The level energies and the electromagnetic properties of heavy transitional nuclei ${}_{}{}^{186,188,190,192}{}_{}{}^{}\mathrm{Os}$ are studied using a semimicroscopic model. In this model the nucleus is assumed to be $\gamma$ rigid but soft in the $\beta$ degrees of freedom. The asymmetry parameter $\gamma$ is determined from the ratio of the experimental energies of the first $2^{+}$ and second $2^{+}$ states assuming a rigid asymmetric rotor model. The symmetry parameter $\beta$ for each $J$ is obtained from the minimization of energy approximately projected from a triaxially symmetric intrinsic wave function. The pairing $+Q·Q$ interaction of Baranger and Kumar is employed to obtain the intrinsic wave function. Within our $\gamma$-rigid model, and without explicitly introducing the hexadecapole term in the Hamiltonian, it has been possible to get good agreement with experiment for the calculated level energies of the ground band, $\gamma$ band, and $K=4^{+}$ band, the $B\left(E2\right)$ values, electromagnetic moments, and $E2$ and $E4$ matrix elements. The success in describing the properties of each of these nuclei taking only one value of $\gamma$ suggests that these nuclei may have rather stable triaxial shapes.

[NUCLEAR STRUCTURE Variation after angular momentum projection, ${}_{}{}^{186,188,190,192}{}_{}{}^{}\mathrm{Os}$, level energies, $B\left(E2\right)$ values, electromagnetic moments, $E2$ and $E4$ matrix elements, stable rigid triaxial shape.]

The two-rotor model of deformed nuclei which predicts an isovector collective $M1\mathrm{}$ state owing to orbital motion is reformulated by a different quantization procedure. This new formulation leads to an eigenvalue equation for the intrinsic motion which is identical to that found in the interacting boson approximation. The close correspondence to the vibrating potential model is also stressed. The values of the excitation energy and $B\left(M1\right)$ strength are now in reasonable agreement with the estimates of those models.

[NUCLEAR STRUCTURE Two-rotor picture, deformed nuclei, collective isovector orbital magnetic excitations.]

A model for collective mode damping in nuclei is devised in the frame of a theory of irreversible evolution. The decay width of a fast nuclear vibration, originated in its coupling to the remaining nuclear degrees of freedom, is calculated in a dynamical fashion. To this aim, a set of equations is proposed that describes the simultaneous dynamics of the oscillation or its associated array of bosons and of the interacting fermions that play the role of a heat reservoir. These are, respectively, a quantal master equation and modified kinetic one. The two of them exhibit their mutual coupling in the non-Hermitian terms of their generators of motion. The equations are worked out in detail in (a) the weak-coupling approximation plus (b) the very-close-to-equilibration regime plus (c) the energy-conserving description of intermediate processes. With hypothesis (c) the heat bath can be regarded as lying in a steady state at all times and the master equation is solved for different temperatures and phonon energies. The damping width of the oscillations is thus quantitatively predicted.

[NUCLEAR STRUCTURE Damping width. High-frequency collective modes. Nonstationary fermionic heat reservoir. Coupled dynamics. Quantal master equation. Modified BBGKY hierarchy. Modified kinetic equation. Single-particle lifetime. Temperature-dependent transition rates. Thermal equilibration. Irreversible evolution with effective collision frequency or relaxation time.]

A modified kinetic equation for nuclear matter, whose generator of irreversible motion contains a contribution from coupling to a fast collective mode in addition to the usual kinetic kernel, is examined. The fermionic collision rates are evaluated in the relaxation-time approximation, assuming a finite amount of excitation is transferred from collective motion to the single-particle degrees of freedom in the form of a temperature increase. Both energy and angular dependence of the collision frequencies are discussed.

[NUCLEAR STRUCTURE Modified kinetic equation; fermion-boson coupling; intrinsic kinetics; phonon-induced kinetics; total collision rate; resonance decay; finite temperature in nuclear matter; distortion of the Fermi sphere.]

The ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$(p,n)${}_{}{}^{10}{}_{}{}^{}\mathrm{C}$ $Q$ value has been determined to be $Q_{\mathrm{pn}}=-4430.17\mathrm{}\pm 0.34$ keV. This seems to resolve the disagreement between two sub-keV measurements reported in the literature.

[NUCLEAR REACTION Measured ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$(p,n)${}_{}{}^{10}{}_{}{}^{}\mathrm{C}$ threshold energy.]

Low energy antiproton absorptions in complex nuclei were found to produce ${\Lambda }^0$ hyperons with a frequency of 1.9±0.4%.

A two-body threshold (${\pi }^{\pm },{\pi }^{\pm }{\pi }^{\mp }$) reaction mechanism is suggested in direct analogy with pion absorption. The mechanism involves boson rescattering via $\Delta$ excitation. The relative importance of this mechanism and the ordinary one-body mechanism in nuclei is studied in the particular case of $S$-wave deuteron targets. The contribution of the two-body mechanism to the threshold reaction cross section is found to be less than 1% of the simple one-body estimate.

A simple alternative derivation of the $S$-wave eigenvalues of the relativistically corrected Schrödinger equation with a Coulomb interaction is presented. It illustrates the utility of Laplace transform methods. The pionic atom problem is discussed.

A simple procedure for correcting Hartree-Fock mean square radii for the effect of zero-point motion of the nuclear surface is proposed. The procedure has been applied to the calculation of the isotope shifts for even-even nuclei at the onset of deformation in the rare earth region, and a consistent improvement of agreement between the measured and calculated isotope shifts results. Preliminary results about the effect of pairing correlations on the nuclear radius have been also obtained. The effects seem to be significant, comparable with standard isotope shifts, and to depend on the effective two-body force used.

We point out that for two extreme reaction mechanisms describing nucleon ejection in the coincidence reaction $A(\mathrm{e},{\mathrm{e}}^{\prime }\mathrm{N})B$ (resonance or quasifree knockout) the interference form factors can be used to make a longitudinal-transverse separation of the inclusive cross section without change in electron scattering angle (resonance case), or a separation of the convection and spin currents in the transverse form factor (quasifree case).

The photonuclear reaction ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}(\mathrm{p},{\gamma }_0){}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ is analyzed within a coupled-channels framework based on a continuum shell model formulation. The microscopic description includes realistic shell model density amplitudes, Pauli-blocking effects, and a large number of nucleon + target open an closed (virtual) channels. Both Pauli corrections and virtual channel contributions are found to be sizable for energies spanning the giant dipole resonance and improved, but not detailed, agreement is obtained with experiment. The sensitivity of the theoretical $E1\mathrm{}$ cross section to model interactions is documented and discussed.

We evaluate the contribution of a $J^P=2^{+}$ dibaryon resonance to the forward $\pi \mathrm{d}$ amplitude, taking into account the fact that the deuteron is a loosely bound structure. Comparison with the data shows that these are well compatible with a resonance in the ${}_{}{}^5{}_{}{}^{}S_2^{}{}_{}{}^{}$, $I=1\mathrm{}$ $\Delta -\mathrm{N}$ channel.

The large angle oscillations in several cases of $\alpha -{}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ scattering are examined in terms of orbital amplitude and the background amplitude interference using the approach developed recently to analyze ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$-${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ scattering.

[NUCLEAR REACTIONS $\alpha -{}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ scattering, anomalous large angle scattering, orbiting phenomena.]

The bandhead energies for the two-particle states in the doubly odd actinide nucleus ${}_{95}{}^{246}{}_{}{}^{}\mathrm{Am}_{151}^{}{}_{}{}^{}$ are calculated involving single particle states up to a summed energy ($E_{\mathrm{p}}+E_{\mathrm{n}}$) up to 300 keV based on an evaluation of the zero range residual interaction energy contribution. The results are compared with the available experimental data. It is concluded that the ground state is the high spin isomer with $K^{\pi }=7^{-}$ corresponding to the configuration {${\frac{5}{2}}^{+}{\mathrm{}\left[642\right]\mathrm{}}_{\mathrm{p}}:{\frac{9}{2}}^{-}{\mathrm{}\left[734\right]\mathrm{}}_{\mathrm{n}}$} with the $K^{\pi }=2^{-}$ of the same configuration as the low spin isomer at an excitation energy of (30 ± 10) keV. The $K^{\pi }=2^{+}\{{\frac{5}{2}}^{-}{\mathrm{}\left[523\right]\mathrm{}}_{\mathrm{p}}:{\frac{9}{2}}^{-}{\mathrm{}\left[734\right]\mathrm{}}_{\mathrm{n}}\}$ bandhead is predicted to lie at about the same energy as the $K^{\pi }=2^{-}$ state. Predictions are made in all regarding the expected location of twelve two-particle states (six Gallagher-Moszkowski pairs).

[NUCLEAR STRUCTURE Odd-odd nucleus ${}_{}{}^{246}{}_{}{}^{}\mathrm{Am}$, two-particle bandhead energies from residual interaction calculations, configuration assignments.]

In a recent paper, Johnson, Larson, Mahaux, and Winters have claimed that the use of the Lorentz-weighted average to determine optical model phase shifts from low-energy neutron scattering data is invalid. We show that this conclusion is unjustified since the $R$-matrix parametrization fails to describe the exact $S$ matrix as $\mathrm{}\left|E\right|\mathrm{}\to \infty$. We also show that the inconsistent numerical averaging precedure used by those authors leads to values of $\overline{R}$ and $s$ which do not agree with those obtained from an optical potential fitted to the same data. Correct use of the Lorentz-weighted average avoids these problems.

We substantiate our previous results on the danger of identifying the energy-average scattering function with the Lorentz-weighted average of a scattering function fitted to the experimental data in a finite energy interval.

In the c.m. energy region 13-15 MeV, the $l=10\mathrm{}$ partial wave dominates the cross section for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{12}{}_{}{}^{}\mathrm{C},\alpha ){}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}(\mathrm{g}.\mathrm{s}.\mathrm{})\mathrm{}$, frequently contributing more than 90% of the total cross section.

Cross sections for compound residues from the fusion reaction ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$+${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ were measured. No "extra push" is necessary to describe the data. Below the fusion barrier, the cross section decreases much faster than observed previously in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$-induced fusion reactions with tin and samarium targets. The present data are incompatible with recently published data for the same system.

An analytical characterization of the conditions for double decoupling in doubly odd deformed nuclei is given within the framework of the rotational model. The stimulus for this investigation originates in the observation of a band in ${}_{}{}^{186}{}_{}{}^{}\mathrm{Ir}$, which is possibly the first case where such a coupling scheme is realized.

Lifetimes in ${}_{}{}^{168}{}_{}{}^{}\mathrm{W}$ have been measured using the recoil distance method. The lifetime of the ${12}^{+}$ yrast state (at the backbend) is 87.4(3.3) ps. The ${14}^{+}$ yrast state has a lifetime of 38.3(1.5) ps. These unusually long lifetimes make ${}_{}{}^{168}{}_{}{}^{}\mathrm{W}$ a particularly favorable case for studying the structure in the backbending region. The $B\left(E2\right)$ of the ${12}^{+}\to {10}^{+}$ yrast transition is retarded by a factor of 50 compared with the rotational value. The transition quadrupole moments for the observed yrast and yrare states are used to deduce yrast-yrare interaction of $10<V<\mathrm{}19\mathrm{}$ keV, in agreement with cranked shell model calculations.