High-spin states in ${}_{}{}^{77}{}_{}{}^{}\mathrm{As}$ have been studied via the ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$(α,p2n) reaction. Proton-gated excitation functions extracted from measured γ-ray yields at beam energies of 32, 36, and 40 MeV and an additional γ-γ coincidence measurement at 40 MeV have been used to assign new γ-ray decay sequences to ${}_{}{}^{77}{}_{}{}^{}\mathrm{As}$. In particular, a positive-parity sequence up to spin 25/2 and a high-lying ΔI=1 sequence up to 21/2 and of probably negative parity have been found to decay into the known 9/$2^{+}$ isomer at 475.5 keV. Nuclear-shape calculations show a triaxial shape for the positive-parity states with moderate quadrupole deformation. The high-lying negative-parity sequence is interpreted as a high-K three-quasiparticle band similar to bands in adjacent nuclei. © 1996 The American Physical Society.

High-spin states in ${}_{}{}^{107,108}{}_{}{}^{}\mathrm{Pd}$ and ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}$ have been studied by charged-particle-γ, γ-γ, and charged-particle-γ-γ coincidence measurements of heavy-ion fusion-evaporation reactions. The high-spin level schemes of these nuclides have been significantly extended and new deformed rotational bands have been found in each nuclide. These bands exhibit the general property of back bends corresponding to the rotational alignment of a pair of ${\mathit{h}}_{11/2}$ quasineutrons. Band properties are compared with predictions of the cranked shell model and total Routhian surface calculations. The properties of the yrast and near-yrast bands of all three nuclides are generally consistent with predictions of moderate (${\mathrm{\beta }}_2$≊0.2) prolate deformation. © 1996 The American Physical Society.

The six superdeformed bands in ${}_{}{}^{193}{}_{}{}^{}\mathrm{Pb}$ have been studied with the EUROGAM 2 γ-ray spectrometer using the ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$${(}^{30}$Si,5n${)}^{193}$Pb reaction. The results are discussed in terms of cranked-Hartree-Fock-Bogolioubov-Lipkin-Nogami calculations. From the @FF ${}_{}{}^{\mathrm{}\left(2\right)\mathrm{}}{}_{}{}^{}$ moment of inertia behavior as function of the rotational frequency and the M1 and E2 decay competition of the superdeformed states, the bands are interpreted as three pairs of signature partners based on quasineutron excitations. Dipole transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, the branching ratio B(M1)/B(E2)=0.15±0.04 ${\mathrm{\mu }}_{\mathit{N}}^2$/${\mathit{e}}^2$ ${\mathrm{b}}^2$ has been extracted. © 1996 The American Physical Society.

Cross sections for the (p,n) reaction on targets of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ and ${}_{}{}^{92}{}_{}{}^{}\mathrm{Zr}$ have been measured at 26 MeV. Our resolution for these measurements was better than 200 keV and allowed the observation of numerous $1^{+}$ states in ${}_{}{}^{92}{}_{}{}^{}\mathrm{Nb}$ below 7 MeV excitation. Our analysis is focused on states below the primary Gamow-Teller state and shows a fragmentation of the Gamow-Teller strength that is only partially accounted for by three different model calculations: a quasiparticle model, a shell model, and a random-phase approximation calculation. All three models yield predictions which include appreciable strength between 3 and 7 MeV excitation, but fail to describe our experimental results in detail. © 1996 The American Physical Society.

Low-lying states in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, up to an excitation energy of about 4 MeV, have been investigated by means of inelastic proton and deuteron scattering experiments at ${\mathit{E}}_{\mathit{p}}$=65 and 400 MeV and at ${\mathit{E}}_{\mathit{d}}$=56 MeV, respectively. Measured cross sections and analyzing powers have been compared with coupled-channels calculations using collective form factors; calculations in both the Schrödinger and Dirac formalisms have been carried out for the proton data. For each probe, the matrix elements have been deduced for transitions from the ground state and from the $2_1^{+}$ state to six quadrupole ($2^{+}$) states to one octupole ($3_1^{\mathrm{-}}$) and two hexadecapole ($4_1^{+}$ and $4_2^{+}$) states. The obtained matrix elements and the previous values from γ decay or electron inelastic scattering have been used to evaluate the isospin character of the transitions. To discuss the quadrupole mixed-symmetry states in ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, the deduced neutron (${\mathit{M}}_{\mathit{n}}$) and proton (${\mathit{M}}_{\mathit{p}}$) components of the matrix elements, or equivalently the isoscalar (${\mathit{M}}_{\mathit{s}}$) and isovector (${\mathit{M}}_{\mathit{v}}$) parts, have been compared with theoretical calculations based on the neutron-proton interacting boson model and on the shell model evaluated in a full f-p configuration space. © 1996 The American Physical Society.

A study of the γ-ray spectrum following thermal-neutron capture by ${}_{}{}^{205}{}_{}{}^{}\mathrm{Pb}$ has revealed 54 γ rays, which have been incorporated into a level scheme consisting of 22 excited states in ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$. This study was carried out with an ∼9 mg lead sample enriched to 78.9% in radioactive ${}_{}{}^{205}{}_{}{}^{}\mathrm{Pb}$. The neutron binding energy of ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ was determined to be 8086.67±0.06 keV, and the thermal-neutron-capture cross section for ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ to be 4.5±0.2 b. The low-lying portion of the level scheme of ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ and the γ-ray branchings of positive-parity states have been compared with shell-model predictions. The overall agreement is excellent for the former and reasonably good for the latter. © 1996 The American Physical Society.

The present paper describes the measurement of high energy gamma rays (∼5–22 MeV) in the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{181}$Ta. Singles measurements were performed at E${(}^{16}$O)=84, 89, and 107 MeV and the multiplicity gated measurements along with the angular distributions were made at E${(}^{16}$O)=94 MeV. It is seen that the Tl nuclei are nonspherical at the excitation energies and spins relevant for the present work. The angular distribution data can be explained only by incorporating large orientation fluctuations. An attempt has been made to extract information on shape fluctuations directly from the γ spectra. Under the restriction of using a common ‘‘intrinsic’’ strength function characterized by ${\mathit{E}}_0$ and ${\mathrm{\Gamma }}_0$ for the multiplicity gated data sets, a small increase of the effective mean deformation and a drive towards triaxiality (γ∼ 30°) with angular momentum is brought out. If this restriction is relaxed, however, the data can be understood also with a smaller value of γ. The singles data can be understood within the same framework with a higher value of ${\mathrm{\Gamma }}_0$. © 1996 The American Physical Society.

Photon scattering experiments on the deformed even-even nuclei ${}_{}{}^{164,166,168,170}{}_{}{}^{}\mathrm{Er}$ provided detailed information on the excitation energies, spins, transition probabilities, and K quantum numbers of low-lying dipole excitations. In the case of ${}_{}{}^{168}{}_{}{}^{}\mathrm{Er}$, in addition, parities of the excited states were determined model independently by measuring the linear polarization of the scattered photons using a sectored Ge-Compton polarimeter with a BGO–anti-Compton shield. The results for M1 excitations are compared with the systematics of low-lying orbital M1 strength (‘‘scissors mode’’) in the neighboring rare-earth nuclei. The observed enhanced E1 transitions are discussed in terms of collective excitations (K=0 octupole bandheads or candidates for $2^{+}$⊗$3^{\mathrm{-}}$ two-phonon excitations) and in the framework of recent microscopic quasiparticle phonon model calculations. © 1996 The American Physical Society.

Three rotational bands in ${}_{}{}^{108}{}_{}{}^{}\mathrm{Sn}$ have been investigated to high frequency via the ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$${(}^{58}$Ni,4p) reaction at 243 MeV. One of these bands is observed to undergo a band crossing at high spin. All three bands show a large decrease in their dynamic moments of inertia, to approximately one third of the rigid body value, as the rotational frequency approaches 1 MeV/ħ. The structures of these bands are thought to be based on 2p-2h proton excitations across the Z=50 shell gap and their properties can be interpreted in terms of cranked Nilsson-Strutinsky calculations. Such calculations predict smooth band terminations for all three struc- tures and a gradual shape change from collective prolate, at low spin, to noncollective oblate at high spin. © 1996 The American Physical Society.

Spin and parity of some low-lying levels in ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$ have been assigned by measuring K-internal conversion coefficients of the deexciting transitions. The new information, combined with the available experimental data, has been analyzed in the framework of the interacting boson model (IBM-2) to clarify the structure of the $2_3^{+}$, $2_4^{+}$, and $3_1^{+}$ levels in terms of their mixed-symmetry character. © 1996 The American Physical Society.

We investigate the pairing treatment with approximate particle number projection, known as the Lipkin-Nogami scheme, in connection with nuclear Hartree-Fock models. A prescription for the pairing strength is developed. It is based on the average gap method and employs a Thomas-Fermi model to estimate the average level density at the Fermi surface. The second-order variation of the Skyrme energy functional gives feedback to the self-consistent density and thus to the mean field. This effect turns out to be small but non-negligible. The Lipkin-Nogami scheme is compared with the conventional treatment of pairing based on the BCS approximation. The bulk observables, especially those which are related to surface properties, depend on the pairing scheme employed, with the differences most noticeable for light nuclei. Pronounced effects on deformation energy surfaces were found, leading to sizable differences in the deformation energies. © 1996 The American Physical Society.

We present a description of the quantum Monte Carlo diagonalization method. This method has been introduced recently as an approach having both the advantage of the quantum Monte Carlo method and that of the direct diagonalization of the Hamiltonian matrix. In addition, the angular momentum projection is implemented so as to remove the degeneracy with regard to magnetic quantum number. We show that with this method the convergence of the eigenvalues is improved and that the wave functions of excited states can be obtained more easily. Moreover, the calculation of transition matrix elements becomes simpler. © 1996 The American Physical Society.

The M1 properties of even ${}_{}{}^{182\mathrm{-}186}{}_{}{}^{}\mathrm{W}$ isotopes are investigated in the interacting boson model-2 (IBM-2). The E2/M1 mixing ratios, g factors, and summed M1 strength are calculated. A least-squares fit of the excitation energies is used to fix the IBM-1 projected Hamiltonian parameters, while the F-spin-breaking terms are adjusted to reproduce the M1 properties of low-lying states. The influence of F-spin mixing on the summed M1 strength is studied using the coherent state technique in perturbation theory. When the standard boson g factors are used, the M1 properties of the low-lying states are described satisfactorily, but the summed M1 strengths are found to be larger than present experimental values. Possible g factor adjustment, which reconciles the calculated and experimental M1 strength, is discussed. © 1996 The American Physical Society.

We discuss the description of a many-body nuclear system using Hamiltonians that contain the nucleon relativistic kinetic energy and potentials with relativistic corrections. Through the Foldy-Wouthuysen transformation, the field theoretical problem of interacting nucleons and mesons is mapped to an equivalent one in terms of relativistic potentials, which are then expanded at some order in $\frac{1}{m_N}$. The formalism is applied to the Hartree problem in nuclear matter, showing how the results of the relativistic mean field theory can be recovered over a wide range of densities.

Ground-state properties of exotic even-even nuclei with extreme neutron-to-proton ratios are described in the framework of self-consistent mean-field theory with pairing formulated in coordinate space. This theory properly accounts for the influence of the particle continuum, which is particularly important for weakly bound systems. The pairing properties of nuclei far from stability are studied with several interactions emphasizing different aspects, such as the range and density dependence of the effective interaction. Measurable consequences of spatially extended pairing fields are presented, and the sensitivity of the theoretical predictions to model details is discussed. © 1996 The American Physical Society.

We reexamine our earlier treatment of favored alpha decays and find that the predicted half-lives are insensitive to the precise geometric form of the nuclear potential. We seek to constrain this potential more tightly by requiring it, in addition, to describe simultaneously the low-lying positive parity spectrum of ${}_{}{}^{212}{}_{}{}^{}\mathrm{Po}$, the E2 and α-decay rates of these states, and the differential cross sections for low-energy elastic α scattering from ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. A recently proposed parametrization of the α-core potential is capable of achieving this goal and, although some ambiguity remains in the choice of parameters, the proposed potential describes all the data well and supports the assignment of ${\mathit{J}}_{\mathit{I}}^{\mathrm{\pi }}$=${18}^{+}$ to the 45.1 s isomer at 2.922 MeV in ${}_{}{}^{212}{}_{}{}^{}\mathrm{Po}$. © 1996 The American Physical Society.

We discuss systematics of the M1 scissors mode within the interacting boson model when the g-boson degree of freedom is included explicitly and microscopically motivated choices of model parameters are adopted. We try to relate the M1 centroid energy to the energetics of deformation. We conclude that, with the introduction of a hexadecapole-hexadecapole interaction and a g-boson admixture in the ground state of only a few percent, we can obtain reasonable estimates of the M1 centroid energy, without invoking a Majorana interaction. If one takes seriously variations in microscopic estimates of boson g factors, then the summed M1 strength near midshell can be interpreted in terms of boson occupation numbers which saturate. © 1996 The American Physical Society.

Spectroscopical properties of the platinum, mercury, and lead isotopes are studied within the Hartree-Fock plus BCS framework with the finite range density-dependent Gogny force. These properties are also studied beyond mean-field theory by combining the use of generator-coordinate-method-like wave functions with the angular momentum projection technique as to generate many-body correlated wave functions that are at the same time eigenstates of the angular momentum operator. We apply this formalism to the calculation of reduced transition probabilities B(E3) from the lowest-lying octupole collective state to the ground state of several isotopes of the platinum, mercury, and lead nuclei whose experimental B(E3) values present a peculiar behavior. The projected calculations show a large improvement over the unprojected ones when compared with the experimental data. The unprojected calculations are unable to predict any structure in the B(E3). © 1996 The American Physical Society.

Elastic cross sections and a complete set of analyzing powers (${\mathit{iT}}_{11}$, ${\mathit{T}}_{20}$, ${\mathit{T}}_{21}$, and ${\mathit{T}}_{22}$) were measured for the first time for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$${(}^6$Li→, ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$)α system at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=11.1 MeV. Elastic cross-section data for the same reaction at 15 MeV are also reported. The angular range covered is from 10° to 160° c.m., and all analyzing powers reported have large magnitudes except for ${\mathit{T}}_{21}$. Additionally, an analyzing power excitation function is reported for the same reaction between ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=10.3 and 11.9 MeV. Results of both optical model and optical model plus α exchange processes including transfers from the 2S and 1D components of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ ground state wave function are presented. The analysis shows the presence of an explicit spin orbit potential, a very small and negative spectroscopic amplitude of the 1D component (${\mathit{b}}_{\mathit{D}}$), and an angular momentum-dependent absorption. © 1996 The American Physical Society.

Elastic and inelastic scattering angular distributions were measured for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{64}$Zn reaction at bombarding energies close to the Coulomb barrier. The experimental data were analyzed within the optical model and coupled-channel model. An extended dispersion relation of integral quantities using a Gaussian weight was applied, instead of the normal relationship. Within this frame, the optical model parameters obtained from the data are in agreement with the dispersion relation and show the threshold anomaly at energies close to the Coulomb barrier. Analysis of the inelastic scattering angular distributions leads to some indications that an inelastic threshold anomaly is beginning to develop at energies lower than the ones for which our data were taken. © 1996 The American Physical Society.