We compute the vacuum polarization correction to the binding energy of nuclear matter in the Walecka model using a nonperturbative approach. We first study such a contribution as arising from a ground-state structure with baryon-antibaryon condensates. This yields the same results as obtained through the relativistic Hartree approximation of summing tadpole diagrams for the baryon propagator. Such a vacuum is then generalized to include quantum effects from meson fields through scalar-meson condensates which amounts to summing over a class of multiloop diagrams. The method is applied to study properties of nuclear matter and leads to a softer equation of state giving a lower value of the incompressibility than would be reached without quantum effects. The density-dependent effective σ mass is also calculated including such vacuum polarization effects.

The particle-plus-rotor model was employed to study the fine structure seen in the $\alpha$ decay of even-even neutron-deficient nuclei in the Hg-Po region. The configuration mixing resulting from the shape coexistence between well-deformed prolate bands and spherical (or quasirotational oblate) structures in the daughter nuclei was considered. Experimental $\alpha$-decay branching ratios are reproduced within one order of magnitude, except for the case of ${}^{180}$Hg, where the daughter nucleus ${}^{176}$Pt is expected to be triaxial in its ground state. The effect of configuration mixing on the relative intensities is discussed in detail, together with the sensitivity of results to the choice of the $\alpha$-nucleus optical model parameters.

Single-particle overlap functions and spectroscopic factors are calculated using the asymptotic behavior in coordinate space of the one-body density matrix corresponding to a many-body wave function in correlated basis function theory. We include state-dependent correlation functions and discriminate between the effects of central, spin-spin, and tensor correlations. The method is applied to ${}^{16}$O. We also discuss the effect of center-of-mass motion on the calculated spectroscopic factors.

Ground state properties of light isotopes from helium to oxygen, such as binding energies, spin parities, rms radii, and halo phenomena are studied systematically by the Skyrme Hartree-Fock model with fit $E_{\sigma }$ as the effective interaction. Since the particle number of the studied system is rather small, pairing interactions between identical nucleons are treated with the particle number conservation method. The results of the calculation show that most of the ground state properties can be well described. Some unsuccessful points of the calculation are pointed out.

In the present work the equation of state (EOS) is derived using three types of potentials for polarized nuclear matter. The potentials used are the extended Seyler-Blanchard (SB), modified Seyler-Blanchard (MSB) and the generalized Seyler-Blanchard (GSB) potentials. It is found that the equation of state derived using SB potential is a stiff EOS whereas the equations of state derived using MSB and GSB potentials are soft ones. The phase diagram for nuclear matter is also studied. The shapes are similar for the three potentials used but the critical temperatures are slightly different.

The energy and angular distribution of secondary neutrons from the interaction of 14-MeV neutrons with sodium and natural lead were investigated. Absolute double-differential neutron emission cross sections were measured for reaction angles from 30° to 150° in steps of 15° in the secondary neutron energy range 2–14 MeV using the time-of-flight facility at the Institut für Radiumforschung und Kernphysik, Vienna. Systematic uncertainties in these measurements could be reduced to less than 3–5% (depending on secondary neutron energy) resulting in total uncertainties of less than 5% for a large part of the studied range in angle and energy for Pb and of less than 8% for Na. These results agree with previous measurements within the relatively large uncertainties of the latter. The measured cross sections were compared to the results of statistical model calculations using the quantum mechanical theory of Feshbach, Kerman, and Koonin for preequilibrium emission and also with calculations based on the exciton model and the Kalbach-Mann systematics for the shape of the angular distributions. For compound nucleus decay, the Hauser-Feshbach model was used in both cases. Very good agreement was found with both predictions in the case of lead; in the case of sodium, noticeable discrepancies do show the limits of applicability of any statistical model for light nuclei.

Coulomb excitation measurements on ${}^{187}$Os and ${}^{189}$Os have been carried out, using beams of $\alpha$ particles, with the aim of measuring $B\left(E2\mathrm{}\right)\mathrm{}$ values for transitions between states in the low-lying bands. The results are compared with predictions in a pseudospin symmetry scheme and, in the case of ${}^{189}$Os, are used to extract two-state mixing amplitudes for states in near degenerate bands.

We study the mechanism of intermediate energy heavy-ion collisions in a picture of tripartition of the total system, using the framework of statistical simultaneous multifragmentation model taking into account the entrance channel characteristics, with no other parameter except density. The relative excitations of the three parts and their contributions to the physical observables such as mean multiplicity for different fragments and total fragment multiplicity are shown as functions of entrance channel properties; namely, the impact parameter, the bombarding energy, and the mass of projectile and target nuclei. As an application we apply the model to describe the data on ${}^{40}$Ar${+}^{40}$Ca and ${}^{40}$Ar${+}^{197}$Au reaction with incident energy varying from 42 to 137 MeV/nucleon, and the intermediate mass fragment production in the ${}^{197}$Au${+}^{197}$Au reaction with incident energies 100 and 400 MeV/nucleon. The importance of the contributions from the spectator parts is clearly demonstrated. It is also shown that the role of the interfragment interaction is important up to bombarding energy 100 MeV/nucleon and a temperature of about 10 MeV.

The particle production in the intermediate-energy heavy-ion collisions is discussed in the framework of the nonequilibrium Green's functions formalism. The evolution equations of the Green's functions for fermions allows for the discussion of the off-shell fermion propagator and of the large momentum component in the initial state. For the case of a homogeneous system numerical calculations of the meson production rate are performed and compared with the semiclassical production rate.

The volume integrals and root-mean-square (rms) radii of the real and imaginary parts of the ambiguous sets of optical potentials that equally fit the differential cross sections for the elastic collisions between ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$ are evaluated when the internal region, where the optical potential is ill defined, is removed from the integrals used in their calculation. It is found that the relative dispersion of the values of these quantities around their means for each set takes the least values when the corresponding integrals are evaluated with a lower cutoff at the rms radius of the imaginary potential when the incident energy is less than 35 MeV/nucleon, and at the rms radius of the real potential at higher incident energies.

We present a simple model for the $\gamma N\to \pi \pi N$ reaction which reproduces the cross sections of the ${\pi }^{+}{\pi }^{-}p,\hspace{0.5em}{\pi }^{+}{\pi }^{-}n,\hspace{0.5em}{\pi }^{+}{\pi }^{0}n$, and ${\pi }^{-}{\pi }^{0}p$ channels over the range of the energies 0.41–0.85 GeV. We use the dynamical model for the resonances $\Delta \mathrm{}\left(1232\right)\mathrm{}$, and $N^{*}$(1520) and the $\rho$ meson. The total photoabsorption off a nucleon is also discussed.

Four-body distorted wave theory appropriate for nucleon-nucleus reactions leading to three-body continuum excitations of two-neutron Borromean halo nuclei is developed. The peculiarities of the halo bound state and three-body continuum are fully taken into account by using the method of hyperspherical harmonics. The procedure is applied for $A=6\mathrm{}$ test-bench nuclei; thus we report detailed studies of inclusive cross sections for inelastic ${}^6$He${(p,p}^{\prime }{)}^6$He${}^{*}$ and charge-exchange ${}^6$Li${\mathrm{}(n,p)\mathrm{}}^6$He${}^{*}$ reactions at nucleon energy 50 MeV. The theoretical low-energy spectra exhibit two resonancelike structures. The first (narrow) is the excitation of the well-known $2^{+}$ three-body resonance. The second (broad) bump is a composition of overlapping soft modes of multipolarities $1^{-}{,2\mathrm{}}^{+}{,1\mathrm{}}^{+}{,0\mathrm{}}^{+}$ whose relative weights depend on transferred momentum and reaction type. Inelastic scattering is the most selective tool for studying the soft dipole excitation mode.

We show that, in contrast to the case of elastic scattering of spinless particles from composite targets, Klein-Gordon dynamics is not a natural choice for the description of their inelastic scattering in the standard coupled channels method. We propose one way to overcome the difficulty, showing that Klein-Gordon dynamics may be used in practical calculations within the coupled channels framework provided that the optical potential is modified in a straightforward manner.

A recent analysis of multifragmentation data in terms of an elementary binary decay appears to imply that the decay is sequential and from a thermalized source. This last feature would imply a lack of sensitivity to dynamical effects, at least in this multifragmentation data. We discuss this analysis using a simultaneous statistical multifragmentation model (SMM), which assumes decay from a thermalized source, and also employing a molecular dynamics model (MDM). The elementary binary decay analysis of the SMM predictions yields results which are qualitatively similar to those of the data. The differences are shown to stem from superposition of processes associated with very different impact parameters. Thus the data are not inconsistent with simultaneous decay. The MDM calculations, on the other hand, disagree with the data. We suggest, however, that more reliable dynamical calculations are needed before accepting an absence of dynamical effects in these data.

We discuss different schemes devised to extend the eikonal approximation to the regime of low bombarding energies (below 50 MeV per nucleon) in heavy-ion collisions. From one side we consider the first- and second-order corrections derived from Wallace's expansion. As an alternative approach we examine the procedure of accounting for the distortion of the eikonal straight-line trajectory by shifting the impact parameter to the corresponding classical turning point. The two methods are tested for different combinations of colliding systems and bombarding energies, by comparing the angular distributions they provide with the exact solution of the scattering problem. We find that the best results are obtained with the shifted trajectories, the Wallace expansion showing a slow convergence at low energies, in particular for heavy systems characterized by a strong Coulomb field.

The effect of nonlocality on heavy ion fusion is discussed at sub-barrier energies. It is shown that a simple barrier penetration model with a nonlocal ion-ion potential cannot reproduce the coupled channel results, though it gives rise to fusion enhancement at low energies. It is further shown that the coupled channel effects are important for fusion around the Coulomb barrier energies whereas the nonlocal effects give a large cross section enhancement at deep sub-barrier energies.

Experiment E878 at the BNL-AGS has measured the invariant cross sections of antiprotons produced near $p_t=0\mathrm{}$ in interactions of 10.8 GeV/$c$ Au beams with targets of Al, Cu, and Au. The data were measured for a wide range of centralities and rapidities using a focusing beamline spectrometer and a high-rate centrality detector. We compare our data with the predictions of simple models and sophisticated transport models to explore the physics of antiproton production and annihilation.

The isotopic production cross sections for ${}^{40}\mathrm{Ca}$ projectiles at 357, 565, and 763 MeV/nucleon interacting in a liquid hydrogen target have been measured by the Transport Collaboration at the LBL HISS facility. The systematics of these cross sections are studied, and the results indicate that nuclear structure effects are present in the isotope production process during the relativistic collisions. The newly measured cross sections are also compared with those predicted by semiempirical and parametric formulas, but the predictions do not fully describe the systematics such as the energy dependence. The consequences of the cross section systematics in galactic cosmic ray studies are also discussed.

In previous work, the E802 Collaboration at the BNL-AGS used negative binomial distribution (NBD) fits to charged particle multiplicity distributions from central collisions of ${}^{16}$O+Cu at $14.6A$ GeV/$c$ to derive the two-particle short-range rapidity correlation length and strength. These turned out to be much shorter and weaker than the values for hadron collisions, which led to a simple and elegant explanation of intermittency. In the present work, a direct measurement of the two-particle correlation of identified pions in the E802/E859 magnetic spectrometer is performed in the interval $1.5<~y<~2.0$ for central ${}^{28}$Si+Au collisions, both in terms of $Q_{\mathrm{inv}}=\sqrt{|q\to {|}^2-q_0^2}$, where ${q=p}_2-p_1=(q\to {,q}_0)$, and also in terms of $\left|{\eta }_2-{\eta }_1\right|$ and ${|y}_2-y_1|$, where $p,\eta ,$ and $y$ are the four-momentum, pseudorapidity, and rapidity of the pions. It is demonstrated that the two-pion correlation in rapidity (and pseudorapidity) is entirely due to the Bose-Einstein interference. The directly measured correlation length in both $\eta$ and $y$ is $\xi =0.19\mathrm{}\pm 0.03$ for two ${\pi }^{-}$, with strength $R(0,0)\mathrm{}\sim 1$%, in agreement with the previous E802 indirect measurements derived from the NBD analysis of intermittency.

The influence of the nuclear Coulomb field on transverse spectra of ${\pi }^{+}$ and ${\pi }^{-}$ measured in Pb+Pb reactions at $158A$ GeV has been investigated. Pion trajectories are calculated in the field of an expanding fireball. The observed enhancement of the ${\pi }^{-}/{\pi }^{+}$ ratio at small momenta depends on the temperature and transverse expansion velocity of the source, the rapidity distribution of the net positive charge, and mainly the time of the freeze-out.