The even-even and even-odd nuclei ${}_{}{}^{126}{}_{}{}^{}–_{}^{132}{}_{}{}^{}$Xe and ${}_{}{}^{131}{}_{}{}^{}–_{}^{137}{}_{}{}^{}$Ba are shown to have a well-realized ${\mathrm{SO}}_8$⊃${\mathrm{SO}}_6$⊃${\mathrm{SO}}_3$ fermion dynamical symmetry. Their low-lying energy levels can be described by a unified analytical expression with two (three) adjustable parameters for even-odd (even-even) nuclei that is derived by using the vector and spinor representations of the fermion model. Analytical expressions are given for wave functions and for E2 transition rates that agree well with data. The distinction between the fermion dynamical symmetry model (FDSM) and the interacting boson model (IBM) ${\mathrm{SO}}_6$ limits is discussed. The experimentally observed suppression of the energy levels with increasing ${\mathrm{SO}}_5$ quantum number τ can be explained as a perturbation of the pairing interaction on the ${\mathrm{SO}}_6$ symmetry, which leads to an ${\mathrm{SO}}_5$ pairing effect for ${\mathrm{SO}}_6$ nuclei. © 1996 The American Physical Society.

A method for calculating nonstatistical particle decay of excited states in odd nuclei is presented. Using the quasiparticle-phonon model partial cross sections and branching ratios for the neutron decay of the high angular momentum states in ${}_{}{}^{209}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}$ excited by means of the (α${,}^3$He) reaction have been evaluated. The calculated branching ratios are compared with existing experimental data. A general agreement is found except for the $5^{\mathrm{-}}$ channel in ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ where practically all the observed decay is attributed to statistical process. © 1996 The American Physical Society.

Properties of proton-rich nuclei around doubly magic ${}_{28}{}^{48}{}_{}{}^{}\mathrm{Ni}_{20}^{}{}_{}{}^{}$ are studied in the framework of the self-consistent mean-field theory (Hartree-Fock, Hartree-Fock-Bogoliubov, and relativistic mean field). Various effective interactions are employed to investigate two-proton separation energies, deformations, single-particle levels, proton average potentials, and diproton partial decay half-lives in this mass region. © 1996 The American Physical Society.

A selection rule for fragmentations of doorway $s$-hole states in light nuclei is given. In the two-body fragmentation process of the doorway $s$-hole states in light nuclei, smaller fragments than the $\alpha$ particle ($p$, $n$, $d$, $t$, and 3He) are allowed, while the fragments such as the $\alpha$ particle and the larger particles are forbidden, in spite of the fact that the $Q$ value for the $\alpha$ fragment is similar to or even larger than those for the $p$, $n$, and $d$ fragments. The selection rule comes from the spatial symmetry of the doorway $s$-hole states. The spectroscopic factors and partial widths for the two-body fragmentations of the $s$-hole state of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ are calculated to show the selection rule. The calculated escape widths for the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}(s-\mathrm{hole})$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(s-\mathrm{hole})$ doorway states are in good correspondence with the experimental widths. This may suggest that the following scenario is realized; in light nuclei the fragmentation process of the $s$-hole state in the doorway stage is superior or competitive to that in the compound nuclear stage.

We study the effect of the unique shell structure as well as the very low particle threshold on collective modes in drip-line nuclei, first performing the Hartree-Fock (HF) calculation with Skyrme interactions and, then, using the random phase approximation solved in the coordinate space with the Green’s function method. We examine also one-particle resonant states in the HF potential. The properties of both isoscalar and isovector monopole giant resonance (GR) are found to change drastically in nuclei around the neutron drip line. The characteristic feature of the isovector dipole modes as well as the isoscalar quadrupole modes in drip-line nuclei is also studied. © 1996 The American Physical Society.

We present calculations for ${}_{}{}^8{}_{}{}^{}\mathrm{-}_{}^8{}_{}{}^{}$Be cluster states in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ based on our recent schematic coupled-channels model. We justify some of the approximations made in the schematic model in this paper. The present calculations broadly support our assumptions about the states in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ which formed the basis of our previous calculations in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$. © 1996 The American Physical Society.

Self-consistent density profiles of two-component hot nuclei in equilibrium with an external gas are calculated in the semi-classical Thomas-Fermi model with a new prescription. The energy functional is calculated with a momentum and density dependent finite range two-body effective interaction. The evolution of equilibrium nuclear masses as a function of temperature and densities of the external neutron and proton gas is investigated in this description. Limiting temperatures of nuclei, their lifetimes against neutron evaporation, temperature dependence of incompressibilites of finite nuclei and a few other observables are also studied in the present model. © 1996 The American Physical Society.

The effects of magnetic interaction between vector mesons and nucleons on the propagation (mass and width) of the ρ meson, in particular moving through very dense nuclear matter, is studied and the modifications, qualitative and quantitative, due to the relevant collective modes (zero-sound and plasma frequencies) of the medium discussed. It is shown that the ρ mesons produced in high-energy nuclear collisions will be on the verge of instability in the region of sufficiently dense nuclear matter, in the presence of such an interaction. © 1996 The American Physical Society.

Formation cross sections of several fission products have been determined using the recoil catcher technique followed by γ-ray spectrometry in ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ induced fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ at ${\mathit{E}}_{\mathrm{lab}}$=95 and 112 MeV. The data show significant admixture of fission from compound nuclei formed by complete fusion as well as targetlike nuclei formed by transfer reactions. Mass distributions for both the fissioning systems have been obtained using the systematics of charge distribution in low and medium energy fission. Mass distribution for complete fusion fission is broad Gaussian whereas it is asymmetric for transfer induced fission. At 95 MeV the transfer fission constitutes about 28% of total fission cross section while at 112 MeV it is about 14%, showing that the transfer fission fraction decreases with increasing projectile energy across the barrier. The evaporation residue cross sections of the targetlike nucleus formed in the ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ ${(}^{19}$F, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$) ${}_{}{}^{233}{}_{}{}^{}\mathrm{Pa}$ reaction were also measured. The evaporation residue cross sections and the calculated decay probabilities of the targetlike nucleus ${}_{}{}^{233}{}_{}{}^{}\mathrm{Pa}$ by PACE2 have been used to estimate the fraction of proton and α transfer fission in the total transfer fission cross section. © 1996 The American Physical Society.

Fusion process in the near and sub-barrier region has been investigated for the systems ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$${+}^{58,60,64}$Ni using the heavy-ion reaction analyzer (HIRA). Fusion excitation functions and the mean angular momenta are obtained from the measured evaporation residue cross sections. Significant enhancements both in the cross section and mean angular momentum data are seen with respect to the predictions of the one-dimensional barrier penetration model. Simplified coupled channel calculations incorporating linear coupling to the inelastic channels (lowest $2^{+}$ and $3^{\mathrm{-}}$ states of both the projectile and the target) are not able to explain the observed enhancements. A systematic analysis of the data indicates lack of isotopic dependence in the sub-barrier fusion for these three systems. © 1996 The American Physical Society.

The branching ratio (BR) for stopped negative pion induced single charge exchange was experimentally determined for ${\mathrm{CH}}_2$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, ${}_{}{}^{45}{}_{}{}^{}\mathrm{Sc}$, and ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$. The BR for ${\mathrm{CH}}_2$ agrees with a previous determination; the others can be understood in terms of the shell model structure of the targets. These results will facilitate studies of the effective pion-nucleon isovector interaction in nuclei at threshold energies. © 1996 The American Physical Society.

In a search for high spin molecular states in ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$, the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$${+}^{16}$O system was studied using a differentially pumped windowless ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ gas target. Angle-averaged excitation functions for the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$${+}^{16}$O and ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$${+}^{12}$C mass partitions were obtained for the energy range 16.39 MeV ≤${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤ 41.67 MeV in 278 keV steps. Inelastic scattering involving the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($0_2^{+}$, 6.05 MeV) state was separated from the nearby ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($3_1^{\mathrm{-}}$, 6.13 MeV) state by detecting the ${\mathit{e}}^{\mathrm{-}}$ and ${\mathit{e}}^{+}$ emitted from the internal pair decay of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($0_2^{+}$) state. A statistical fluctuation analysis indicates that several correlated structures are present in the current system. Tentative spin assignments based on angular distribution measurements suggest the resonant structures could be associated with rotational states of ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$. The results are compared with a cranked cluster model calculation. © 1996 The American Physical Society.

The same hot nuclear system (ΣZ=18) has been studied for two different entrance channels with reaction products detected in a forward array of scintillators: central collisions of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ on a ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ target at 25A and 35A MeV and peripheral pickup reactions of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ on a ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ target at 43A MeV. The detection-efficiency-corrected charge distributions, multiplicity of charged particles and cross sections as a function of excitation energy are compared. The reaction mechanism is investigated, through comparison to simulations with statistical observables. The central reaction ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$${+}^{12}$C at 35A MeV is well characterized by a dissipative binary collision scenario. Data at 25A MeV show less evidence of such dynamical characteristics. The intermediate-mass fragments (3≤Z≤8) production for each reaction is compared to model calculations for different values of excitation energy. The systems formed in the central collision at 25A MeV and the pickup reaction at 43A MeV show similar source characteristics, both statistically and in momentum space. However, the yields of the various exit channels, from evaporation and/or fission to multifragmentation and vaporization, differ for the two reactions. © 1996 The American Physical Society.

Elastic and inelastic scattering data from the scattering of 160 MeV protons from ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, and of 200 MeV protons from ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ have been analyzed using a fully microscopic distorted wave approximation. The analyses involve large space (multi-ħω) shell model wave functions, an effective nucleon-nucleon interaction that is energy and medium dependent, and fully microscopic (nonlocal) optical potentials built with that same effective interaction. The results for ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ correlate with analyses of elastic and inelastic electron scattering form factors indicating that improvements are needed in the shell model interactions used to obtain the nuclear wave functions. © 1996 The American Physical Society.

We propose to introduce a new stochastic process in molecular dynamics in order to improve the description of the nucleon emission process from a hot nucleus. We give momentum fluctuations originating from the momentum width of the nucleon wave packet to the nucleon stochastically when it is being emitted from the nucleus. We show by calculating the liquid-gas phase equilibrium in the case of antisymmetrized molecular dynamics, that with this improvement, we can recover the quantum mechanical statistical property of the nucleus for the particle emission process. © 1996 The American Physical Society.

This paper presents an extension of our previous method to study neutron breakup angular distributions in heavy-ion reactions. We distinguish two cases: elastic and inelastic breakup. In the first case the target is left in its ground state while in the second case the target is left in an excited state. The elastic breakup angular distribution is calculated here by using the eikonal form of the final neutron wave function rather than the plane wave function of our previous work. In this way we include the effect of distortion by the final-state interaction of the neutron with the target optical potential. The neutron angular distribution for inelastic breakup is also obtained for the first time by considering explicitly some possible final excited states of the target. Our theoretical results for the angular distribution and for the total reaction cross section are compared with recent experimental data for the reaction ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$${(}^{20}$Ne${,}^{19}$Ne${)}^{49}$Ca at ${\mathit{E}}_{\mathrm{inc}}$=48A MeV. It appears that the inelastic breakup contribution is essential to explain the experimental neutron multiplicity and to properly describe the three body physical background of the inclusive spectra. © 1996 The American Physical Society.

The spectra of preequilibrium particles, taking into account the energy dependence of the single-particle level density, are calculated using the particle-hole (exciton) level density. We demonstrate the significant effect of the finite depth of the potential well (continuum effect) on partial emission spectra for configurations with a small exciton number. © 1996 The American Physical Society.

Scalar and vector interactions, with the scalar interaction coupled to a composite spin-1/2 system so as to cause a shift of its mass, are shown to obey a low-energy theorem which guarantees that the second order interaction is the same as for a point Dirac particle, in which case the second-order interaction comes from z graphs. Off-shell and contact interactions appropriate to the composite system cancel and this is verified in a model of a composite fermion. The low-energy theorem and its generalizations provide a justification for the use of the Dirac equation as it has been used in relativistic nuclear scattering and mean field theories. © 1996 The American Physical Society.

Deep inelastic collisions of stable and radioactive projectiles with heavy targets are considered. The effects of shell structure and the N/Z ratio of a projectile on the excitation energy distribution between interacting nuclei, and on the centroid position and variance of the charge (mass) distribution of binary reaction products are explored. The role of nucleon exchange and particle-hole excitation mechanisms in the transformation of relative motion kinetic energy into the internal excitation energy of nuclei is studied. It is shown that a change in the mass number of the projectile nucleus causes a sufficient change both in the sharing of the excitation energy between fragments and in the charge (mass) distribution of reaction products. The nucleon exchange process between interacting nuclei is the main mechanism of dissipation.

It is shown that the electric polarizability of the deuteron produces negligible effect on the cross section of deuteron induced rearrangement reactions even at extremely low energies. This assessment is based on simple analytical formulas, derived on the basis of N-particle scattering theory by means of the general two-potential formalism, including Coulomb and exchange effects. It is shown on the basis of general physical arguments that the polarizability effects at very low energies are entirely contained in a multiplicative enhancement factor that differs from 1 by at most a few percent. As a result enhancement of (d,p) relative to (d,n) reactions is not possible by the Oppenheimer-Phillips mechanism.