Cross sections have been measured for the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(γ,pp)n reaction with tagged photons in the range ${\mathit{E}}_{\gamma }$ =161–208 MeV using the Saskatchewan-Alberta Large Acceptance Detector (SALAD). The protons were detected over a range of polar angles of 40°–140° and azimuthal angles of 0°–360° with an energy threshold of 40 MeV. Comparisons are made with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon processes, including final-state interactions, are unable to account for the measured cross sections. The addition of three-nucleon absorption diagrams gives roughly the right strength, but the distribution in phase space is in disagreement with the data. © 1996 The American Physical Society.

We investigate characteristic properties of effective meson-exchange potentials to achieve a simultaneous description of the nucleon-nucleon (NN) and hyperon-nucleon (YN) interactions in the (3q)-(3q) resonating-group formulation of the spin-flavor ${\mathrm{SU}}_6$ quark model. The quark Hamiltonian includes a phenomenological confinement potential of ${\mathit{r}}^2$ type, the full Fermi-Breit interaction with explicit quark-mass dependence, and the central, spin-spin, and tensor components of the meson-exchange potentials generated from the scalar and pseudoscalar meson nonet exchanges. A small number of parameters are determined to fit the S- and P-wave NN phase shifts and the low-energy cross-section data for YN scattering. Satisfactory agreement with experiment is obtained, including the NN phase shifts up to J=4 partial waves, the deuteron properties, the effective-range parameters, and the total and differential cross sections of the ΛN and ΣN systems. In the intermediate-energy region with ${\mathit{p}}_{\mathrm{lab}}$≥ 400–500 MeV/c, some discrepancies from the one-boson exchange potentials, such as the Nijmegen and Jülich models, are found for the YN scattering observables. In particular, the total nuclear cross sections of ${\mathrm{\Sigma }}^{+}$p elastic scattering show a smooth decreasing behavior in this energy region. In Λp elastic total cross sections, the cusp structure appearing at the ΣN threshold through the one-pion tensor force is influenced by the antisymmetric ${\mathit{LS}}^{\left(\mathrm{-}\right)}$ force generated from the Fermi-Breit interaction. © 1996 The American Physical Society.

The pion-exchange contribution to the electromagnetic current of the deuteron is calculated perturbatively in the context of a field-theoretical approach on the light front. The deuteron charge and magnetic and quadrupole form factors are calculated for the Paris and Bonn-B two-nucleon potential models. The description of the experimental deuteron charge form factor is improved for both potentials models over the impulse approximation. © 1996 The American Physical Society.

We evaluate the contribution to the nucleon-nucleon interaction due to correlated πρ exchange in the π, ω, and ${\mathit{A}}_1$/${\mathit{H}}_1$ channels by means of dispersion-theoretic methods based on a realistic meson-exchange model for the interaction between π and ρ mesons. This process has substantial effects: In the pionic channel it counterbalances the suppression generated by a soft πNN form factor of monopole type with a cutoff mass of about 1 GeV; in the ω channel it provides a strong repulsive contribution, leaving little room for explicit quark-gluon effects. © 1996 The American Physical Society.

We discuss the two-pion-exchange (TPE) potential which emerges from a box diagram with one nucleon (the spectator) restricted to its mass shell, and the other nucleon line replaced by a subtracted, covariant πN scattering amplitude which includes Δ, Roper, and ${\mathit{D}}_{13}$ isobars, as well as contact terms and off-shell (nonpole) dressed nucleon terms. The πN amplitude satisfies chiral symmetry constraints and fits πN data below ∼ 700 MeV pion energy. We find that this TPE potential can be well approximated by the exchange of an effective sigma and delta meson, with parameters close to the ones used in one-boson-exchange models that fit NN data below the pion production threshold. © 1996 The American Physical Society.

Certain soft photon amplitudes which have been recently suggested as alternatives to the usual Low form of the soft photon approximation are studied and it is demonstrated that problems exist in their relation to the corresponding nonradiative amplitude. The nonradiative amplitude, which is an input to soft photon calculations, is in certain cases required to be evaluated outside of its physical phase space region. Also, for the case of two-body identical particle bremsstrahlung processes, the symmetrized or antisymmetrized form of these soft photon amplitudes cannot be written in terms of the symmetrized or antisymmetrized amplitude for the nonradiative process. It is found that the usual Low form of the soft photon theorem is essentially unaffected by these problems. © 1996 The American Physical Society.

Two new superdeformed bands, which possess almost constant dynamic moments of inertia as a function of rotational frequency, have been observed in ${}_{}{}^{197}{}_{}{}^{}\mathrm{Pb}$. They are thought to be based upon the favored and unfavored signature components of an N=7 neutron intruder orbital. Previous work tentatively assigned a superdeformed band to ${}_{}{}^{198}{}_{}{}^{}\mathrm{Pb}$. The present work has confirmed this assignment. The properties of all the bands are discussed in terms of extended total Routhian surface calculations. © 1996 The American Physical Society.

The properties of $2^{+}$ states at 2156 keV and 2231 keV in ${}_{}{}^{112}{}_{}{}^{}\mathrm{Cd}$ have been measured using the (n,${\mathit{n}}^{\prime }$γ) reaction. These states have strong decay branches, determined to be almost pure M1, to the $2_1^{+}$ level with B(M1) values of 0.044(5)${\mathrm{\mu }}_{\mathit{N}}^2$ and 0.055(5)${\mathrm{\mu }}_{\mathit{N}}^2$ respectively, and are interpreted as the main fragments of the $2^{+}$ mixed-symmetry state, the vibrational analogue of the ‘‘scissors-mode’’ state in deformed nuclei. These observations provide the first firm evidence of mixed-symmetry states in a good U(5) nucleus. © 1996 The American Physical Society.

High-spin states in ${}_{}{}^{145}{}_{}{}^{}\mathrm{Pm}$ have been studied with TESSA3 operating at NSF Daresbury, through the ${}_{}{}^{134}{}_{}{}^{}\mathrm{Xe}$${(}^{15}$N,4n) compound nucleus reaction using a solid xenon target. To determine conversion coefficients and properties of low-spin excitations electron-γ and γ-γ coincidences have been measured following the ${}_{}{}^{146}{}_{}{}^{}\mathrm{Nd}$(d,3n) reaction. An extended level scheme of ${}_{}{}^{145}{}_{}{}^{}\mathrm{Pm}$ is proposed and interpreted in terms of single-particle excitations with collective quadrupole and octupole vibrations coupled to them. © 1996 The American Physical Society.

For the first time an (n,γ) reaction has been extensively studied using a large array of Compton-suppressed Ge detectors (the TESSA array). The nucleus ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$ was studied and the data show substantial improvement, in both quantity and quality, over previous coincidence data. Even though ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$ is perhaps the best studied deformed nucleus, over 250 new coincidence relations and a number of new levels in ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$ were disclosed, demonstrating the usefulness of this approach. Nuclear physics applications relate to the extension of nearly complete spectroscopy to higher excitation energies and to the study of statistical and chaotic features of the decay of low spin compound nuclear levels. © 1996 The American Physical Society.

Nuclear resonance fluorescence experiments were performed on the rare earth nuclei ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$ to study the fragmentation of the M1 scissors mode in odd deformed nuclei and to establish a kind of systematics. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.1 MeV) and high resolution Ge-γ spectrometers detailed information was obtained on excitation energies, decay widths, transition probabilities, and branching ratios. The results are compared to those observed recently for the neighboring odd nuclei ${}_{}{}^{161,163}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$. Whereas in the odd Dy isotopes the dipole strength is rather concentrated, both Gd isotopes show a strong fragmentation of the strength into about 25 ${(}^{155}$Gd) and 90 transitions ${(}^{157}$Gd) in the energy range 2–4 MeV. The nucleus ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$ linking the odd Dy and Gd isotopes exhibits an intermediate strength fragmentation. In general the observed total strength in the odd nuclei is reduced by a factor of 2–3 as compared to their neighboring even-even isotopes. The different fragmentation behavior of the dipole strengths in the odd Dy and Gd isotopes is unexplained up to now. © 1996 The American Physical Society.

Coulomb-nuclear interference data for incident energies between 9 and 17 MeV were obtained in the form of elastic and inelastic (to the $2_1^{+}$ states) excitation functions of backscattered (θ≃172.8°) alpha particles on ${}_{}{}^{100,102,104}{}_{}{}^{}\mathrm{Ru}$. The analysis was done in a distorted-wave Born approximation within a deformed optical model approach. B(E2) values, obtained from the charge deformation lengths ${\mathrm{\delta }}^{\mathit{C}}$ extracted from the low energy data, are compatible for the three isotopes within ∼ 2σ with published values. The nuclear quadrupolar deformation lengths ${\mathrm{\delta }}^{\mathit{N}}$, obtained from the analysis of the interference region of the excitation functions, and also of one angular distribution at 22 MeV measured for ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$ are generally lower than the corresponding charge deformation lengths, the difference increasing with increasing A of the isotope, ${\mathrm{\delta }}^{\mathit{N}}$ being 18% lower than ${\mathrm{\delta }}^{\mathit{C}}$ for ${}_{}{}^{104}{}_{}{}^{}\mathrm{Ru}$ ($2_1^{+}$). Nuclear deformation lengths associated with the $3_1^{\mathrm{-}}$ states of ${}_{}{}^{100,102,104}{}_{}{}^{}\mathrm{Ru}$ and with the $4_2^{+}$ state of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$ at 2.367 MeV were also obtained as a by-product of the present work. © 1996 The American Physical Society.

The alpha and coincident gamma decays of ${}_{}{}^{216}{}_{}{}^{}\mathrm{Fr}$ and ${}_{}{}^{212}{}_{}{}^{}\mathrm{At}$ in secular equilibrium with 0.8 s ${}_{}{}^{224}{}_{}{}^{}\mathrm{Pa}$ and 26.1 ms ${}_{}{}^{220}{}_{}{}^{}\mathrm{Ac}$ have been studied with emphasis on the level scheme of ${}_{}{}^{212}{}_{}{}^{}\mathrm{At}$. The level structure has been interpreted in terms of the shell model configurations π(${\mathit{h}}_{9/2}$${)}_{9/2}^3$ν(${\mathit{g}}_{9/2}$), π(${\mathit{h}}_{9/2}$${)}_{0+}^2$(${\mathit{f}}_{7/2}$)ν(${\mathit{g}}_{9/2}$), and π(${\mathit{h}}_{9/2}$${)}_{9/2}^3$ν(${\mathit{i}}_{11/2}$). These configurations are then compared with the calculated configurations in ${}_{}{}^{212}{}_{}{}^{}\mathrm{At}$ and with the corresponding experimental configurations in ${}_{}{}^{210}{}_{}{}^{}\mathrm{Bi}$ and ${}_{}{}^{212}{}_{}{}^{}\mathrm{Bi}$. In all three cases plots of the experimental energies vs the spin show the expected inverted parabola shape, but as we move farther away from the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ closed shells, the inverted parabolas become more compressed. © 1996 The American Physical Society.

NMR and spin echo measurements were performed on dilute samples of ${}_{}{}^{187}{}_{}{}^{}\mathrm{Os}$ and ${}_{}{}^{189}{}_{}{}^{}\mathrm{Os}$ in Fe. The magnetic hyperfine splitting frequencies ${\nu }_{\mathit{M}}$=|g${\mathrm{\mu }}_{\mathit{N}}$${\mathrm{B}}_{\mathrm{HF}}^{\left(\mathrm{eff}\right)}$/h| at 4.2 K were determined to be 111.17(1) and 371.00(5) MHz for ${}_{}{}^{187}{}_{}{}^{}\mathit{Fe}$ and ${}_{}{}^{189}{}_{}{}^{}\mathit{Fe}$, respectively. Taking into account the known magnetic moments, the hyperfine anomaly is deduced to be ${}_{}{}^{187}{}_{}{}^{}\mathrm{\Delta }_{\mathrm{Fe}}^{189}{}_{}{}^{}$ = +0.0195(2), which is of opposite sign and much smaller than expected theoretically. © 1996 The American Physical Society.

The lifetimes of nine low-lying excited states in ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ have been measured with the Doppler-shift attenuation method in the ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$(p,nγ) reaction. A comparison of experimental quantities with predictions based on the interacting boson-fermion model unravels the states due to the coupling of a ${\mathit{g}}_{9/2}$ proton hole to the quadrupole vibrations of the core. © 1996 The American Physical Society.

High spin states have been populated in ${}_{}{}^{115}{}_{}{}^{}\mathrm{Sb}$ with the reaction ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$${(}^{19}$F,4n${)}^{115}$Sb at a beam energy of 82 MeV. Two intruder rotational bands (ΔI=2) extending to spins 43/$2^{\mathrm{-}}$ and 51/$2^{\mathrm{-}}$ have been observed. The extracted relative B(E2) value supports the nearly diabatic nature of these two bands. The transition energies in these two bands are nearly similar and the bands have a relative spin alignment of 4ħ at low frequencies. The bands have been interpreted to be based on the π${\mathit{h}}_{11/2}$ orbital coupled to the deformed 2p-2h states of the ${}_{}{}^{114}{}_{}{}^{}\mathrm{Sn}$ core. Possible quasiparticle configurations for the excited negative parity band are discussed and it is suggested that this band is based on aligned ${\mathit{h}}_{11/2}$ neutrons, the yrare extension of which is observed diabatically down to very low spin values. Two strongly coupled (ΔI=1) rotational bands involving the (π${\mathit{g}}_{9/2}$${)}^{\mathrm{-}1}$ configuration have been observed, of which the known strongly coupled band has been extended from 23/$2^{+}$ to 37/$2^{+}$ in spin and the extracted B(M1;I→I-1)/B(E2;I→I-2) values for this band are compared with the Dönau-Frauendorf formula. Alignment features due to the ${\mathit{h}}_{11/2}$ neutrons in the observed bands are compared with the core nucleus ${}_{}{}^{114}{}_{}{}^{}\mathrm{Sn}$ and cranked shell model calculations. Equilibrium deformations for a range of odd Sb isotopes have been calculated with various intrinsic proton configurations. © 1996 The American Physical Society.

The theory of self-consistent effective interactions in nuclei is extended for a system with a velocity dependent mean potential. By means of the field coupling method, we present a general prescription to derive effective interactions which are consistent with the mean potential. For a deformed system with the conventional pairing field, the velocity dependent effective interactions are derived as the multipole pairing interactions in doubly stretched coordinates. They are applied to the microscopic analysis of the giant dipole resonances (GDR’s) of ${}_{}{}^{148,154}{}_{}{}^{}\mathrm{Sm}$, the first excited $2^{+}$ states of Sn isotopes and the first excited $3^{\mathrm{-}}$ states of Mo isotopes. It is clarified that the interactions play crucial roles in describing the splitting and structure of GDR peaks, in restoring the energy weighted sum rule, and in reducing the values of B(Eλ). © 1996 The American Physical Society.

A geometrical mapping of the semimicroscopic algebraic cluster model is given. The geometrical variables are the relative radius vector and the quadrupole deformation parameters. The last ones correspond to absolute β and γ values, while the orientation of the deformed nucleus in the laboratory can be changed. We show that the position of the minimum of the nuclear molecular potential is determined by the minimal number of π bosons describing the relative motion. The minimal number of π bosons is determined by the implementation of the Pauli principle. Applications to simple systems ${(}^{16}$O+α and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+α) are presented. © 1996 The American Physical Society.

The intrinsic E2 matrix elements 〈K=2|E2|K=0〉 for 25 deformed nuclei, covering from neodymium to uranium, have been deduced from measured interband E2 matrix elements between the ground band and γ band after correcting for the first-order angular momentum dependence of the coupling between the rotation and intrinsic motion. Fairly precise centroids for the triaxiality of the intrinsic E2 moments are obtained, and these correlate well with the triaxiality implied by the excitation energies. The strong correlation of the triaxiality derived from the E2 properties and level energies provides a quantitative measure of triaxial quadrupole deformation of the nuclear shape for these states. © 1996 The American Physical Society.

The structure of the low-lying levels in the mirror nuclei ${}_{}{}^{57}{}_{}{}^{}\mathrm{Ni}$ and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Cu}$ is described within the extended unified model. The problem of single-particle energies in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ is treated in detail. ‘‘Bare’’ single-particle energies are extracted from existing experimental data for the energy levels in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Ni}$ and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Cu}$ by carefully considering the influence of the coupling to excitations of the core. Important contributions arise, influencing especially the results on the spin-orbit splitting. The differences between the Coulomb energy shifts of various orbitals in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ are discussed and compared with those resulting from Hartree-Fock calculations carried out using a broad range of Skyrme interactions. The parameters of the Woods-Saxon potential reproducing these neutron ‘‘bare’’ single-particle energies and the charge root-mean-square radius of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$ are extracted. It is demonstrated that the contributions associated with the Thomas-Ehrman effect and the electromagnetic spin-orbit interaction are important and large enough to account for the differences between the Coulomb energy shifts of the single-particle levels in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$. © 1996 The American Physical Society.