange between the valence nucleons seems to possess the necessary properties as qualitatively shown by a simple molecular model. © 1996 The American Physical Society.

The ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(α${,}^3$He${)}^5$He(g.s.) reaction was studied at incident laboratory energies of 158 and 200 MeV for c.m. scattering angles ranging from ∼16° to 100°. The shape of the ${}_{}{}^5{}_{}{}^{}\mathrm{He}$(g.s.) peak in the measured ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ energy spectra is well reproduced by distorted-wave Born approximation calculations. Whereas calculations utilizing six-parameter optical potentials yield good qualitative agreement with the measured angular distributions, unacceptably poor agreement is found when nine-parameter potentials are utilized. Absolute normalization with ${\mathit{D}}_0^2$ strengths extracted from corresponding zero-range and finite-range calculations overpredicts the measurements by factors of ∼2 to 3. © 1996 The American Physical Society.

Statistic approach based on the kinetics of a first-order phase transition is proposed for the description of multifragmentation phenomenon. We start from the thermalized gaslike system of nucleons and calculate the fragmentation yields according to classical nucleation theory. The fragmentation distributions are derived from the steady-state solution of a Fokker-Planck equation for the distribution function. Curvature energy and Coulomb energy in the form provided by the Wigner-Seitz approximation are included into the fragment formation energy. The fit to experimental data is performed. It is shown that the Coulomb interaction may lead to the deviation of the fragmentation yields from the power-law dependence. This prediction may be verified experimentally in heavy-ion central collisions at the energies about 40A MeV. © 1996 The American Physical Society.

Pi meson interactions with free nucleons as known from recent experiments have been fit by Arndt et al. in the form of phase shifts. In the present work these phase shifts are fit by a simple parametrization, and observables computed from the resulting parametrized phase shifts are compared successfully to important examples of recent measurements. The parameters of this work are readily suited to pion-nucleus optical model codes built upon the impulse approximation. Some examples of the effect of the new pion-nucleon data upon these pion-nucleus calculations are shown, relative to older results based upon far fewer pion-nucleon data. © 1996 The American Physical Society.

Elastic and inelastic scattering data of 1.37 GeV α particles on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{40,42,44,48}{}_{}{}^{}\mathrm{Ca}$ are analyzed within the framework of the Glauber theory. Collective excitations to one-phonon levels are treated using the Tassie model. The effect of the coupling between the elastic and inelastic channels is considered. It is shown that a phase variation of the nucleon-nucleon elastic scattering amplitude leads to a large increase in the calculated differential cross section. The presence of a phase variation leads to a substantial improvement. © 1996 The American Physical Society.

In this paper we address the adequacy of various approximate methods of including Coulomb distortion effects in (e,${\mathit{e}}^{\prime }$) reactions by comparing to an exact treatment using Dirac-Coulomb distorted waves. In particular, we examine approximate methods and analyses of (e,${\mathit{e}}^{\prime }$) reactions developed by Traini et al. using a high energy approximation of the distorted waves and phase shifts due to Lenz and Rosenfelder. This approximation has been used in the separation of longitudinal and transverse structure functions in a number of (e,${\mathit{e}}^{\prime }$) experiments including the newly published ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$(e,${\mathit{e}}^{\prime }$) data from Saclay. We find that the assumptions used by Traini and others are not valid for typical (e,${\mathit{e}}^{\prime }$) experiments on medium and heavy nuclei, and hence the extracted structure functions based on this formalism are not reliable. We describe an improved approximation which is also based on the high energy approximation of Lenz and Rosenfelder and the analyses of Knoll and compare our results to the Saclay data. At each step of our analyses we compare our approximate results to the exact distorted wave results and can therefore quantify the errors made by our approximations. We find that for light nuclei, we can get an excellent treatment of Coulomb distortion effects on (e,${\mathit{e}}^{\prime }$) reactions just by using a good approximation to the distorted waves, but for medium and heavy nuclei simple additional ad hoc factors need to be included. We describe an explicit procedure for using our approximate analyses to extract so-called longitudinal and transverse structure functions from (e,${\mathit{e}}^{\prime }$) reactions in the quasielastic region. © 1996 The American Physical Society.

Isoscalar collective modes in a relativistic meson-nucleon system are investigated in the framework of the time-dependent Thomas-Fermi method. The energies of the collective modes are determined by solving consistently the dispersion relations and the boundary conditions. The energy weighted sum rule satisfied by the models considered allows the identification of the giant resonances. The percentage of the energy weighted sum rule exhausted by the collective modes is in agreement with experimental data, but the agreement with the energy of the modes depends on the model considered. © 1996 The American Physical Society.

Average quantities related to the characteristics of the nucleon-nucleon (NN) interaction along the trajectory of the projectile in preequilibrium reactions are calculated by using the semiclassical method to follow the incoming particle’s path in the nuclear target. The radial dependences of the nucleon’s mean free path and the probability for the first NN collision have pointed out the surface character of the first NN interaction in multistep reactions even at low energies. In the local density approximation an average Fermi energy and an average strength of the effective NN interaction ${\mathit{V}\mathit{¯}}_0$ along the trajectory of the incident nucleon are obtained with respect to both the nuclear density and the first NN-collision probability. Good agreement is found between the average strengths obtained with the Hartree-Fock potential plus the dispersive component and by using the parametrization based on the Brueckner-Hartree-Fock nuclear matter calculations. It is also shown that the nuclear-density dependence of the effective NN interaction may account for the low-energy phenomenological ${\mathit{V}}_0$ values which are much more increased in comparison with any prediction. © 1996 The American Physical Society.