The interacting boson model is extended to describe the spectroscopy of superdeformed bands. Microscopic structure of the model in the second minimum is discussed and superdeformed bosons are introduced as the new building blocks. Solutions of a quadrupole Hamiltonian are implemented through the 1/N expansion method. Effects of the quadrupole parameters on dynamic moment of inertia and electric quadrupole transition rates are discussed and the results are used in a description of superdeformed bands in the Hg-Pb and Gd-Dy regions. © 1996 The American Physical Society.

The experimental study of the Δ${\mathit{T}}_{\mathit{z}}$=+1 component of the isovector monopole resonance of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ has revealed itself as an experimental puzzle. We present in this work a theoretical calculation of the strength distribution of this resonance and of its neutron decay properties, within the framework of a model in which random phase approximation states are coupled with continuum configurations and with a set of ‘‘doorway states’’ in order to describe the basic escape and damping mechanisms of the nuclear vibration. Our results are compared with existing measurements and with other theoretical analyses. © 1996 The American Physical Society.

Spectroscopic factors for one-nucleon knock-out from ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ are calculated for states with low excitation energy in ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ with the Bonn-C potential. A method is proposed to deal with both short- and long-range correlations consistently. For this purpose a Green’s function formalism is used and the self-energy in the Dyson equation is approximated as the sum of an energy-dependent Hartree-Fock (HF) term and dispersion and correlation terms of higher order in the G-matrix interaction. This G matrix is obtained by solving the Bethe-Goldstone equation with a Pauli operator which excludes just the model space treated in the subsequent calculation of the self-energy. The energy dependence of the HF energies induces an additional reduction of the spectroscopic factors for quasiparticle states close to the Fermi level by about 10%. Experimental data may signal the need of some further improvement in the treatment of intermediate- and long-range correlations. © 1996 The American Physical Society.

Microscopic calculations based on the cranked shell model extended by the random-phase-approximation are performed to investigate the quadrupole and octupole correlations for excited superdeformed bands in ${}_{}{}^{190}{}_{}{}^{}\mathrm{Hg}$, ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$, and ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$. The K=2 octupole vibrations are predicted to be the lowest excitation modes at zero rotational frequency. At finite frequency, however, the interplay between rotation and vibrations produces different effects depending on neutron number: The lowest octupole phonon is rotationally aligned in ${}_{}{}^{190}{}_{}{}^{}\mathrm{Hg}$, is crossed by the aligned two-quasiparticle bands in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Hg}$, and retains the K=2 octupole vibrational character up to the highest frequency in ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$. The γ vibrations are predicted to be higher in energy and less collective than the octupole vibrations. From a comparison with the experimental dynamic moments of inertia, a new interpretation of the observed excited bands invoking the K=2 octupole vibrations is proposed, which suggests those octupole vibrations may be prevalent in superdeformed Hg nuclei. © 1996 The American Physical Society.

The validity of BCS theory has been questioned because the appearance of normally deformed identical bands in odd and even nuclei seems to contradict the conventional understanding of the blocking effect. This problem is examined with the projected shell model (PSM), which projects good angular momentum states and includes many-body correlations in both deformation and pairing channels. Satisfactory reproduction of identical band data by the PSM suggests that it may be necessary to go beyond the mean field to obtain a quantitative account of identical bands. © 1996 The American Physical Society.

We consider a simple but nontrivial many-body system interacting through short-range forces, and confirm a property of such systems that was recently proposed on general grounds, namely, that single-particle overlap functions, spectroscopic factors, and separation energies of the bound (A-1)-particle eigenstates can be derived from the one-body density matrix of the A-particle system in its ground state. The basis of natural orbitals for the system is constructed and its properties are discussed. We also investigate the high-momentum content of the bound-state overlap functions and momentum distribution. It is found that the mean field provides a good approximation for the bound-state overlap functions even in the region of large momenta, where the total momentum distribution is already enhanced by several orders of magnitude over the mean-field result. © 1996 The American Physical Society.

Several BCS-type approximations encountered in the literarature were correlated using a 1/N expansion over a set of dispersion type energy equations, which are the exact solution of a system of fermions interacting through a pure pairing force. One of the interesting results obtained is that the BCS+QRPA approximation coincides with the first order term of the 1/N expansion. © 1996 The American Physical Society.

The inclusive A(e,${\mathit{e}}^{\prime }$) cross section for x≃1 was measured on ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, C, Fe, and Au for momentum transfers ${\mathit{Q}}^2$ from 1 to 6.8 (GeV/c${)}^2$. The scaling behavior of the data was examined in the region of transition from y scaling to x scaling. Throughout this transitional region, the data exhibit ξ scaling, reminiscent of the Bloom-Gilman duality seen in free nucleon scattering. © 1996 The American Physical Society.

This paper aims at comparing the deexcitation mechanism of heavy and light hot nuclei. Two experimental samples of events corresponding to two systems differing mostly by their total mass (Au and Ca) are thus compared with two types of statistical model (instantaneous multifragmentation and binary sequential decay). The sizes of the nuclei contained in the two samples are different, but the constraints which characterize them are very similar. The first sample (Au+Cu at 600 MeV/nucleon) was obtained using the ALADIN spectrometer at SIS and the second one (Ca+Cu at 35 MeV/nucleon) using the AMPHORA multidetector at SARA. The same rigorous analysis protocol was used to make the four comparisons between experimental and simulated data. It is shown that the Au decay can only be reproduced by the multifragmentation codes (MMMC, Berlin, and SMM, Copenhagen-Moscow) and the Ca decay exclusively by the sequential emission code (GEMINI). © 1996 The American Physical Society.

Gamma rays, light charged particles, and evaporation residues emitted from hot nuclei formed in the ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$${+}^{90}$Zr reaction at 27 MeV/nucleon have been measured at the GANIL facility with the 4π barium fluoride multidetector MEDEA. The combination of the residue and particle measurements shows that nuclei with masses around 115 and excitation energies between 350 and 550 MeV are produced. The γ spectra measured in coincidence with the evaporation residues exhibit three components: a low-energy statistical component, a high-energy contribution due to nucleon-nucleon bremsstrahlung during the initial stages of the collision, and a contribution from the decay of the giant dipole resonance built on highly excited states. The characteristics of the bremsstrahlung component are in agreement with previously published systematics. The γ yield from the decay of the giant dipole resonance remains constant over the excitation energy range studied. A comparison with other experiments shows that the N/Z asymmetry in the entrance channel does not affect the γ yield. Statistical calculations performed using the code CASCADE and supposing a fixed width and full sum rule strength for the dipole resonance strongly overpredict the data. The hypothesis of a continuously increasing width of the resonance with temperature gives a better agreement with experiment near the centroid of the resonance but overpredicts the γ spectra at higher energies. The best account of the data is given by assuming a cutoff of γ emission from the resonance above an excitation energy of approximately 250 MeV. This cutoff is discussed in terms of the time necessary to equilibrate the dipole oscillations with the hot compound nucleus. Finally, some evidence is given for a possible new low-energy component of the dipole strength at very high temperatures. © 1996 The American Physical Society.

The spatial-temporal extent of the emitting system for central collisions in ${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$ + ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ at E/A = 35, 55, and 70 MeV is probed using fragment-fragment velocity correlation functions. Selection on fragment pairs of high velocity yields a stronger fragment-fragment Coulomb interaction than for inclusive fragment pairs. This result is consistent with fragment emission from a source of decreased spatial-temporal extent. The universality of this association and the effect of different selection criteria are explored. The sensitivity of the spatial-temporal extent deduced by the correlation function technique to measurement uncertainties, assumed source characteristics, and rotational effects is assessed. © 1996 The American Physical Society.

We present an analysis of recent measurement of the 56 MeV deuteron breakup data on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ targets taken by Okamura et al.. The cross section measured at ${\mathrm{\theta }}_{\mathit{p}}$=${\mathrm{\theta }}_{\mathit{n}}$=0° was claimed to be a strong evidence of Coulomb breakup, especially in light nuclei. However, the single-step pure Coulomb breakup formalism fails to describe both the magnitude and the shape of the triple differential cross section even in light nuclei. At 56 MeV incident deuteron energy, the nuclear interference effect is found to be significant in all the above nuclei. The prior form distorted-wave Born-approximation calculations with unusual optical potentials in the exit channel can reproduce the highly asymmetric shapes of the triple differential cross section at the extreme forward angle, but, fail to give their exact magnitude. The possible role of multistep processes is discussed. © 1996 The American Physical Society.

The mechanism of the Coulomb breakup reactions of the nuclei with neutron-halo structure is investigated in detail. A time-dependent Schrödinger equation for the halo neutron is numerically solved by treating the Coulomb field of a target as an external field. The momentum distribution and the post-acceleration effect of the final fragments are discussed in a fully quantum mechanical way to clarify the limitation of the intuitive picture based on the classical mechanics. The theory is applied to the Coulomb breakup reaction of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$${+}^{208}$Pb. The breakup mechanism is found to be different between the channels of ${\mathit{j}}^{\mathrm{\pi }}$=1/$2^{\mathrm{-}}$ and 3/$2^{\mathrm{-}}$, reflecting the underlying structure of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$. The calculated result reproduces the energy spectrum of the breakup fragments reasonably well, but explains only about a half of the observed longitudinal momentum difference. © 1996 The American Physical Society.

We examine commonly used approaches to deal with the scattering of electrons from a bound nucleon. Several prescriptions are shown to be related by gauge transformations. Nevertheless, due to current nonconservation, they yield different results. These differences reflect the size of the uncertainty that persists in the interpretation of (e,${\mathit{e}}^{\prime }$p) experiments. © 1996 The American Physical Society.

Geometric and incompressibility constraint relations are used explicitly in reducing the number of collective variables of the dinuclear phase of the fissioning system to calculate the barrier penetrability factor. Consistently, we define an effective inertial coefficient for the relative motion of the fissioning system. With this inertial coefficient, half-lives of the exotic and alpha decays are successfully reproduced for all available experimental data, using only one well-controlled nuclear parameter, the nuclear radius constant. © 1996 The American Physical Society.

We calculate the spatial correlation of electrons and positrons emitted by internal pair conversion of Coulomb excited nuclei in heavy-ion collisions. The alignment of the nucleus results in an anisotropic emission of the electron-positron pairs that is closely related to the anisotropic emission of γ rays. However, the angular correlation in the case of internal pair conversion displays additional features which provide the possibility for a deeper understanding of the nuclear structure. Our results are of particular interest for the electron-positron coincidence experiments currently analyzed at GSI (Darmstadt) and at Argonne. © 1996 The American Physical Society.

We have calculated the (p,${\mathrm{\pi }}^{\mathrm{-}}$) spectra for calcium isotopes based on a two-nucleon pion-production model. In the near-threshold region, the observed selective excitation of high-spin states is well explained. The experimental data are reproduced within a factor of 2–3. In the higher-energy region, our calculation predicts a pronounced selectivity for stretched two-particle–one-hole high-spin states. © 1996 The American Physical Society.

The optical model potential for nucleon elastic scattering is investigated within the context of a three-body model. It is shown that the standard full-folding optical model potential is obtained as a special case of an approximate solution to such a three-body problem. A new full-folding-type optical model potential together with its corresponding factorized tρ version are then obtained in the above three-body picture under some plausible approximations. © 1996 The American Physical Society.

In this paper we show by solving the inversion problem at high energies that the fundamental McIntyre parametrization of the S matrix, for heavy-ion collisions, will correspond to a Woods-Saxon-type optical model potential. The parameters of such a Woods-Saxon potential are directly related to the corresponding parameters of the McIntyre parametrization. The inversion solution results were tested for the available experimental data and were found to be in good agreement. © 1996 The American Physical Society.

Statistical properties of antisymmetrized molecular dynamics (AMD) are classical in the case of nucleon-emission processes, while they are quantum mechanical for the processes without nucleon emission. In order to understand this situation, we first clarify that there coexist mutually opposite two statistics in the AMD framework: One is the classical statistics of the motion of wave packet centroids and the other is the quantum statistics of the motion of wave packets which is described by the AMD wave function. We prove the classical statistics of wave packet centroids by using the framework of the microcanonical ensemble of the nuclear system with a realistic effective two-nucleon interaction. We show that the relation between the classical statistics of wave packet centroids and the quantum statistics of wave packets can be obtained by taking into account the effects of the wave packet spread. This relation clarifies how the quantum statistics of wave packets emerges from the classical statistics of wave packet centroids. It is emphasized that the temperature of the classical statistics of wave packet centroids is different from the temperature of the quantum statistics of wave packets. We then explain that the statistical properties of AMD for nucleon-emission processes are classical because nucleon-emission processes in AMD are described by the motion of wave packet centroids. We further show that when we improve the description of the nucleon-emission process so as to take into account the momentum fluctuation due to the wave packet spread, the AMD statistical properties for nucleon-emission processes change drastically into quantum statistics. Our study of nucleon-emission processes can be conversely regarded as giving another kind of proof of the fact that the statistics of wave packets is quantum mechanical while that of wave packet centroids is classical. © 1996 The American Physical Society.