The symmetric and near-symmetric mass fission yields from the ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{24}$Mg reaction have been studied in a particle-particle-γ coincidence measurement. Evidence is presented for a selective population of states in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ fragments arising from the symmetric fission of the ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ compound nucleus. A statistical-model calculation of the expected strength to specific mutual excitations of the fission fragments is presented and compared to the experimental results. This calculation is found to describe the structures observed at high excitation energy in the fission Q-value spectra quite well. Analysis of the γ-ray spectra indicates, however, that a specific set of states in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, corresponding to a highly deformed prolate band, is populated more strongly than expected based on a purely spin-weighted, statistical decay of the compound nucleus. It is suggested that the population pattern of states in the fission fragments may reflect nuclear structure effects at the point of scission.

The fully energy-damped cross sections of the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$${+}^{24}$Mg reaction at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=44.4 MeV have been measured for all of the major fission channels. High-resolution Q-value spectra have been obtained for the large-angle yields in the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$${+}^{24}$Mg and ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$${+}^{28}$Si channels. Calculations based on the transition-state model are found to reproduce the fully damped cross sections in all of the observed mass channels. The pronounced structure that is observed in the excitation-energy spectra for the more symmetric mass channels, even for the strongly damped yields, is shown to be qualitatively reproduced by assuming a spin-weighted population of the fragment states. There is no evidence, however, that the structure of the nascent fission fragments at scission may influence the population of states in the fragments. These results, taken together with earlier measurements of the resonance behavior of this system, suggest the coexistence of fission from the normal, compact compound nucleus with that from the deformed configurations believed to be responsible for the resonance behavior.

An investigation of energy deposition mechanisms in heavy-ion reactions at intermediate energies is presented. Theoretical simulations are performed in the framework of the semiclassical Landau-Vlasov model. They emphasize the influence of the initial nonequilibrium conditions, and the connection with the incident energy is discussed. Characteristic times involved in the energy thermalization process and finite size effects are analyzed.

Starting with the elementary amplitudes having spin-flip and spin-nonflip interactions for ${\mathrm{\pi }}^{+}$n→Λ${\mathit{K}}^{+}$, hypernuclear production cross sections and polarizations have been estimated in detail for the (${\mathrm{\pi }}^{+}$,${\mathit{K}}^{+}$) reaction on available p-shell nuclear targets ${(}^{10,11}$B, ${}_{}{}^{12,13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$). The experimental excitation spectra of ${}_{\mathrm{\Lambda }}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{\mathrm{\Lambda }}{}^{13}{}_{}{}^{}\mathrm{C}$, and ${}_{\mathrm{\Lambda }}{}^{16}{}_{}{}^{}\mathrm{O}$, including the typical cross section data, are satisfactorily reproduced in the distorted wave impulse approximation with the configuration-mixed wave functions. The angular dependence of hypernuclear polarization is predicted for each typical state, showing, for example, that the use of the mixed wave functions changes appreciably the previous estimate with the simple one-particle–one-hole wave function for ${}_{\mathrm{\Lambda }}{}^{12}{}_{}{}^{}\mathrm{C}$. The Λ-spin polarization in nuclear medium is also estimated. Typical states are shown to have large polarizations at the hypernuclear production stage.

The geometry-dependent hybrid model of preequilibrium nuclear reactions is used together with the droplet model description of neutron and proton densities to investigate the effect of excess surface neutrons on the nucleon emission spectra. It is found that in alpha-particle–induced reactions the effect of neutron-skin on nucleon emission spectra is negligible. For nucleon-induced reactions the effect is more significant but not strong enough to remove the discrepancies between calculation and experiment.

The existing up to data global data on angle integrated energy spectra (AIES) and energy integrated angular distributions (EIAD) of protons from (n,p) reactions at 14 MeV nominal neutron energy have been analyzed to develop systematics in relative preequilibrium contributions (RPC). The experimental estimates of RPC were obtained either from asymmetric components of the extrapolated EIAD or from the analysis of AIES, by comparison with equilibrium contributions obtained from the Hauser-Feshbach model. The experimentally obtained RPC have been compared with those expected from recent Feschbach-Kerman-Koonin theory as extended by Kalbach et al. as well as with theoretical predictions by Brown and Muirhead. The mass number systematics of RPC manifest shell and even-odd effects. The fraction of the precompound nucleus decays obtained from RPC, using the Blann-Griffin model, is constant and small for A≥72, and increases to about 1 for small A’s. The analysis with the Kalbach model yields the dependence of the level density parameters ${\mathit{a}}_1$ and ${\mathit{a}}_2$ on A; also values of ${\mathit{a}}_1$ and ${\mathit{a}}_2$ manifest prominent structures near closed shell nuclei. The values of ${\mathit{a}}_1$ for compound nucleus are systematically higher than ${\mathit{a}}_2$, for the residual nuclei by an amount much larger than expected by usual A dependence, indicating excitation energy dependence of the single particle level density g. The excitation energy dependence of g when explored in the light of Rosenzweig’s positive power law, with parameters n and k, reveals that for most cases 1.2≤n≤1.5, with no systematic dependence of n on A, compared to reported theoretical values of n≃1.6 by Rosenzweig, and 0.8≤k≤3.8 with a trend of slow increase of k with A.

The recent accurate measurements of fusion cross sections at subbarrier energies have enabled extraction of barrier distributions which could provide a novel test for collective models. We carry out a systematic study of subbarrier fusion in rare-earth nuclei using a scheme which incorporates the interacting boson model for nuclear structure effects. The existing data on vibrational and rotational nuclei are fitted with a consistent set of parameters which are then used to predict the fusion cross sections and distribution of barriers in transitional nuclei. Contrary to vibrational and rotational nuclei where the cross sections increase smoothly with increasing deformation or mass number, transitional nuclei exhibit sharp changes in the barrier distribution due to shape-phase transitions. An accurate measurement of fusion cross sections for Pt and Os nuclei could thus provide a sensitive test for competing models in this region.

Resonance energy pion-nucleon elastic scattering in the ${\mathit{S}}_{11}$ channel is modeled by assuming that the pion plus nucleon couple to two resonances, the resonances couple to inleastic channels, but there is no direct coupling of the pion-nucleon channel to the inelastic channels. The model is solved by matrix N/D methods. The coupling of the inelastic channels to the elastic channel is taken directly from data. Using form factors from the constituent quark model, we find the model is able to reproduce the experimentally determined pion-nucleon phases in the ${\mathit{S}}_{11}$ channel over the resonance region and that the resonance part of the amplitude is negligible at low energies.

Local phase equivalent interactions to the nonlocal Kerman, McManus, and Thaler multiple scattering nucleon-nucleus optical potentials are determined. Both first- and second-order contributions in the free nucleon-nucleon transition amplitude are included. The second-order terms modify the derived potentials in a nontrivial way and reduce the strnegths of the real and imaginary central potentials in the nuclear interior. The nonlocality of the first- and second-order multiple scattering potentials is quantified by the evaluation of an associated Perey factor. Calculations give a mean free path of λ=3–4 fm for 100–200 MeV incident nucleons on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$.

The imaginary part of optical potentials for fixed exciton number to be used for preequilibrium reactions is microscopically calculated. The semiclassical approximation and a simple delta function type interaction are adopted. Only the lowest order processes are taken into account, and the incident particle is restricted to nucleons. Five types of processes contribute to the imaginary potential; the first is the creation of a particle-hole pair by the incident nucleon. This process is considered in usual optical potentials, but in this work the intermediate states are restricted to Q space. The imaginary parts corresponding to the other four processes are obtained in analytic forms, to which the effects of particle-hole states in the target nucleus are taken into account. The resultant imaginary parts are, with good accuracy, independent of the excitation energy and related to the exciton number in a very simple way. The transmission coefficients are calculated using obtained imaginary parts and compared with the ones based on conventional imaginary potentials. The effect of inelastic scattering before absorption is studied in a perturbation expansion and the second-order terms are found to be comparable with the zeroth-order terms.

Motivated by recent experiments we have studied ${\nu }_{\mathit{e}}$ and ${\nu }_{\mathrm{\mu }}$ induced charged- and neutral-current reactions on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ within the continuum random phase approximation employing two different residual interactions. We find good agreement with the measured cross sections for the ${\nu }_{\mathit{e}}$ reactions, while our calculation signficantly differs from recently measured ${\nu }_{\mathrm{\mu }}$ cross sections. Our calculation indicates that the measurements of the total ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\nu }_{\mathit{e}}$,${\mathit{e}}^{\mathrm{-}}$${)}^{12}$${\mathrm{N}}^{\mathrm{*}}$ cross sections are consistent with the total ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(μ,${\nu }_{\mathrm{\mu }}$${)}^{12}$${\mathrm{B}}^{\mathrm{*}}$ capture rate.

Relativistic heavy-ion colliders like the Brookhaven Relativistic Heavy Ion Collider or CERN Large Hadron Collider open up the possibilities for γγ physics. This provides tests of the standard model, possible extensions and especially QCD in the resonance and the continuum regime. The model independent quantity to study these processes is the γγ luminosity function due to the experimentally limited detector acceptance. Due to irreducible γ-hadron interactions, there are serious background problems at such colliders. We discuss this background due to γA and especially γg reactions.

We study the space-time dependence of muon pair production from dense, nonequilibrium pion matter in the framework of a Boltzmann type transport model which properly includes effects of Bose statistics. Starting from an initially supersaturated pion gas with a large positive effective chemical potential we find that this pion chemical potential stays approximately constant during the evolution. As a consequence, such a scenario leads to an increased dilepton yield near the kinematical threshold for muon pair production via pion annihilation. Depending on the lifetime of the dense hadron matter formed in relativistic heavy ion collisions, this component can be observed above the background of muon pairs from resonance decays and the ρ,ω bump. In such a way the measurement of the dimuon invariant mass spectrum could help to understand the initial state and the dynamical evolution of ultrarelativistic heavy ion collisions.

Antiproton production in nucleus-nucleus collisions at energies below the threshold for its production from the nucleon-nucleon interaction in free space is studied in the relativistic Vlasov-Uehling-Uhlenbeck model. The antiproton self-energies in the medium are obtained in the mean-field approximation from the G-parity transformation of the nucleon self-energies. Due to the decrease of antiproton mass in the medium as a result of the attractive scalar mean field, there is an enhanced production of primordinal antiprotons which are, however, mostly annihilated by nucelons as they propagate through the matter. The calculated final antiproton momentum spectrum is seen to agree reasonably with the experimental data.

We systematically study the NN→baryon+baryon quark-exchange terms. The effective numbers for the different baryon-baryon components in the deuteron are calculated.

We derive closed form kaon-nucleon scattering amplitudes using the ‘‘quark Born diagram’’ formalism, which describes the scattering as a single interaction (here the OGE spin-spin term) followed by quark line rearrangement. The low energy I=0 and I=1 S-wave KN phase shifts are in reasonably good agreement with experiment given conventional quark model parameters. For ${\mathit{k}}_{\mathrm{lab}}$>0.7 GeV, however, the I=1 elastic phase shift is larger than predicted by Gaussian wave functions, and we suggest possible reasons for this discrepancy. Equivalent low energy KN potentials for S-wave scattering are also derived. Finally we consider OGE forces in the related channels KΔ, ${\mathit{K}}^{\mathrm{*}}$N, and ${\mathit{K}}^{\mathrm{*}}$Δ, and determine which have attractive interactions and might therefore exhibit strong threshold enhancements or ‘‘${\mathit{Z}}^{\mathrm{*}}$-molecule’’ meson-baryon bound states. We find that the minimum-spin, minimum-isospin channels and two additional ${\mathit{K}}^{\mathrm{*}}$Δ channels are most conducive to the formation of bound states. Related interesting topics for future experimental and theoretical studies of KN interactions are also discussed.

We calculate the vector-isovector hadronic current correlation function in the presence of nuclear matter. We find that the rho mass decreases by about 20% at nuclear matter density and the rho width is reduced by about 35%. These results are in general accord with the results of calculations made using QCD sum rules. In our calculations, and in the QCD sum-rule calclations, the main effect considered is reduction of the value of the quark condensate in the presence of matter. That reduction is here assumed to be 35% at nuclear matter density.

The self-energies of a Λ hyperon propagating in nuclear matter are studied using finite-density QCD sum-rule methods. A correlator of Λ interpolating fields is evaluated in the nuclear matter ground state using both an operator product expansion and a dispersion relation with a spectral ansatz. The Lorentz vector self-energy of the Λ is found to be substantially smaller than the corresponding nucleon vector self-energy. The magnitude of Lorentz scalar self-energy of the Λ is also smaller than the corresponding value for nucelon; however, this prediction is sensitive to the assumed density dependence of certain four-quark condensates. The scalar and vector self-energies tend to cancel, but not completely. The implications for the Λ spin-oribt force in a finite nucleus are discussed.

The ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$(α,n${)}^{12}$C reaction determines the rate for neutron-catalyzed helium burning and thus plays an important role in supernovae ejecta under α- and n-rich conditions which may lead to the r process. Cross sections for this reaction have been measured for 0.16≤${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤1.87 MeV using a 4π neutron detector, and are used to calculate the thermonuclear reaction rates for temperatures between 0.1 and 10 GK. During these measurements, the thick-target yields were determined for 0.50≤${\mathit{E}}_{\mathrm{\alpha }}$≤2.30 MeV, the stopping powers for α’s in beryllium determined for 0.24≤${\mathit{E}}_{\mathrm{\alpha }}$≤2.12 MeV, and the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$(p,p) elastic scattering cross sections measured at ${\mathrm{\theta }}_{\mathrm{lab}}$=142.4° for 2.34≤${\mathit{E}}_{\mathit{p}}$≤2.66 MeV.

The charge, velocity, and angular distributions of three coincident fragments measured for the system ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ on ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ at an incident energy of 31 MeV/nucleon evidence a deep inleastic collision process followed by fission. The estimated total kinetic energy loss is about 600–800 MeV which represents roughly 60–80% damping of the initial kinetic energy.