We compare the elastic-scattering cross sections for electrons and positrons from ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. The results demonstrate that the electron-nucleus interaction can be described to good accuracy as a single hard-photon exchange process, and validate the theoretical framework used to interpret electron-scattering data in terms of ground-state charge densities of nuclei.

Multifragment events from 35, 40, 45, and 55 MeV/nucleon ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$${+}^{12}$C, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$, ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Cu}$, and ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ reactions can be assigned to sources characterized by their velocity. At each bombarding energy, the probabilities of threefold, fourfold and fivefold events increase substantially with decreasing source velocity, but are independent of the target mass. To remove the bombarding-energy dependence, a simple transformation has been applied which gives the excitation energy of the fused system in the simple incomplete-fusion picture. These ‘‘excitation functions’’ appear to independent of both the system and bombarding energy.

A serious breakdown of the distorted-wave impulse approximation has recently been observed in the proton-knockout reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(e,e’p${)}^3$H. In the phenomenological description of the data, charge-excnange contributions were included through a coupled-channel calculation. New data obtained in a wide range of kinematical configurations are presented in order to study the anomaly in more detail. Its magnitude and dependence on momentum transfer are well accounted for by a newly presented microscopic description of various interaction effects, including meson-exchange currents and charge-exchange processes.

The ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ spectrum resulting from ultrerelativistic heavy-ion collisions is shown to have a sharp resonance at an invariant mass M≊0.2${\mathit{T}}_{\mathit{c}}$ (≊40 MeV of typical ${\mathit{T}}_{\mathit{c}}$=200 MeV). Observation of this resonance would signal the existence of the quark-gluon plasma, confirm the first-order nature of the transition, and determine the value of ${\mathit{T}}_{\mathit{c}}$.

The existence of an I=0, ${\mathit{J}}^{\mathit{P}}$=$1^{\mathrm{-}}$ quasibound state in the ηNN-πNN coupled system is theoretically predicted with a mass of about 2430 MeV and a width 10–20 MeV. The three-body equation for the ηNN-πNN coupled system. The primary two-body interactions generating the bound state are (1) the NN interaction in the ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$${\mathrm{-}}^3$${\mathit{D}}_1$ state and (2) the πN and ηN coupled interaction in the ${\mathit{S}}_{11}$ state. A remarkable enhancement of the elastic cross section of η-d scattering near the ηd threshold is found, whose origin is in the pole structure of the I=0, ${\mathit{J}}^{\mathit{P}}$=$1^{\mathrm{-}}$ ηd scattering amplitude on the complex energy plane.

In the proton-induced fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, a binary structure was observed in the velocity and the total-kinetic-energy distributions of fission fragments with A=128–131. From the present results, it is concluded that there are at least two kinds of scission configurations: elongated shapes probably associated with the symmetric liquid-drop path, and more compact shapes associated with the asymmetric shell-influenced path.

The relative polarization of positrons emitted in pure Fermi ${(}^{14}$O) and Gamow-Teller ${(}^{10}$C) decays has been measured using a new technique based on time-resolved positronium decay. The result ${\mathit{P}}_{\mathrm{GT}}$/${\mathit{P}}_{\mathrm{F}}$=0.9996±0.0037 provides constraints on possible right-handed and scalar-tensor current admixtures to the weak interaction of -4<ζδ×${10}^4$<7 and ‖(${\mathit{C}}_{\mathit{S}}$/${\mathit{C}}_{\mathit{V}}$)-(${\mathit{C}}_{\mathit{T}}$/${\mathit{C}}_{\mathit{A}}$)‖=(1±9)×${10}^{\mathrm{-}3}$ (90% C.L.), respectively.

A dramatic reduction in background was achieved in the latest Pacific Northwest Laboratory–University of South Carolina germanium detectors. Two 1.05-kg natural-isotopic-abundance detectors were operated for 1.92 kg yr. The residual spectrum, after straightforward corrections, has a significant region resembling the theoretical spectrum of the two-neutrino ββ decay of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$. A fit to the data yields ${\mathit{T}}_{1/2}^{2\nu }$${(}^{76}$Ge)=(1.$1_{\mathrm{-}0.3}^{+0.6\mathrm{}}$)×${10}^{21}$ yr at the 95% C.L., which agrees with shell-model predictions.

The p→${+}^{13}$C→ (polarized target) elastic-scattering spin observables ${\mathit{A}}_{000\mathit{n}}$ (target analyzing power) and ${\mathit{A}}_{00\mathit{n}\mathit{n}}$ (spin-correlation parameter) were determined at 497.5 MeV over the laboratory angular range 12°–30° with statistical uncertainties typically ±(0.02–0.06). Results of distorted-wave Born approximation calculations, based on either the relativistic or the nonrelativistic impulse approximation, are in reasonable agreement with these new data.

We have measured the fusion excitation function and γ-ray multiplicities ${\mathit{M}}_{\gamma }$ for the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$${+}^{154}$Sm system. We have also measured ${\mathit{M}}_{\gamma }$ for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{166}$Er system that leads to the same compound nucleus, ${}_{}{}^{182}{}_{}{}^{}\mathrm{Os}$, as a calibration of the connection between ${\mathit{M}}_{\gamma }$ and the first moment of the spin distribution, 〈l〉. We find that the deduced 〈l〉 for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$${+}^{154}$Sm agrees reasonably well with theoretical calculations, and in particular its energy dependence exhibits the barrier bump expected when a shape degree of freedom is strongly coupled to the relative motion.

A mass spectrometer consisting of two Penning traps has been set up for short-lived isotopes at the on-line mass separator ISOLDE at CERN. The ion beam is collected and cooled in the first trap. After delivery to the second trap, high-accuracy direct mass measurements are made by determining the cyclotron frequency of the stored ions. Measurements have been performed for ${}_{}{}^{118}{}_{}{}^{}–_{}^{137}{}_{}{}^{}$Cs. A resolving power of over ${10}^6$ and an accuracy of 1.4×${10}^{\mathrm{-}7}$ have been achieved, corresponding to about 20 keV.

We study the fluctuation properties of the spectrum and the electromagnetic E2 transitions of low-lying collective states in even-even nuclei. Using the framework of the interacting-boson model we discuss the transition between rotational and γ-unstable nuclei. Near those two limits the system exhibits regular behavior but in the transition region it shows chaotic behavior, where the fluctuations are characterized by the Gaussian orthogonal ensemble. Analysis of the classical mean-field dynamics of the five nuclear quadrupole shape degrees of freedom confirms the quantal results.

Geiger-Nuttall plots of the accurate modern data on partial half-lives for α decay yield very striking linear correlations. The plots for isotopic sequences which cross the neutron magic number N=126 show clearly the presence of different linear relations for N<126 and N>126. We indicate that this observation and all other data on ground-state to ground-state α decays for even-even nuclei with 76≤Z≤100 may be accounted for very well by a simple model with fixed parameters. An important ingredient in the model is the proposal that preformed α particles in the parent nuclei move in orbits with large values of a global quantum number.

The M1 excitation of ${\mathit{K}}^{\mathrm{\pi }}$=$1^{+}$ states in ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ through inelastic electron scattering is studied within a quasiparticle random-phase-approximation approach with quadrupole-quadrupole, spin-spin, and rotational-vibrational interactions. The calculated distorted-wave Born approximation (e,e’) form factors and the low-energy spectrum of $1^{+}$ states are in good agreement with the experimental data. The strongest experimentally observed $1^{+}$ states can be interpreted as isovector rotational vibrations, in which several quasiparticle pairs perform a scissors type of vibrational motion.

The total cross section for the reaction pp→pp${\mathrm{\pi }}^0$ was measured at nine center-of-mass energies from 1.5 to 23 MeV above threshold. The experiment was carried out with the Indiana Cooler, a recently constructed storage ring. The experimental advantages of an electron-cooled proton beam were utilized. The data cover an energy range where only the lowest possible angular momentum state contributes in the exit channel. The measured energy dependence of the total cross section is not compatible with that predicted by models of s-wave pion production and rescattering.

Recent observations of the giant dipole resonance in states at high angular momentum and temperature have shown an effective narrowing of the resonance. It is pointed out that this may be interpreted as a signature of rotation of the nucleus about a symmetry axis. In this limit, the giant dipole orientation freezes in the plane of rotation, the analog of the Foucault pendulum positioned at the North Pole. All effects of rotation, other than those of pure deformation, cease to be present. In this region it is thus possible to probe the nuclear shape directly, uncontaminated by rotational effects.

The purity of intrinsic states of nuclei with respect to a proton-neutron boson symmetry (F spin) is shown to be largely determined by the difference between proton and neutron deformations and not by whether the Hamiltonian is an F-spin scalar. Upper and lower bounds on F-spin mixing in the ground-state band of ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ are estimated using recent pion single-charge-exchange data.

Shell-model calculations have been carried out for isospin-forbidden β-delayed proton emission for 1s-0d-shell nuclei. The process in which a nucleus near the proton drip line ${\mathrm{\beta }}^{+}$ decays by a superallowed transition to the isobaric analog state in the daughter nucleus is considered. The first complete set of calculations for the subsequent isospin-forbidden particle decay of the isobaric analog state is presented. The decay channels considered include the processes of single-proton, sequential two-proton, direct two-proton, and α decay. The calculations reproduce the known data and suggest further experiments.

Nuclear-resonance-fluorescence spectra have been measured in the chain of ${}_{}{}^{148,150,152,154}{}_{}{}^{}\mathrm{Sm}$ isotopes. Together with supplementary information from inelastic electron scattering and other reaction studies, orbital M1 transition strengths have been deduced for a number of $1^{+}$ states located around an excitation energy of 3 MeV. The systematic study, carried out for the first time, for nuclei within a large range of the deformation parameter δ shows that the orbital M1 strength varies quadratically with δ. This result is interpreted in terms of models containing explicitly neutron and proton degrees of freedom.

We report here isovector M1 strength distributions in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ obtained in a high-resolution (e,e’) experiment at the Darmstadt Electron Linear Accelerator. The M1 strength is strongly enhanced relative to the Gamow-Teller strength for analog transitions observed in recent (p,n) and (n,p) experiments at the Indiana University Cyclotron Facility and TRIUMF. We show that this enhancement can likely be attributed to meson-exchange contributions to the spin part of the M1 operator.