In the limit of unbroken SU(3) symmetry, we exhibit a selection rule which forbids the decay of certain hypernuclear states involving a coherent admixture of $\Lambda$ and $\Sigma$ hyperons. This may provide an explanation of the narrow widths of some hypernuclear excitations observed in the $\Sigma$ continuum.

The ${}_{}{}^2{}_{}{}^{}\mathrm{H}(\gamma , p)n$ cross section was measured for the first time simultaneously at ${\theta }_p^{\mathrm{c}.\mathrm{m}.\mathrm{}}=0\mathrm{}°, 90°, \mathrm{and} 180°$. The photon energies were 170 and 210 MeV. A quasimonochromatic photon beam was used and the photon spectrum measured on line by a pair spectrometer. The results confirm the consistency among all experimental data from monochromatic photon beams and indicate that the angular distributions are less isotropic than predicted by theory. This problem might be solved by the introduction of relativistic corrections.

The observed pattern of the population and decay of the superdeformed band of ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ is related to the large splitting of the giant dipole resonance based on the superdeformed minimum as well as to the low level density associated with it, and to the sudden onset of static pairing correlations taking place at the rotational frequency $\hslash \omega \approx 0.3$ MeV. A new method for spectroscopic studies of superdeformed nuclei is suggested.

Relativistic mean-field theory is used to describe ground-state deformations of nuclei. The relativistic Hartree equations are solved by expansion of the nucleon wave functions and the meson field in a harmonic-oscillator basis. Using parameters adjusted in spherical nuclei we find in axially symmetric deformed nuclei good agreement with conventional nonrelativistic calculations based on density-dependent Skyrme forces.

A longitudinal/transverse separation of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(e,e’p) reaction in the quasielastic region has been performed in parallel kinematics. Above the two-particle emission threshold a broad bump is seen which has been attributed to knockout from the deeply bound s shell. The s-shell region shows a large transverse enhancement compared to distorted-wave impulse-approximation calculations based on free-proton form factors. This enhancement increases with missing energy, indicating that a nonquasielastic reaction mechanism plays a significant role. The p shell is consistent with the free-proton behavior.

The attenuation of the Doppler shift has been measured for γ rays emitted from the superdeformed states in the nucleus ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$, recoiling in a ${}_{}{}^{108}{}_{}{}^{}\mathrm{Pd}$ target and its gold backing. The transitions from the high-spin members of the superdeformed band are fully shifted, indicating feeding times of not more than a few femtoseconds. The partially shifted low-spin transitions yield a quadrupole moment of 19±3 e⋅b, in excellent agreement with the value expected for a superdeformed shape.

The two-neutrino mode of double-beta decay in ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ has been observed in a time-projection chamber at a half-life of ${(1.1\mathrm{}}_{\mathrm{-}0.3}^{+0.8\mathrm{}}$)×${10}^{20}$ yr (68% confidence level). This result from direct counting confirms the earlier geochemical measurements and helps provide a standard by which to test the double-beta-decay matrix elements of nuclear theory. It is the rarest natural decay process ever observed directly in the laboratory.

Two-dimensional γ-ray energy correlations have been measured at high spins in ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ with the anti-Compton spectrometer array OSIRIS. Pronounced ridges have been found consisting of stretched E2 transitions. The separation of the ridges gives a dynamical moment of inertia 2${\mathrm{\theta }}_{\mathrm{band}}^{\mathrm{}\left(2\right)\mathrm{}}$=(150±6)${\mathrm{ħ}}^2$/MeV, suggesting a superdeformation with an axis ratio c/a=1.7 (${\beta }_2$≊0.6). The quadrupole moment of the superdeformed nucleus is $Q_0$${=11\mathrm{}}_{\mathrm{-}4}^{+14\mathrm{}}$ e⋅b. .AE

A model-independent multipole analysis of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$(e,e’f) coincidence data, taken at four momentum transfers (0.2≤$q_{\mathrm{eff}\mathrm{\le }0.7}$ ${\mathrm{fm}}^{\mathrm{-}1}$; ω=4–22 MeV) yields both E1, E2/E0, and E3 form factors and strength distributions. The E2/E0 strength distribution in the fission channel shows two distinct bumps centered at ω≃10 and 14 MeV, exhausting up to 12 MeV (19±2)% of the isoscalar E2 sum rule. The extracted form factors can be described within a hydrodynamical model by use of parameters $c_{\mathrm{tr}/c_0=1.2\mathrm{}}$ and 1.0 for E1 and E2, respectively.

Potential-energy surfaces are obtained for the three dynamical-symmetry chains of SO(8) within the fermion-dynamical-symmetry model. A remarkable feature is that the SO(7) dynamical symmetry is transitional and β soft. The transitional or ‘‘critical’’ behavior of SO(7) is vividly demonstrated.

We have found a narrow line (intrinsic width ≤2.5 keV) at 1062±1 keV in the summed-energy 180°-c.m.-correlated two-photon spectrum from 5.95-MeV/nucleon U+Th collisions. Including possible systematic uncertainties, the production cross section is found to be 50±25 μb, averaged over the 1-mg/${\mathrm{cm}}^2$ Th target thickness. This line may belong to the decay of a neutral system which also produces correlated electron-positron pairs found by others in the same reaction.

Total reaction cross sections of neutron-rich nuclei from C to Mg in a thick Si target have been measured by the detection of the associated γ rays in a 4π geometry. This cross section strongly increases with neutron excess, indicating an increase of as much as 15% of the reduced strong absorption radius with respect to stable nuclei.

Recently observed nuclear spin-spin dependence of total cross sections for ${\mathrm{n}}_{\mathrm{pol}{+\mathrm{}}^{27}}$${\mathrm{Al}}_{\mathrm{pol}}$ is accounted for by coupled-channels calculations that include the effects of the static quadrupole moment of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and without any spin-spin potentials. Effective spin-spin interactions, of both spherical and tensor kinds, can thus be generated by static nuclear deformation.

The University of California, Irvine ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ double-beta-decay experiment has now accumulated 7960 h of live time. After imposition of kinematic cuts, thirteen candidate events remain in the sum-energy window between 2.0 and 3.0 MeV. If these are interpreted as neutrinoless double-beta decay with the emission of a Goldstone boson (known as the Majoron), we obtain a half-life limit of 4.4×${10}^{20}$ yr (90% confidence level). This gives, dependent upon the various calculations of the nuclear matrix elements, a limit of the majoron coupling constant ${\mathrm{g}}_{\mathrm{ee}}$≤(2–30)×${10}^{\mathrm{-}4}$. .AE

Relativistic Dirac-Brueckner calculations, based on a one-boson-exchange interaction, are presented for the equation of state of asymmetric nuclear matter. For decreasing values of the ratio Z/A it is shown that the saturation shifts to lower densities and the compression modulus at saturation drops. however, at high densities for all values of Z/A a stiff equation of state is observed, in contrast to the outcome of supernova calculations. The possibility of the occurrence of a ${\mathrm{\pi }}^{\mathrm{-}}$ condensate is discussed.

The ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ charge and magnetic form factors have been extracted from cross-section measurements in the region 0.3≲Q≲2.9 ${\mathrm{fm}}^{\mathrm{-}1}$. The measurements have random uncertainties of about 2% and systematic uncertainties of about 2% for ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and 1.5% for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The small systematic uncertainties allow accurate determination of the isoscalar and isovector trinucleon form factors. The isoscalar charge and isovector magnetic form factors are in reasonable agreement with current theoretical models, whereas the isovector charge and isoscalar magnetic form factors show significant deviations from the models.

It is shown, by use of the O(6) limit of the interacting-boson model, that the γ-unstable state can be generated from the rigid triaxial intrinsic state with γ=30°. This equivalence between the two descriptions holds under a certain condition on the number of bosons, which is satisfied in realistic cases. The β-γ deformation potential calculated from this triaxial intrinsic state has a minimum also at γ=30°, though quite shallow. The validity of the potential and the triaxial intrinsic state, which has been questioned because of the shallow minimum, is thus confirmed.

The isovector parts of the effective nucleon-nucleon interaction are studied by examination of the reaction ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$(p,n) at intermediate energies near zero momentum transfer with use of recently developed G-matrix and free–t-matrix interactions. The spin-independent coupling ($V_{\tau }$) exhibits a strong energy and density dependence which, in the case of the G matrix based on the Bonn potential, significantly improves the agreement between calculated values of ‖$V_{\sigma \tau }$/$V_{\tau }$${\Vert }^2$ at q=0 and those recently extracted from the reaction ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$(p,n).

Results of a multidimensional (${\beta }_2$, γ, ${\beta }_4$, ω, Z, and N) search for nuclear superdeformed configurations are presented. Calculations based on a realistic deformed average field give a relatively strong dependence of the ‘‘super’’ elongation on the particle number. This dependence is shown to be a cyclic function of the particle number. It originates from the pseudospin and pseudo-SU(3) symmetries which are obeyed approximately in a realistic average field.

High-energy γ-ray spectra have been measured in coincidence with products from the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ge}$ at 15 and 24 MeV/nucleon. The evolution of the giant dipole resonance (GDR) is studied as a function of excitation energy ($E^{\mathrm{*}}$). Up to $E^{\mathrm{*}}$≊300 MeV the measured GDR strength is consistent with full collectivity. At higher $E^{\mathrm{*}}$ a strong inhibition of the GDR γ decay is observed. At the highest transition energies, the spectra are consistent with bremsstrahlung emission associated with nucleon-nucleon collisions.