The absorption of 239 MeV positive pions on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, N, and Ar is studied. A strong enhancement is seen in the differential cross sections in the kinematic region associated with free $\pi -p$ scattering. This enhancement is interpreted as a clear signature of initial state interactions in the absorption of pions on nuclei.

An exclusive measurement of slow, massive residues from the ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$+${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$ reaction at E/A=35 MeV has been performed in coincidence with projectile-like fragments, neutrons, as well as light- and intermediate-mass charged products. The highly efficient (double $4\pi$) detector setup used included the University of Rochester SuperBall neutron detector and the Washington University Microball. The observed large yield of slow, massive residues shows characteristics consistent with a production scenario similar to that of binary dissipative collisions. The residues result from the statistical decay of primary targetlike fragments, produced even in the most dissipative collisions identified in the present experiment.

The sub-Coulomb breakup of ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ on a ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ target is studied at an incident energy of 26 MeV. These data provide a measure of the $E2\mathrm{}$ component of the breakup cross section as a function of the astrophysical factor ${\mathrm{S}}_{17}$ for radiative capture of protons on ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$. Implications for the solar neutrino problem are discussed.

We have calculated the Gamow-Teller matrix elements of 64 decays of nuclei in the mass range $A=41-50\mathrm{}$. In all the cases the valence space of the full $\mathrm{pf}$ shell is used. Agreement with the experimental results demands the introduction of an average quenching factor $q=0.744\mathrm{}\pm 0.015$ slightly smaller but statistically compatible with the $\mathrm{sd}$-shell value, thus indicating that the present number is close to the limit for large $A$.

The low-mass dielectron signal from heavy-ion collisions at the CERN-SPS reported by the CERES Collaboration is in excess of estimated hadronic decays, suggesting a possible contribution from two-pion annihilation or other hadronic reactions. In the absence of dramatic medium modifications, annihilation alone is unable to account for the data. We explore the role of pion plus resonance scattering [$\pi \rho \to a_1\mathrm{}\left(1260\right)\mathrm{}\to \pi e^{+}{e}^{-}$], which has favorable kinematics to populate masses between $2m_{\pi }$ and $m_{\rho }$. While it seems to account for some of the remaining excess beyond annihilation, it fails to allow quantitative interpretation of data.

Characteristic properties of pionium $A_{2\pi }$ and associated low energy $s$-wave cross sections $\sigma ({\pi }^0{\pi }^0\to {\pi }^0{\pi }^0)$, $\sigma ({\pi }^{+}{\pi }^{-}\to {\pi }^0{\pi }^0)$, and $\sigma ({\pi }^0{\pi }^0\to {\pi }^{+}{\pi }^{-})$ are investigated with a coupled-channels potential model. Some experimental results and conclusions should be reconsidered.

The angles at which the n-p elastic scattering neutron analyzing power ${\mathit{A}}_{00\mathit{n}0}$ crosses zero were measured with precision at four TRIUMF energies below 300 MeV. The mean interaction energies are also measured with greater precision than in previous experiments. The results are ${\mathit{E}}_{\mathit{n}}$=175.26±0.23 MeV, ${\mathrm{\theta }}_{\mathit{zx}}$=98.48°±0.28°; ${\mathit{E}}_{\mathit{n}}$=203.15±0.20 MeV, ${\mathrm{\theta }}_{\mathit{zx}}$=91.31°±0.18°; ${\mathit{E}}_{\mathit{n}}$=217.24±0.19 MeV, ${\mathrm{\theta }}_{\mathit{zx}}$=87.64°±0.18°; and ${\mathit{E}}_{\mathit{n}}$=261.00±0.16 MeV, ${\mathrm{\theta }}_{\mathit{zx}}$=80.18°±0.19°. After correction for charge symmetry breaking effects, the energy where the averaged neutron-proton analyzing power crosses zero at ${\mathrm{\theta }}_{\mathit{zx}}$=90° is found to be ${\mathit{E}}_{\mathit{n}}$=206.8±0.6 MeV. © 1996 The American Physical Society.

The reaction pd${\to }^3$Heη has been studied with the SPES2 spectrometer, for proton energies between 0.2 MeV and 11 MeV above threshold. The total cross section rises from 0.25 to 0.40 μb in this region. The observed energy dependence can be attributed to the strong final state interaction. The angular distribution is nearly isotropic, consistent with S wave production. A measurement of the pd${\to }^3$He${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$ cross section in the threshold region is also presented. The setup, which is capable of producing ${10}^5$ tagged η/s with only a few percent background, is used to investigate rare η decays. © 1996 The American Physical Society.

Low-lying states of A=6 hypernuclear doublets ${(}_{\mathrm{\Lambda }}^6$He and ${}_{\mathrm{\Lambda }}{}^6{}_{}{}^{}\mathrm{Li}$) and A=7 triplets ${(}_{\mathrm{\Lambda }}^7$He, ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Li}$, and ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Be}$) are studied with an accurate three-body model calculation in which all the rearrangement Jacobian coordinates are equally taken into account. Since most of the hypernuclear states concerned are weakly bound states, focus is placed on the binding energies with respect to the particle breakup thresholds and on the density distributions in the surface and exterior regions. With the α+Λ+N model, the observed binding energies of the ground states of ${}_{\mathrm{\Lambda }}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{\mathrm{\Lambda }}{}^6{}_{}{}^{}\mathrm{Li}$ are well reproduced. ${}_{\mathrm{\Lambda }}{}^6{}_{}{}^{}\mathrm{He}$ is found to have a three-layer structure of the matter distribution: the α nuclear core, a Λ skin, and a neutron halo. The A=7 hypernuclei are shown to be well described with the ${}_{\mathrm{\Lambda }}{}^5{}_{}{}^{}\mathrm{He}$ + N+N model. Using a realistic NN interaction, the correlation between the valence nucleons is fully taken into account; this is essentially important to make the proton-rich three-body system ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Be}$ = ${}_{\mathrm{\Lambda }}{}^5{}_{}{}^{}\mathrm{He}$ + p+p bound although none of the two-body subsystems is bound. The observed binding energies of ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Be}$ are well reproduced, and energies are predicted for ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{He}$ whose core nucleus is a neutron halo nucleus, ${}_{}{}^6{}_{}{}^{}\mathrm{He}$. We discuss the validity of the assumption of frozen deuterons adopted in the previous α+d+Λ models for the T=0 states of ${}_{\mathrm{\Lambda }}{}^7{}_{}{}^{}\mathrm{Li}$. © 1996 The American Physical Society.

Chiral symmetry is consistently implemented in the two-nucleon problem at low-energy through the general effective chiral Lagrangian. The potential is obtained up to a certain order in chiral perturbation theory both in momentum and coordinate space. Results of a fit to scattering phase shifts and bound state data are presented, where satisfactory agreement is found for laboratory energies up to about 100 MeV. © 1996 The American Physical Society.

High-spin states of the odd-A ${}_{}{}^{179}{}_{}{}^{}\mathrm{Ir}$ have been studied via heavy-ion fusion reaction. Bands built on five different Nilsson states (π${\mathit{h}}_{9/2}$, π${\mathit{i}}_{13/2}$, π${\mathit{d}}_{5/2}$, π${\mathit{h}}_{11/2}$, and π${\mathit{f}}_{7/2}$) have been established, among which the π${\mathit{f}}_{7/2}$ (1/$2^{\mathrm{-}}$[530]) band is identified for the first time in Ir isotopes. Experimental aligned angular momenta, band-crossing frequencies, and relative transition rates are analyzed and compared with the cranked shell model and the particle-rotor model calculations. Effects of intruder states, particularly π${\mathit{h}}_{9/2}$ and π${\mathit{i}}_{13/2}$, have been discussed on three aspects: deformation driving, shape evolution, and quasiparticle alignment. It is found that the π${\mathit{i}}_{13/2}$ orbital has a strong, positive-${\mathrm{\beta }}_2$ driving effect on the shape of Ir nuclei, while the driving force of the π${\mathit{h}}_{9/2}$ orbital is minimized when the Fermi level reaches the shell. The total Routhian surface calculations suggest that the π${\mathit{h}}_{11/2}$ and π${\mathit{d}}_{5/2}$ configurations in ${}_{}{}^{179}{}_{}{}^{}\mathrm{Ir}$ possess relatively large ${\mathrm{\beta }}_2$ deformation (∼0.26), which is consistent with the particle-rotor calculations. The possible contribution of the π${\mathit{h}}_{9/2}$ crossing to the gradual alignment gains observed in the π${\mathit{h}}_{11/2}$ and π${\mathit{d}}_{5/2}$ bands is discussed from the influence of deformation and interaction strength. © 1996 The American Physical Society.

Four superdeformed bands have been confirmed in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Tl}$. Two of these bands have ${\mathcal{I}}^{\mathrm{}\left(2\right)\mathrm{}}$ dynamic moments of inertia which are nearly constant with rotational frequency ħω. The other two bands show the characteristic rise of ${\mathcal{I}}^{\mathrm{}\left(2\right)\mathrm{}}$ with increasing ħω seen in most superdeformed bands of the A = 190 region of superdeformation. From comparisons with the odd-A neighbors, it was found that the alignments of these bands relative to a ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$ core can be accounted for from the additive contributions of the assigned quasiproton and quasineutron orbitals. © 1996 The American Physical Society.

High spin states in ${}_{}{}^{68}{}_{}{}^{}\mathrm{As}$ have been studied using the ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$${(}^{14}$N,2n${)}^{68}$As, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ ${(}^{12}$C,pn${)}^{68}$As, and ${}_{}{}^{46}{}_{}{}^{}\mathrm{Ti}$${(}^{25}$ Mg,p2n${)}^{68}$As reactions. New high spin states have been observed including a band structure above the known 2160 keV isomeric state. A microscopic description based on the EXCITED VAMPIR model for negative and positive parity states in ${}_{}{}^{68}{}_{}{}^{}\mathrm{As}$ is presented. A possible mechanism to populate the observed high spin isomeric state is proposed. The complex structure of the ${}_{}{}^{68}{}_{}{}^{}\mathrm{As}$ nucleus is caused essentially by specific shape coexistence effects. © 1996 The American Physical Society.

The magnetic moments of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{17}{}_{}{}^{}\mathrm{B}$ were measured by using spin-polarized radioactive nuclear beams which were obtained from the projectile fragmentation reaction. The observed magnetic moment of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$, ‖μ${(}^{17}$N)‖=(0.352±0.002)${\mathrm{\mu }}_{\mathit{N}}$, where ${\mathrm{\mu }}_{\mathit{N}}$ is the nuclear magneton, falls outside the Schmidt lines. By virtue of a simplifying feature of nuclear structure inherent in a ${\mathit{p}}_{1/2}$ valence nucleus, the deviation from the Schmidt value is attributed on firm ground to admixing of the configurations in which two neutrons in the sd shell are coupled to ${\mathit{J}}^{\mathrm{\pi }}$=$2^{+}$. This interpretation is confirmed in standard shell-model calculations. The calculations reproduce fairly well the experimentally inferred amount of $2^{+}$ admixture, as well as the experimental magnetic moment itself. The magnetic moment for ${}_{}{}^{17}{}_{}{}^{}\mathrm{B}$ was determined as ‖μ${(}^{17}$B)‖=(2.545±0.020)${\mathrm{\mu }}_{\mathit{N}}$. The result is substantially smaller than the π${\mathit{p}}_{1/2}$ single-particle value, and the shell-model calculations indicate that the quenching of μ largely stems from ${\mathit{J}}^{\mathrm{\pi }}$=$2^{+}$ configurations of the sd neutrons. The observed amount of quenching, however, is larger than the shell-model predictions, suggesting an enhanced contribution of the $2^{+}$ neutron configurations. This result is explained if the pairing energy for neutrons in the sd shell of a neutron-rich nucleus is assumed to diminish by about 30%. We also find that the use of the reduced pairing energy improves agreements in the magnetic moment and low-lying energy levels of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ as well. © 1996 The American Physical Society.

The level structure and configurations of ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$ have been studied by observing the alpha decay of ${}_{}{}^{220}{}_{}{}^{}\mathrm{Fr}$, which is in secular equilibrium with ${}_{}{}^{224}{}_{}{}^{}\mathrm{Ac}$. Six of the 16 observed states are tentatively assigned to the configuration π(${\mathit{h}}_{9/2}$${)}^3$υ(${\mathit{g}}_{9/2}$${)}^5$, and four to the mixed configuration π(${\mathit{h}}_{9/2}$${)}^3$υ(${\mathit{g}}_{9/2}$${)}^4$(${\mathit{i}}_{11/2}$)+π(${\mathit{h}}_{9/2}$${)}^2$(f ${}_{7/2}{}^{}{}_{}{}^{}$)υ(${\mathit{g}}_{9/2}$${)}^5$. These two configurations in ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$ are compared with the corresponding configurations in ${}_{}{}^{212}{}_{}{}^{}\mathrm{Bi}$ and ${}_{}{}^{210}{}_{}{}^{}\mathrm{Bi}$. The shape transitions are traced from the quadrupole-octupole deformed ${}_{}{}^{224}{}_{}{}^{}\mathrm{Ac}$ through ${}_{}{}^{220}{}_{}{}^{}\mathrm{Fr}$ and ${}_{}{}^{216}{}_{}{}^{}\mathrm{At}$, to ${}_{}{}^{212}{}_{}{}^{}\mathrm{Bi}$, described by the spherical shell model, using the alpha decay sequence beginning with ${}_{}{}^{228}{}_{}{}^{}\mathrm{Pa}$. © 1996 The American Physical Society.

In this study we appraise the capability of the relativistic density-dependent Hartree theory to reproduce the properties of finite nuclei by using parametrizations of relativistic Brueckner-Hartree-Fock calculations for infinite nuclear matter, performed in the full Dirac space. We apply the density-dependent relativistic Hartree approach of Brockmann and Toki as well as its covariant extension including rearrangement contributions, recently developed by Lenske and Fuchs. Within this parameter-free theory the results for finite systems are satisfactory. We used the one-boson-exchange potentials A and B constructed by Brockmann and Machleidt. © 1996 The American Physical Society.

We study the effect of isospin impurity on the superallowed Fermi β decay using the microscopic Hartree-Fock and random phase approximation (or Tamm-Dancoff approximation) model taking into account charge symmetry and independence breaking interactions. It is found that the isospin impurity of N=Z nuclei gives enhancement of the sum rule of Fermi transition probabilities. On the other hand, the superallowed transitions between odd-odd J=0 nuclei and even-even J=0 nuclei are quenched because of the cancellation of the isospin impurity effects of mother and daughter nuclei. An implication of the calculated Fermi transition rate on the unitarity of the Cabibbo-Kobayashi-Maskawa mixing matrix is also discussed. © 1996 The American Physical Society.

A summed M1 strength of the scissors mode derived within the proton-neutron interacting boson model under the assumption of F-spin symmetry is expressed in terms of classical quantities borrowed from the two-rotor model. In this classical form, the M1 strength shows explicitly a quadratic dependence on deformation in agreement with experiments. © 1996 The American Physical Society.

An explicit correction term to the ensemble averaged trajectory for the phase space single-particle distribution function due to the presence of the higher order long range correlations is considered. It is demonstrated that this extension of the usually employed transport approaches (BUU, BNV, LV, etc.) has a diffusion structure. The role of higher order correlations in the cases of nuclear collective motion and the mean field decomposition effect in nuclear fragmentation at high temperatures is analyzed. In particular we show the importance of higher order correlations for the transition from zero to first sound regime for propagation of the collective excitation in hot nuclear matter. © 1996 The American Physical Society.

Nucleon self-energies and spectral functions are calculated at the saturation density of symmetric nuclear matter at finite temperatures. In particular, the behavior of these quantities at temperatures above and close to the critical temperature for the superfluid phase transition in nuclear matter is discussed. It is shown how the singularity in the thermodynamic $T$ matrix at the critical temperature for superfluidity (Thouless criterion) reflects in the self-energy and correspondingly in the spectral function. The real part of the on-shell self-energy (optical potential) shows an anomalous behavior for momenta near the Fermi momentum and temperatures close to the critical temperature related to the pairing singularity in the imaginary part. For comparison the self-energy derived from the $K$ matrix of Brueckner theory is also calculated. It is found that there is no pairing singularity in the imaginary part of the self-energy in this case, which is due to the neglect of hole-hole scattering in the $K$ matrix. From the self-energy the spectral function and the occupation numbers for finite temperatures are calculated.