The pion double-charge-exchange reactions on the Te isotopes are discussed in the generalized seniority scheme. The elementary process of charge exchange is described in a double scattering process within the plane wave limit. The transition rates are calculated for double-isobaric-analog state as well as for ground-state reactions. © 1996 The American Physical Society.

We exploit analytic properties of the optical potential for elastic scattering of α particles on nuclei to extract information on the effective interaction that should be used to describe the motion of a cluster of two neutrons and two protons bound to a nuclear system. This prescription solves long-standing ambiguities in the formalisms used for the study of α decay. © 1996 The American Physical Society.

Recent experimental data on positive-parity yrast states, based on π${\mathit{h}}_{11/2}$⊗ν${\mathit{h}}_{11/2}$ configuration, of odd-odd ${}_{}{}^{132,134,136,138}{}_{}{}^{}\mathrm{Pr}$ nuclei have been analyzed in a systematic way with the energies and transition probabilities calculated within the framework of two quasiparticles coupled to an axially symmetric core model. It is found that very good agreement is obtained in ${}_{}{}^{132}{}_{}{}^{}\mathrm{Pr}$ assuming a prolate deformation. For ${}_{}{}^{134,136}{}_{}{}^{}\mathrm{Pr}$ also our calculation favors a prolate shape up to a quite high spin. Theoretical electromagnetic transition properties for both positive and negative deformation have been predicted for ${}_{}{}^{138}{}_{}{}^{}\mathrm{Pr}$. © 1996 The American Physical Society.

The validity of microscopic thermal treatments in comparison with microcanonical results is analyzed for the case of a pairing interaction. The approximate evaluation of microcanonical averages by means of saddle point and moment expansion methods is also examined. Results are shown for a 20-level model, where exact microcanonical averages and level densities are compared with exact thermal results and with those given by the BCS and static path approximations. © 1996 The American Physical Society.

The decay properties of proton-rich nuclei with Z=19–30 are investigated using measured binding energies of the analog neutron-rich nuclei and Coulomb energy shifts deduced from a parametrization of measured Coulomb displacement energies. Predicted binding energies and separation energies are compared where possible with previous calculations; in most cases the calculations agree within the quoted uncertainties. The positions of the one-proton and diproton drip lines are determined from the calculated separation energies. It is suggested that good candidates for the observation of correlated two-proton emission are ${}_{}{}^{34}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{38,39}{}_{}{}^{}\mathrm{Ti}$, ${}_{}{}^{45}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{54}{}_{}{}^{}\mathrm{Zn}$. © 1996 The American Physical Society.

The fully energy-damped yields for the ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ reactions at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$=47.0 MeV and 45.5 MeV, respectively, are explored using particle-particle-γ coincidence data. These reactions reach a similar excitation energy of ${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$=59.5 MeV in the ${}_{}{}^{48}{}_{}{}^{}\mathrm{Cr}$ compound nucleus as was obtained in an earlier particle-particle coincidence study of the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ reaction. The overall mass and total kinetic energy distributions of the fission fragments are found to be well reproduced by statistical-model calculations. These calculations are also found to reproduce structure seen in the excitation-energy spectra for the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ and ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ exit channels for all three reactions. In previous excitation-function measurements, strong heavy-ion resonance behavior has been observed in elastic and inelastic cross sections for the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ system. There has been speculation that peaks observed in the corresponding excitation-energy spectra at more negative Q values may also be a consequence of this resonance phenomenon. The observation of very similar behavior with the asymmetric-mass entrance channels makes it less likely, though, that the peaks arise from any special configuration of the compound system. Instead, an analysis of the γ-ray data and the results of statistical-model calculations support the conclusion that most of the observed high-lying structure can be accounted for in terms of statistical fission from a fully energy- and shape-equilibrated compound nucleus. For the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ entrance channel, however, comparisons with the statistical model indicate a reduction of high-angular-momentum partial cross sections, leading to the ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ + ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ fission channel. For the first time, we are able to deduce the nature of the competition between the resonance and statistical-fission mechanisms in this mass region. © 1996 The American Physical Society.

The quasielastic scattering of ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ on a ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ target has been measured at an incident energy of 540 MeV. The new experimental data are used to extract an effective interaction for ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ scattering. The uncertainty in this interaction was previously a major obstacle to extracting information on the structure of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ from existing ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ +${\mathrm{}}^{12}$C quasielastic-scattering data. © 1996 The American Physical Society.

A complete set of analyzing powers (AP’s), ${\mathit{iT}}_{11}$, ${\mathit{T}}_{20}$, ${\mathit{T}}_{21}$, and ${\mathit{T}}_{22}$, for 50 MeV ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^6$Li→${,}^6$Li) elastic scattering and inelastic scattering to the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($2^{+}$, 4.44 MeV), ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($0^{+}$, 7.65 MeV), and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($3^{\mathrm{-}}$, 9.64 MeV) states over the center-of-mass (c.m.) angular range 10°–115° is reported. In addition, cross sections for the excited states $3^{+}$(2.18 MeV), $2^{+}$(4.31 MeV), and $1^{+}$(5.65 MeV) of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ were measured by using the inverse-kinematics reaction ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${(}^{12}$C${,}^{12}$C) at 100 MeV. A combined analysis of the new 50 MeV data and previous 30 MeV data has been carried out using the coupled-channels (CC) code FRESCO. Comparison of FRESCO calculations with those using the CC code CHUCK are presented. The CC calculations use an optical potential with double-folded (DF) real central, Woods-Saxon imaginary central, and Thomas real spin-orbit (SO) potentials. Calculations include reorientation terms and coupling to the first three excited states of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and the first two nonzerospin states of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. The ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ coupling strengths were fixed by the measured ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ excited-state cross sections. The elastic-scattering cross sections and A.P.’s are described well. The need for an explicit SO potential is apparent in the elastic and inelastic-scattering AP’s ${\mathit{iT}}_{11}$, more so at 30 MeV than at 50 MeV. The rank-2 AP’s up to 50° c.m. arise mainly from ground-state reorientation effects. The DF potential normalization constant N approaches unity for the 50 MeV data. At both energies, the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($2^{+}$) cross sections are underestimated at large angles, and the description of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($3^{\mathrm{-}}$) cross sections is poor in detail. The ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($3^{\mathrm{-}}$) AP’s and the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($2^{+}$) ${\mathit{iT}}_{11}$ are not reproduced at either energy. © 1996 The American Physical Society.

One- and two-charge-transfer reactions were studied in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${+}^{197}$Au and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{197}$Au systems at bombarding energies around the Coulomb barrier. Transfer probabilities for the same Coulomb reduced radius, extracted from angular distribution measurements, show a strong dependence on the bombarding energy and are interpreted within a semiclassical model. Quasielastic scattering data are analyzed in terms of an energy-dependent optical model. © 1996 The American Physical Society.

The excitation energy of the primary products from the reaction ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ on ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ at 672 MeV was determined by the kinematic coincidence technique. The fraction of the total excitation energy of the system stored in the projectilelike fragment was found to decrease with increasing energy loss. However, thermal equilibrium is not reached, even at the highest energy damping. A small correlation between excitation energy partition and reaction exit channel was observed. Monte Carlo simulations of the present experiment confirmed that some of this correlation is due to the finite resolution of the measured parameters. © 1996 The American Physical Society.

Measurements of the angular distributions of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${(}^{11}$B${,}^6$Li${)}^{17}$O reaction were performed at three energies of a ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ beam: 28, 35, and 40 MeV. The results were analyzed in the frame of the exact finite range distorted wave Born approximation of the first and the second order assuming the simultaneous and sequential transfer of the neutron and the α particle. Such an analysis was also performed for previously measured angular distributions of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(d${,}^7$Li${)}^7$Be reaction at ${\mathit{E}}_{\mathrm{lab}}$ = 78 MeV. In both reactions under investigation dominance was found of the simultaneous transfer of the α particle and the nucleon correlated to the ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ ${(}^5$Li) cluster in the ground or the first excited state. © 1996 The American Physical Society.

Pairs of supersymmetric transformations allow one to remove square-integrable solutions from a complex potential without modifying its complex phase shifts. This technique is applied to ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{16}$O scattering where deep and shallow optical potentials provide essentially similar fits of excitation functions over the 10–35 MeV range in the center-of-mass system. After removing complex normalizable solutions from the deep optical potential of Kondō et al., the resulting potential resembles the shallow potential of Chatwin et al., except for the fact that it is singular at the origin. The transformation succeeds in removing bound states and also narrow resonances from the real part of the deep potential. Different theoretical aspects of scattering with complex potentials are also discussed such as the behavior of complex phase shifts in the vicinity of a resonant or normalizable state, and the Levinson theorem. In order to explain phase-shift behaviors, resonance locations are calculated by the complex rotation method, combined with numerical methods valid for determining square-integrable solutions of a Schrödinger equation involving a complex potential. In phase-equivalent pairs of supersymmetric transformations, the apparent contradiction between removing a square-integrable solution which corresponds to a pole of the scattering matrix, and keeping this S matrix unchanged is explained with the Jost function. © 1996 The American Physical Society.

The reduced (e,${\mathit{e}}^{\prime }$p) cross section is calculated for kinematics that probe high missing momenta. The final-state interaction is handled within a nonrelativistic many-body framework. One- and two-body nuclear currents are included. Electron distortion effects are treated in an exact distorted-wave calculation. It is shown that at high missing momenta the calculated (e,${\mathit{e}}^{\prime }$p) cross sections exhibit a pronounced sensitivity to ground-state correlations of the random-phase approximation type and two-body currents. The role of these mechanisms is found to be relatively small at low missing momenta. © 1996 The American Physical Society.

We have developed a general semiempirical total reaction cross section formula for proton-nucleus reactions. It is applicable for all materials with nuclear charge Z greater than 5, and all proton kinetic energies from the lowest measured energies up to ≊20 GeV. While the regime of applicability is greatly enlarged, agreement between the calculated cross sections and the experimental data is comparable to or better than all earlier published results in their regime of applicability. © 1996 The American Physical Society.

We present a modification of the wall formula for one body dissipation in order to include the effect of irregularity in the shape of the one body potential on the dissipation mechanism. We arrive at a dissipation rate which is a scaled version of the wall formula developed earlier by Blocki et al. We show that the scaling factor is determined by a measure of chaos in the single particle motion. As an illustration, we obtain this measure of chaos for particles in a multipole deformed cavity with a view to use the scaled wall formula to calculate the damping widths of multipole vibrations of the cavity wall. Considering the amplitudes of the vibrations typical of the giant resonances in nuclei, it is observed that the effect of the shape dependence is to strongly suppress the damping caused by the original wall formula. © 1996 The American Physical Society.

A new precompound Monte Carlo simulation model is presented. It permits unlimited multiple precompound emission for each interaction, may be used to give exclusive spectra and yields, and does not rely upon those exciton state densities shown to be inconsistent with the assumed two body transitions in the hybrid and exciton model formulations. Results of the new model are compared with ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ (p,xn) single differential spectra for incident energies from 25 to 160 MeV, and for ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ (p,x) and ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ (p,x) excitation functions for incident proton energies up to 200 MeV. The new model gives significant improvement over the hybrid model for calculation of excitation functions. Multiple precompound decay predictions are compared for the two models. It is shown that the restriction to two precompound nucleons per decay in the hybrid model becomes a poor approximation for incident proton energies above ≊80 MeV. © 1996 The American Physical Society.

To facilitate the relativistic heavy-ion calculations based on transport equations, the binary collisions involving a Δ resonance in either the entrance channel or the exit channel are investigated within a Hamiltonian formulation of πNN interactions. An averaging procedure is developed to define a quasiparticle Δ* and to express the experimentally measured NN→πNN cross section in terms of an effective NN→NΔ* cross section. In contrast to previous works, the main feature of the present approach is that the mass and the momentum of the produced Δ*’s are calculated dynamically from the bare Δ⇆πN vertex interaction of the model Hamiltonian and are constrained by the unitarity condition. The procedure is then extended to define the effective cross sections for the experimentally inaccessible NΔ*→NN and NΔ*→NΔ* reactions. The predicted cross sections are significantly different from what are commonly assumed in relativistic heavy-ion calculations. The Δ potential in nuclear matter has been calculated by using a Bruckner-Hartree-Fock approximation. By including the mean-field effects on the Δ propagation, the effective cross sections of the NN→NΔ*, NΔ*→NN and NΔ*→NΔ* reactions in nuclear matter are predicted. It is demonstrated that the density dependence is most dramatic in the energy region close to the pion production threshold. © 1996 The American Physical Society.

The particle density distribution emerging from the solution of the Vlasov equation for relativistic, noninteracting particles with spherically symmetric initial conditions is shown to exhibit a shell-like structure for late fixed times in the center-of-mass (CM) frame of the system. A similar phenomenon was recently observed employing the test-particle method to solve the Vlasov equation for quarks with Nambu–Jona-Lasinio-type interactions, and was attributed to the attractive forces among the particles. Contrary to this claim, it is demonstrated here that this effect is of purely relativistic origin and is sensitive only to the mass of the particles and their initial phase-space distribution. © 1996 The American Physical Society.

The distribution of low mass dielectrons likely to be produced in relativistic heavy ion collisions due to bremsstrahlung processes in quark matter and hadronic matter is obtained. Effort is made to minimize uncertainty and restore Lorentz covariance as compared with past calculations performed in a similar vein. It is found that the excess of dielectrons in S+Au collisions at 200A GeV measured by the CERES Collaboration at the CERN Super Proton Synchrotron may have an important contribution from these processes at very low masses. ©1996 The American Physical Society.

A nonthermal expansive component has recently been interpreted from the observed light fragment spectra in Au + Au collisions. We have used the BUU transport model to generate several different freezeout surfaces and applied a coalescence algorithm to approximate the complete final state. We vary the microscopic details leading to specific equations of state, reduce cross sections, and isolate the effect of compression on the spectra for protons and helium isotopes. We find a radial flow signal consistent with experiment in the energy range 0.25A to 1.15A GeV, but find it to be rather insensitive to the microscopic details of the model calculations. © 1996 The American Physical Society.