Refine
Year of publication
Document Type
- Article (31421) (remove)
Language
- English (16026)
- German (13387)
- Portuguese (696)
- French (387)
- Croatian (251)
- Spanish (250)
- Italian (134)
- Turkish (113)
- Multiple languages (36)
- Latin (35)
Has Fulltext
- yes (31421)
Keywords
- Deutsch (503)
- taxonomy (451)
- Literatur (299)
- new species (194)
- Hofmannsthal, Hugo von (185)
- Rezeption (178)
- Übersetzung (163)
- Filmmusik (155)
- Johann Wolfgang von Goethe (131)
- Vormärz (117)
Institute
- Medizin (5408)
- Physik (1958)
- Biowissenschaften (1152)
- Biochemie und Chemie (1113)
- Extern (1108)
- Gesellschaftswissenschaften (803)
- Frankfurt Institute for Advanced Studies (FIAS) (753)
- Geowissenschaften (593)
- Präsidium (453)
- Philosophie (448)
We investigate the possibility that high-energy photons are channeled, when passing through an oriented single crystal, due to Delbrück scattering. For this purpose the exact electron propagator for the single-string model is constructed. Starting from a separation of variables, we solve the Dirac equation for a cylindrical electrostatic potential. The propagator for such external fields is constructed from solutions of the radial Dirac equation. This propagator is applied to a calculation of the S matrix for Delbrück scattering. We specify the conditions under which photon channeling takes place. Unfortunately these conditions are only matched for a very small fraction of those photons being produced by channeled electrons.
We investigate the influence of nuclear masses, radii, and interaction potentials on 12C radioactivity of 114the best representative of a new island of cluster emitters leading to daughter nuclei around the doubly magic 100Sn. Three different models are considered: one derived by Blendowske, Fliessbach, and Walliser (BFW) from the many-body theory of alpha decay, as well as our analytical (ASAF) and numerical (NuSAF) superasymmetric fission models. A Q value larger by 1 MeV or an ASAF potential barrier reduced by 3% are producing a half-life shorter by 2 orders of magnitude. A similar effect can be obtained within BFW and NuSAF by a decrease of the action integral with less than 10% and 5%, respectively. By increasing the radius constant within ASAF or BFW models by 10%, the half-life becomes shorter by 3 orders of magnitude.
The properties of nuclear matter are studied in the framework of quantum hadrodynamics. Assuming an ω-meson field, periodic in space, a self-consistent set of equations is derived in the mean-field approximation for the description of nucleons interacting via σ-meson and ω-meson fields. Solutions of these self-consistent equations have been found: The baryon density is constant in space, however, the baryon current density is periodic. This high density phase of nuclear matter can be produced by anisotropic external pressure, occurring, e.g., in relativistic heavy ion reactions. The self-consistent fields developing beyond the instability limit have a special screw symmetry. In the presence of such an ω field, the energy spectrum of the relativistic nucleons exhibits allowed and forbidden bands, similar to the energy spectrum of the electrons in solids.
We investigate the production of heavy quarks in continuum and bound states in nuclear collisions. Creation rates for free bb and tt quark pairs and for bottomonium and toponium in the ground state are computed at energies of the BNL Relativistic Heavy Ion Collider, CERN Large Hadron Collider (LHC), and Superconducting Super Collider. Central and peripheral heavy-ion collisions are discussed. For top-quark creation we assumed a mass range of 90≤mt≤250 GeV. The creation rate for top quarks in peripheral collisions is estimated to be by a factor 40 to 130 smaller compared with corresponding central collisions. For mt=130 GeV we calculated a creation rate of about 4760 top-quark pairs per day at the LHC (3.5 TeV/nucleon) for Pb-Pb collisions.
We discuss the multiplicity distribution of electron-positron pairs created in the strong electromagnetic fields of ultrarelativistic heavy-ion transits. Based on nonperturbative expressions for the N-pair creation amplitudes, the Poisson distribution is derived by neglecting interference terms. The source of unitarity violation is identified in the vacuum-to-vacuum amplitude, and a perturbative expression for the mean number of pairs is given.
The Gottfried sum-rule violation reported by the New Muon Collaboration was interpreted as an indication for a flavor asymmetry of the sea quark in the nucleon. We investigate the alternative possibility that isospin symmetry between the proton and the neutron is breaking. We examine systematically the consequences of this possibility for several processes, namely, neutrino deep inelastic scattering, the charged pion Drell-Yan process, the proton Drell-Yan process, and semi-inclusive deep inelastic scattering, and conclude that a decision between the two alternative explanations is possible.
A new region of proton-rich parent nuclei decaying by spontaneous cluster emission with a measurable branching ratio relative to alpha decay is predicted within the analytical superasymmetric fission model. After a brief presentation of the model and of the seven mass tables used to calculate the released energy, the obtained results are discussed. Measurable half-lives and branching ratios are estimated for 12C, 16O, 28Si, and other cluster radioactivities of some nuclides having proton and neutron numbers in the range Z=56–64 and N=58–72. Such nuclei far from stability could be produced in reactions induced by radioactive beams.
Ionization, pair creation, and electron excitations in relativistic heavy-ion collisions are investigated in the framework of the coupled-channel formalism. Collisions between heavy projectiles and Pb82+ are considered for various bombarding energies in the region E=500 up to 2000 MeV/u. Useful symmetry relations for the matrix elements are derived and the influence of gauge transformations onto the coupled-channel equations is explored.
Using relativistic Green’s-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei.
A method is presented to define unique continuum states for the two-center Dirac Hamiltonian. In the spherical limit these states become the familiar angular-momentum eigenstates of the radial Coulomb potential. The different states for a fixed total energy ‖E‖>m may be distinguished by considering the asymptotic spin-angular distribution of states with unique scattering phases. The first numerical solutions of the two-center Dirac equation for continuum states are presented.
We present calculations for the impact-parameter dependence of K-shell ionization rates in p¯-Cu and in p¯-Ag collisions at various projectile energies. We show that the effect of the attractive Coulomb potential on the Rutherford trajectory and the antibinding effect caused by the negative charge of the antiproton result in a considerable increase of the ionization probability. Total ionization cross sections for proton and antiproton projectiles are compared with each other and with experimental ionization cross sections for protons.
Positron creation in crossed-beam collisions of high-energy, fully stripped heavy ions is investigated within the coupled-channel formalism. In comparison with fixed-target collisions of highly stripped heavy-ion projectiles positron production probabilities are enhanced by more than one order of magnitude. The increase results from the possibility to excite electrons from the negative energy continuum into all bound states. The positron spectrum is shifted towards higher energies because of the absence of electron screening. Rutherford scattering as well as nuclear collisions with time delay are investigated. We also discuss the filling of empty bound states by electrons from pair-production processes.
We study a relativistic model of the nucleus consisting of nucleons coupled to mesonic degrees of freedom via an effective Lagrangian whose parameters are determined by a fit to selected nuclear ground-state data. We find that the model allows a very good description of nuclear ground-state properties. Because of the relativistic nature of the model, the spin properties are uniquely fixed. We discuss variations of the parametrization and of the data which suggest that the present fit has exhausted the limits of the mean-field approximation, and discuss extensions which go beyond the mean field.
The properties of symmetric nuclear matter are investigated in the nonlinear relativistic mean field theory of nuclear matter. We consider the constraints imposed by four nuclear ground state properties on the coupling constants and on the equation of state at zero and at finite temperature. We find that the compression constant K(ρ0) as well as the temperature is irrelevant for the stiffness of the equation of state for m*(ρ0)≤0.7. The main point is that the relativistic mean field theory exhibits acausal and unphysical behavior for compressibilities below K(ρ0)=200 MeV. Every set of coupling constants with a negative quartic coupling constant c is unstable against small quantum fluctuations.
We investigate the influence of additional nonlinear terms in the Dirac Lagrangian on strongly bound electron states in heavy and superheavy atoms. Upper bounds for the coupling constants are deduced by comparison with precision spectroscopy data in QED. We demonstrate that nonlinear interactions may cause significant modifications of electron binding energies in superheavy quasiatomic systems which would not be visible in ordinary atomic-physics measurements.
We calculate angular correlations between coincident electron-positron pairs emitted in heavy-ion collisions with nuclear time delay. Special attention is directed to a comparison of supercritical and subcritical systems, where angular correlations of pairs produced in collisions of bare U nuclei are found to alter their sign for nuclear delay times of the order of 2 × 10-21 s. This effect is shown to occur exclusively in supercritical systems, where spontaneous positron creation is active.
Parity mixing of electron states should be extremely strong for heliumlike uranium. We calculate its size and discuss whether it could be determined experimentally. We analyze one specific scheme for such an experiment. The required laser intensities for two-photon spectroscopy of the 23P0–2 1S0level splitting is of the order of 1017 W/cm2. A determination of parity mixing would require at least 1021 W/cm2.
The magnetic dipole scattering of neutrinos by the electrostatic potentials of single atoms as well as crystals is investigated. It is shown that scattering by a rigid cubic lattice can amplify the neutrino-atom cross section by a factor of N1/3, N being the number of scatterers. However, comparing the results with typical weak-interaction cross sections, the effect seems to be not observable in experiment.
The angular distribution of electrons and positrons emitted in internal pair conversion is calculated. Coulomb-distorted waves are used as electron wave functions. Nuclear transitions of various multipolarities L>0 and of magnetic (ML) and of electric (EL) type are considered as well as E0 conversion. Analytical expressions for the angular correlation are derived, which are evaluated numerically assuming a finite extension of the nucleus and, for the EL and ML conversion, also in the point-nucleus approximation. The calculated angular correlations are compared with results obtained within the Born approximation and, for the E0 case, with experimental data.
We consider the contribution of nuclear polarization to the Lamb shift of K- and L-shell electrons in heavy atoms and quasiatoms. Our formal approach is based on the concept of effective photon propagators with nuclear-polarization insertions treating effects of nuclear polarization on the same footing as usual QED radiative corrections. We explicitly derive the modification of the photon propagator for various collective nuclear excitations and calculate the corresponding effective self-energy shift perturbatively. The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational states is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear-polarization effects are of minor importance for Lamb-shift studies in 82208Pb.
Antileukoproteinase (ALP) is a physiological inhibitor of granulocytic serine proteases that has been shown to have anti-inflammatory properties in addition to its antiproteolytic activity. On the basis of its potential to block anti-collagen type II (CII) antibody-induced arthritis (CAIA) and to suppress the conformational activation of β2-integrins in leukocytes, the present study was undertaken to investigate its interference with leukocyte adherence to cytokine-activated endothelium. The potential of recombinant ALP to block the interactions of leukocytes with the endothelial lining was concomitantly investigated in vitro and in vivo. Thus, intravital fluorescence microscopic imaging of leukocyte rolling and firm adhesion to postcapillary venules were performed in the knee joints of DBA1/J mice after intravenous injection of anti-CII mAbs. An IL-1β-activated endothelial layer formed by a murine glomerular cell line (glEND.2) was used to assay the interaction with human leukocytes in vitro. Electromobility shift and luciferase reporter gene assays permitted the analysis of cytokine-induced activation of the NF-κB pathway. Fluorescence-activated cell sorting was applied to determine endothelial E-selectin expression. Leukocyte rolling and firm adhesion to the synovial endothelium in an early response to the anti-CII antibody transfer were significantly decreased in ALP-pretreated mice. Concomitantly, ALP suppressed the IL-1β-induced NF-κB activation and the upregulation of E-selectin expression in glEND.2 cells in vitro. These findings support the notion that the newly uncovered properties of ALP to interfere with cytokine signalling and upregulation of adhesion molecules in endothelial cells are likely to contribute to the therapeutic potential of ALP in immune-complex-induced tissue injury.
The molecular particle-core model is applied to the scattering of 13C on 13C. The model divides the 13C+ 13C system into two 12C cores and two valence neutrons. The valence neutrons are described with molecular eigenfunctions of the symmetric two-center shell model. Coupled channel calculations are carried out for the inelastic single and mutual excitation of the first (1/2+ state of 13C and the neutron transfer to the 12C+14C system. The results reproduce the experimental data. The analysis of the S matrix shows that the gross structure of the transfer excitation function is related to resonances in the relative motion of the elastic and transfer channels.
We discuss the possibility that nuclei with very large baryon numbers can exist in the form of large quark blobs in their ground states. A calculation based on the picture of quark bags shows that, in principle, the appearance of such exotic nuclear states in present laboratory experiments cannot be excluded. Some speculations in connection with the recently observed anomalous positron production in heavy-ion experiments are presented.
We present a mechanism for the separation of strangeness from antistrangeness in the deconfinement transition. For a net strangeness of zero in the total system, the population of s quarks is greatly enriched in the quark-gluon plasma, while the s¯ quarks drift into the hadronic phase. This separation could result in ‘‘strangelet’’ formation, i.e., metastable blobs of strange-quark matter, which could serve as a unique signature for quark-gluon plasma formation in heavy-ion collisions. PACS: 25.70.Np, 12.38.Mh
If the local color symmetry in a quark-gluon matter is broken, the expectation value of the gluon field 〈Aμa(x)〉 may be different from zero. Such a gluon-condensed phase has been found in mean field approximation. The gluon-condensed phase is characterized by a static, periodic chromomagnetic field, which is coupled to a periodic spin-color density distribution of quarks and antiquarks. Transitions of first and second order type have been found between the gluon-condensed and normal phases, the latter characterized by the vanishing value of the mean gluon field.
We formulate a group-theoretical projection technique for the quantum-statistical description of systems with exactly conserved charges corresponding to local non-Abelian gauge symmetries. The formalism is specified for SU(N) internal symmetry and a partition function related to a mixed canonical–grand-canonical ensemble is defined. Its perturbation expansion is derived, and we point out potential applications. We also study single-particle Green’s functions for the calculation of mixed ensemble averages with the help of a generalized Wick’s theorem and find that a connected-graphs expansion is impossible.
Conversion processes in light nuclei with transition energies above the e+, e- pair creation threshold are investigated within an analytical framework. In particular, we evaluate the ratio of electron transition probabilities from the negative energy continuum into the atomic K shell and into the positive energy continuum, respectively. The possible role of monoenergetic positron conversion with respect to the striking peak structures observed in e+ spectra from very heavy collision systems is examined.
The inelastic excitation of the (1/2)+ (871 keV) state of 17O in the reaction of 13C on 17O is described by a time-dependent quantum mechanical model with two diabatic states and a classical treatment of the radial relative motion. The structures in the angle-integrated cross section are interpreted as caused by the barriers of the angular momentum-dependent potentials. The transition strength is enhanced by the Landau-Zener effect between the levels considered.
Phenomenological consequences of a hypothetical light neutral particle in heavy ion collisions
(1986)
We discuss the possibility that the line structure observed in the spectrum of the positrons produced in heavy ion collisions is due to the decay of a new neutral elementary particle. We argue that this can be ruled out unless one is willing to accept fine tuning of parameters, or to assume the dominance of nonlinear effects.
We compute the energy spectrum of photons which originate from the quark-annihilation process ss¯→γg in quark-gluon plasma. The spectrum peaks at an energy Eγmax∼2ms∼400 MeV in the rest frame of the plasma. We expect one photon from the above process in the energy range of 2ms±0.25ms per hundred quark-gluon plasmas of a size R=3 fm and a lifetime τ=6 fm/c formed in nuclear collisions.
By using the analytical superasymmetric fission model it is shown that all ‘‘stable’’ nuclei lighter than lead with Z>40 are metastable relative to the spontaneous emission of nuclear clusters. An even-odd effect is included in the zero point vibration energy. Half-lives in the range 1040–1050 s are obtained for Z>62. The region of metastability against these new decay modes is extended beyond that for α decay and in some cases, in the competing region, the emission rates for nuclear clusters are larger than for α decay.
The great majority of the known nuclides with Z>40, including the so-called stable nuclides, are metastable with respect to several modes of spontaneous superasymmetric splitting. A model extended from the fission theory of alpha decay allows one to estimate the lifetimes and the branching ratios relative to the alpha decay for these natural radioactivities. From a huge amount of systematic calculations it is concluded that the process should proceed with maximum intensity in the trans-lead nuclei, where the minimum lifetime is obtained from parent-emitted heavy ion combinations leading to a magic (208Pb) or almost magic daughter nucleus. More than 140 nuclides with atomic number smaller than 25 are possible candidates to be emitted from heavy nuclei, with half-lives in the range of 1010–1030 s: 5He, 8–10Be, 11,12B, 12–16C, 13–17N, 15–22O, 18–23F, 20–26Ne, 23–28Na, 23–30Mg, 27–32Al, 28–36Si, 31–39P, 32–42S, 35–45Cl, 37–47Ar, 40–49 K, 42-51. . .Ca, 44–53 Sc, 46–53Ti, 48–54V, and 49–55 Cr. The shell structure and the pairing effects are clearly manifested in these new decay modes.
Strange particle abundances in small volumes of hot hadronic gas are determined in the canonical ensemble with exact strangeness and baryon number conservation. Substantial density and baryon number dependence is found. A p¯d experiment is examined and applications to p¯-nucleus annihilations are considered.
Strong indirect evidence exists for the existence of attractive forces between nuclei making surface contact. Experimentally, the recent observations of spontaneous positron production in heavy-ion collisions can only be understood if nuclei stick together for times long compared to the collision time. We show that any such tendency for nuclei to attract implies the existence of nuclear molecules with entirely new kinds of collective modes. We present a simple model for these modes and apply it to 238U-238U.
Atomic excitations are used to obtain information on the course of a nuclear reaction. Employing a semiclassical picture we calculate the emission of δ electrons and positrons in deep inelastic nuclear reactions for the example of U+U collisions incorporating nuclear trajectories resulting from two different nuclear friction models. The emission spectra exhibit characteristic deviations from those expected for elastic Coulomb scattering. The theoretical probabilities are compared with recent experimental data by Backe et al. A simple model is used to estimate the influence of a threebody breakup of the compound system upon atomic excitations.
For the scattering of 28Si on 28Si coupled channel calculations of the elastic scattering and inelastic single excitation of the first 2+ state of 28Si are carried out. The real coupling potentials are calculated in the framework of an adiabatic model. The resulting cross sections reveal structures in agreement with the observed ones and support their interpretation as nuclear molecular resonances.
We compare a proximity-type potential for two interacting nuclei with the double-folding method. Both spherical and deformed systems are considered. Special "orientation windows" are found for two deformed nuclei giving rise to nuclear cohesion. If the same nucleon-nucleon interaction is utilized, the proximity and the double-folding potentials agree fairly well for a spherical + deformed system. However, deviations are found in the case of two deformed nuclei.
A new spontaneous-symmetry-breaking mechanism is formulated for SU(3), which is used to describe the formation of bags around quarks. The Higgs field is replaced by the scalar product of two colored fermion fields. This model gives mass only to one gluon (equivalent to Aμ8) when spontaneously broken. The consequences of this scheme are discussed, and it is argued that it can explain several puzzling high-energy heavy-ion experiments.
Excitations of the atomic shell in heavy-ion collisions are influenced by the presence of a nuclear reaction. In the present Rapid Communication we point out the equivalence between a semiclassical description based on the nuclear autocorrelation function with an earlier model which employs a distribution of reaction times f(T). For the example of U+U collisions, results of coupled-channel calculations for positron creation and K-hole excitations are discussed for two schematic reaction models.
The collision process is described by hydrodynamical equations. The escape of nucleons which do not take part in the thermal equilibrium is considered by including drain terms in these equations. The energy spectra of the escaped nucleons and of nucleons evaporated after the breakup of the fluid are compared. NUCLEAR REACTIONS Relativistic heavy ion reactions, nuclear hydrodynamics, nucleon spectra.
The nucleons taking part in heavy ion reaction are considered as a three-component fluid. The first and second components correspond to the nucleons of the target and the projectile, while the thermalized nucleons produced in the course of the collision belong to the third component. Making use of the Boltzmann equation, hydrodynamical equations are derived. An equation of state for anisotropic nuclear matter obtained from a field theoretical model in mean field approximation is applied in a one dimensional version of the three-component fluid model. The speed of thermalization is analyzed and compared to the results of cascade and kinetic models. NUCLEAR REACTIONS Relativistic heavy-ion reactions, hydrodynamic description.
The energy shift of K electrons in heavy atoms due to the self-energy correction has been calculated. This process is treated to all orders in Zα, where Z denotes the nuclear charge. For the superheavy system Z=170, where the K-shell binding energy reaches the pair-production threshold (E1sb∼2mc2), a shift of +11.0 keV is found. This shift is almost cancelled by the vacuum polarization, leaving a negligible effect for all quantum-electrodynamical corrections of order α but all orders of Zα.
Two-center level diagrams for the neutron orbitals in the scattering of 16O on 25Mg and of 17O on 24Mg are calculated by using a deformed potential for 24,25Mg. Possible consequences of the nuclear Landau-Zener mechanism, namely the promotion of nucleons at avoided level crossings, and of the rotational coupling between crossing molecular single-particle orbitals are studied for inelastic excitation and neutron transfer. The important excitation and transfer processes, which are enhanced by the promotion process and the rotational coupling, are presented. NUCLEAR REACTIONS Heavy ion scattering, theory of nucleon transfer, molecular wave functions, asymmetric two center shell model, single particle excitation, deformed nuclei.
Collisions of very heavy ions at energies close to the Coulomb barrier are discussed as a unique tool to study the behavior of the electron-positron field in the presence of strong external electromagnetic fields. To calculate the excitation processes induced by the collision dynamics, a semiclassical model is employed and adapted to describe the field-theoretical many-particle system. An expansion in the adiabatic molecular basis is chosen. Energies and matrix elements are calculated using the monopole approximation. In a supercritical (Z1+Z2≳173) quasiatomic system the 1s level joins the antiparticle continuum and becomes a resonance, rendering the neutral vacuum state unstable. Several methods of treating the corresponding time-dependent problem are discussed. A projection-operator technique is introduced for a fully dynamical treatment of the resonance. Positron excitation rates in s1/2 and p1/2 states are obtained by numerical solution of the coupled-channel equations and are compared with results from first- plus second-order perturbation theory. Calculations are performed for subcritical and supercritical collisions of Pb-Pb, Pb-U, U-U, and U-Cf. Strong relativistic deformations of the wave functions and the growing contributions from inner-shell bound states lead to a very steep Z dependence of positron production. The results are compared with available data from experiments done at GSI. Correlations between electrons and positrons are briefly discussed.
We define a new scalar-tensor theory with an effective gravitational coupling constant depending on a scalar field. The coupling is such that the gravitational interaction decreases with the strength of the scalar field. We show that this is not sufficient to prevent the gravitational collapse of sufficiently massive dense objects.
We calculate the spin polarization of 1sσ vacancies and emitted δ electrons induced by the strong magnetic field (|Bmax|∼1016G) in collisions of very heavy ions (Z1+Z2=178). The electron excitations are determined by the solution of coupled-channel equations within the quasimolecular basis states including the vector potential. The formulation is extended to the many-electron case. Spin polarizations of the order of 5-10% for impact energies below the Coulomb barrier are predicted.
The origin and importance of electron-translation effects within a molecular description of electronic excitations in heavy-ion collisions is investigated. First, a fully consistent quantum-mechanical description of the scattering process is developed; the electrons are described by relativistic molecular orbitals, while the nuclear motion is approximated nonrelativistically. Leaving the quantum-mechanical level by using the semiclassical approximation for the nuclear motion, a set of coupled differential equations for the occupation amplitudes of the molecular orbitals is derived. In these coupled-channel equations the spurious asymptotic dynamical couplings are corrected for by additional matrix elements stemming from the electron translation. Hence, a molecular description of electronic excitations in heavy-ion scattering has been achieved, which is free from the spurious asymptotic couplings of the conventional perturbated stationary-state approach. The importance of electron-translation effects for continuum electrons and positrons is investigated. To this end an algorithm for the description of continuum electrons is proposed, which for the first time should allow for the calculation of angular distributions for δ electrons. Finally, the practical consequences of electron-translation effects are studied by calculating the corrected coupling matrix elements for the Pb-Cm system and comparing the corresponding K-vacancy probabilities with conventional calculations. We critically discuss conventional methods for cutting off the coupling matrix elements in coupled-channel calculations.
Different collective deformation coordinates for neutrons and protons are introduced to allow for both stretching and γ transitions consistent with experiments. The rotational actinide nuclei 234-238U and 232Th are successfully analyzed in this model. NUCLEAR STRUCTURE 232Th, 234-238U calculated B (E2) values, collective model.
Dynamics of Coulomb fission
(1980)
A general formalism is described for the treatment of Coulomb fission, within the framework of the semiquantal theory. We develop a model for the fission probabilities of levels excited in Coulomb excitation. This model contains penetration of the double-humped fission barrier, competition from gamma and neutron emission, and the spreading of the collective states into noncollective compound states. For 74184W + 92238U, the fission probability at θc.m.=180° is increased by a factor of 3.9, 3.3, and 2.0 at E/ECoul=0.77, 0.85, and 0.935, respectively, compared to the simplified sharp cutoff model used in earlier model calculations. The enhancement comes from barrier penetration. The damping of the fission probability due to spreading into noncollective compound states is small. Prompt Coulomb fission (near the distance of closest approach) is studied in a one-dimensional model. The results clearly imply that prompt fission is negligible. We have also studied the sudden approximation for collective rotational levels in connection with Coulomb fission. At high spins (I≈20), it leads to significant errors. Contrary to the basic assumption of the sudden approximation that the nuclear symmetry axis remains fixed during the collision, it is shown that Coulomb excitation results in a strong alignment of the nuclear symmetry axis perpendicular to the beam axis at small internuclear distances. NUCLEAR REACTIONS, FISSION Semiquantal theory of prompt and asymptotic Coulomb fission, study of double-humped barrier penetration, damping effects, neutron and γ emission. Calculated σ(Ep, θc.m.=180°).
Phase transitions in nuclear matter A method for the description of spin-isospin phase transitions in nuclear matter is developed. It allows a complete description of the pion condensation phase transition in the framework of the Landau-Migdal Fermi liquid theory. The equation of the order parameter is derived and the condensation energy is calculated. We study the influence of pion condensation on the nuclear equation of state and the temperature dependence of pion condensation. NUCLEAR STRUCTURE Description of pion-condensed ground state by Green's function technique.
In heavy ion collisions, the molecular single-particle motion may cause specific structures in the energy dependence of the cross sections which arise by the promotion of nucleons at level crossings according to the Landau-Zener excitation mechanism. In order to examine this effect in asymmetric heavy ion collisions, we have calculated level diagrams of the two-center shell model for the target projectile combinations 13C + 16O and 12C + 17O and analyzed with respect to inelastic excitation and neutron transfer. We select certain reactions as possible candidates for showing enhanced cross sections for nucleon excitation and transfer due to real and avoided level crossings near the Fermi level.
This paper reports calculations of the influence of a reaction time T>10-21 s in deep-inelastic Xe-Pb collisions on the energy spectrum of δ electrons ejected in the same collision. It is shown that the lifetime of the superheavy composite system causes pronounced oscillations of width ε=h/T in the electron distribution, which survive the inclusion of multistep excitations and the folding with a lifetime distribution function. This effect may serve as an atomic clock for deep-inelastic collisions.
This Letter discusses inner-shell excitation in collisions of very heavy ions (Z1+Z2≳140) in the framework of the quasimolecular model. The importance of multistep excitations and of coupling between continuum states is demonstrated. The 1sσ vacancy probabilities resulting from coupled-channels calculations exceed perturbation theory by a factor 3-5, thus giving good agreement with recent experimental results.
Binding energies and wave functions of inner-shell electronic states in superheavy quasimolecules with (Zp+Zt)α>1 are calculated. Ionization during a collision of very heavy ions is investigated within a molecular basis generated by the solutions of the two-center Dirac equation. Transitions to vacant bound states as well as direct excitation to the continuum are taken into account. We present theoretical values for the ionization probability as a function of impact parameter, bombarding energy, and combined nuclear charge. Our computed results are compared with recent experimental data. It is suggested that relativistic binding energies of electrons in superheavy quasimolecules can be determined experimentally via the impact-parameter dependence of ionization and the anisotropy of quasimolecular radiation.
Within an extended semiquantal theory we perform large-sized coupled-channel calculations involving 260 collective levels for Coulomb fission of 238U. Differential Coulomb fission cross sections are studied as a function of bombarding energy and impact parameter for several projectiles. In the Xe + U case, total cross sections are also given. We find a strong dependence on projectile charge number, PCF(180°)∼(Zp)6 in the region 50≤Zp≤92 for a fixed ratio E/ECoul, which might be helpful to separate Coulomb fission experimentally from sequential fission following transfer reactions. Since the cross sections are sensitive to the moment of inertia ⊖ at the saddle point, Coulomb fission can serve as a tool to investigate the dependence of ⊖ on elongation. The fragment angular distribution exhibits deviations from 1/sinθf which are pronounced at low incident energies. Our theory indicates that the recently measured Xe + U fission cross sections contain a major fraction of Coulomb-induced fission at E≤0.85 ECoul. NUCLEAR REACTIONS, FISSION Calculated Coulomb fission cross sections σ(Ep,θp) for 54Xe, 67Ho, 82Pb, 92U→92238U, fragment angular distribution, fission energy spectrum, mean spin value 〈Jf〉.
The theory of nucleon transfer in heavy ion reactions is formulated on the basis of the molecular particlecore model for a system consisting of two cores and one extracore nucleon. The extracore nucleon is described by the molecular wave functions of the asymmetric two-center shell model. The cores, which are assumed to be collectively excitable, are treated with vibrator-rotator models. Potentials for shape polarization are contained in the asymmetric two-center shell model and the interaction between the cores. The excitation and transfer of the extracore nucleon is induced by the radial and rotational couplings. The coupled channel equations, which include the recoil effects in first approximation, are derived in a form suitable for numerical calculations of cross sections. NUCLEAR REACTIONS Heavy ion scattering, theory of nucleon transfer, molecular wave functions, two-center shell model, collective and single-particle excitation.
Inner-shell ionization induced by nuclear Coulomb excitation in collisions of very heavy ions
(1978)
K- and L-shell ionization of 238U with Xe and U projectiles is investigated. Internal conversion following nuclear Coulomb excitation which is particularly important for deformed heavy nuclei is compared with direct ionization of inner-shell electrons in superheavy quasimolecules. Both processes exhibit different impact-parameter dependences. As a result of internal conversion, about 0.1-0.3 K holes per central collision are created.
On the basis of the two-center shell model a theory is developed for the excitation of loosely bound nucleons in heavy ion collisions. These nucleons move in the two-center shell model potential generated by all the nucleons and are described by molecular wave functions. The model is applied to calculate the cross sections for the elastic and inelastic 13C-13C scattering. The cross sections show intermediate structures caused by the excitation of quasibound resonances in the molecular nucleus-nucleus potential. NUCLEAR REACTIONS 13C(13C,13C) molecular wave functions, dynamical two-center shell model, quasimolecular resonances, radial and Coriolis coupling, coupled channel calculations for σ(θ).
Quasimolecular resonance structures in the 12C-12C system are studied in the framework of the coupled channel formalism in the energy range Ec.m.=5-14 MeV. The influence of the coupling of the first excited 2+ state in 12C on the resonance structures is investigated by choosing various types of coupling potentials. The intermediate structures in the reflection and transition coefficients and cross sections can be interpreted with the double resonance mechanism. NUCLEAR REACTIONS 12C(12C, 12C), quasimolecular states, coupling potentials, coupled channel calculations for σ(θ).
The theory of direct electron-positron pair production in the collision of heavy ions is formulated in the framework of the quasimolecular model. The pair production process acquires a collective nature for (Z1+Z2)α>1 and can be understood as the shakeoff of the strong vacuum polarization cloud formed in the quasimolecule. The total cross section is, e.g., 76 μb for Pb + Pb at Coulomb barrier energies.
Internal conversion of γ rays from Coulomb-excited nuclear levels cannot be neglected compared with the spontaneous and induced positron production in overcritical electric fields. It is shown that both processes are separable by their different distributions with respect to the ion angle and the positron energy.
We show that information about quasimolecular electronic binding energies in transient atomic systems of Z=Z1+Z2 up to 184 can be obtained from three sources: (1) the impact-parameter dependence of the ionization probability; (2) the ionization probability in head-on collisions as a function of total nuclear charge Z; (3) the delta-electron spectrum in coincidence with K-vacancy formation in asymmetric collisions. Experiments are proposed and discussed.
Determination of the effective 12C + 12C potential from the sub-Coulomb single-particle resonances
(1974)
The sub-Coulomb resonances observed in the total reaction yield of the 12C + 12C system at 4.9, 5.6, and 6.2 MeV are explained as single-particle resonances. The "true" effective 12C + 12C potential is determined directly as the real potential which reproduces best the position and the spacing of the observed sub-Coulomb resonances. This potential is found from a parametrization of the two limiting adiabatic and sudden potentials.
the development of the mass asymmetry vibrations in the final stages of the fission process is studied with an approximate treatment of the coupling to relative motion. A parametrized friction is introduced and its effects are studied. Numerical results are presented for 236U, together with estimates for the kinetic energy of the fragments. RADIOACTIVITY, FISSION 236U; calculated mass distribution, kinetic energy distribution. Collective dynamics, shell correction method, cranking model.
It is shown that nuclear matter is compressed during the encounter of heavy ions. If the relative velocity of the nuclei is larger than the velocity of first sound in nuclear matter (compression sound for isospin T=0), nuclear shock waves occur. They lead to densities which are 3-5 times higher than the nuclear equilibrium density ρ0, depending on the energy of the nuclei. The implications of this phenomenon are discussed.
The mechanisms of spontaneous and induced emission of radiation are derived from the Dirac equation in a rotating coordinate system. The molecular-orbital x-ray spectra exhibit a strong asymmetry with respect to the beam axis. The asymmetry peaks for the high-energy transitions, which can be used for spectroscopy of two-center orbitals.
In critical or nearly critical heavy-ion collisions, induced as well as spontaneous energyless e-e+ pair creation result in the decay of the neutral vacuum. Induced transitions from the negative-energy continuum into a vacant molecular 1s level can occur even in the absence of diving and produce a substantial enhancement and broadening of the previously considered spontaneous positron spectrum. Total cross sections of 5 b have been calculated for U-U collisions.
With the mass asymmetry described by the dynamical collective fragmentation coordinate ξ, and with use of the asymmetric two-center shell model, the fission mass distributions for 226Ra, 236U, and 258Fm (which are typical representatives for triple-, double-, and single-humped distributions) are explained.
In view of new high-precision experiments in atomic physics it seems necessary to reexamine nonlinear theories of electrodynamics. The precise calculation of electronic and muonic atomic energies has been used to determine the possible size of the upper limit Emax to the electric field strength, which has been assumed to be a parameter. This is opposed to Born's idea of a purely electromagnetic origin of the electron's mass which determines Emax. We find Emax≥1.7×1020 V/cm.
A general formalism for the scattering of heavy ions, which is especially convenient to study the antisymmetrization effects, is developed. Antisymmetrization effects are investigated by expanding the completely antisymmetrized wave function according to the number of exchanged nucleons. The particle-core model for the scattering of nuclei with loosely bound nucleons is presented. A formula for the additional contribution to the effective potential due to antisymmetrization effects is obtained by calculating the expectation value of the Hamiltonian with intrinsic wave functions. Application of the formalism is illustrated for the 14N + 14N scattering problem and its usefulness is demonstrated.
A fully gauge-invariant, Lorentz-covariant, nonlocal, and nonlinear theory, for coupled spin-½ fields, ψ, and vector fields, A, i.e., "electrons" and "photons," is constructed. The field theory is linear in the ψ fields. The nonlinearity in the A fields arises unambiguously from the requirement of gauge invariance. The coordinates are generalized to admit hypercomplex values, i.e., they are taken to be Clifford numbers. The nonlocality is limited to the hypercomplex component of the coordinates. As the size of the nonlocality is reduced toward zero, the theory goes over into the inhomogeneous Dirac theory. The nonlocality parameter corresponds to an inverse mass and induces self-regulatory properties of the propagators. It is argued that in a gauge-invariant theory a graph-by-graph convergence is impossible in principle, but it is possible that convergence may hold for the complete solution, or for sums over classes of graphs.
The 1s bound state of superheavy atoms and molecules reaches a binding energy of -2mc2 at Z≈169. It is shown that the K shell is still localized in r space even beyond this critical proton number and that it has a width Γ (several keV large) which is a positron escape width for ionized K shells. The suggestion is made that this effect can be observed in the collision of very heavy ions (superheavy molecules) during the collision.
A continuum shell-model calculation based on the collective correlation model has been made for the giant resonance of 12C using the eigenchannel reaction theory. The low-lying negative-parity states of 11C and 11B have been taken into account by corehole coupling. Partial, total, and integrated photoabsorption cross sections are calculated for the region of the giant dipole resonance.
With the use of the cranking formula, the coordinate-dependent mass parameters of the kinetic-energy operator in fission processes and heavy-ion collisions are calculated in the two-center oscillator model. It is shown that the reduced mass and also the classical moment of inertia are obtained for large separations of the fragments. For small separations, however, the mass parameter for the motion of the centers of mass of the fragments is larger than the reduced mass by an order of magnitude.
An upper limit to the electric field strength, such as that of the nonlinear electrodynamics of Born and Infeld, leads to dramatic differences in the energy eigenvalues and wave functions of atomic electrons bound to superheavy nuclei. For example, the 1s1/2 energy level joins the lower continuum at Z=215 instead of Z=174, the value obtained when Maxwell's equations are used to determine the electric field.
The dynamic collective model has been extended to quadrupole giant resonances in spherical nuclei. The splitting of giant dipole and giant quadrupole resonances due to their coupling to surface vibrations has been calculated for Sn isotopes. Agreement with recent γ-absorption measurements of the Livermore group has been found.
A two-center shell model with oscillator potentials, l→·s→ forces, and l→2 terms is developed. The shell structures of the original spherical nucleus and those of the final fragments are reproduced. For small separation of the two centers the level structure resembles the Nilsson scheme. This two-center shell model might be of importance in problems of nuclear fission.
Higher-order effects are calculated in the framework of the eigenchannel theory for elastic and inelastic electron-nucleus scattering in the energy region 100≤E≤250 MeV. A dispersion effect of about 12% is found for the elastic scattering on Ni58 at a momentum transfer q≈500 MeV/c. For inelastic scattering, the reorientation effect is discussed, in addition to the dispersion effect. The total higher-order effect changes the form factor for a hindered first-order transition by 50% at its minima. Furthermore, the dependence of the higher-order effects on the transition potentials of the virtual excitations, the model dependence, and the dependence on the energy E of the electron and the momentum transfer q are discussed. A closed formula for the S matrix is developed by calculating the eigenchannels in stationary perturbation theory.
With a schematic model for the nuclear matter we give a unified treatment of the real and imaginary parts of the elastic O16-O16 scattering potential. The model connects the parameters of the potential with the density and binding properties of the O16-O16 system and reproduces the structure of the excitation function quite well. It is shown that the nuclear compressibility can be obtained from the scattering data, and in the case of the S32 compound system there results an effective compressibility (finite quenching of the nuclei) of about 200 MeV.
The total particle-particle SJ matrix of O16 for spin J=1- and excitation energies between 15 and 27 MeV has been calculated in the eigenchannel reaction theory for several parameters of the Saxon-Woods potential and the two-body force. The many-body problem has been treated in the 1-particle-1-hole approximation. The photon channels have been included by perturbation theory. Surprisingly, the most important structure of the experimental cross sections is reproduced quite well in this simple approximation.
The Coulomb-fission cross sections for 132Xe and 148Nd incident on 238U are calculated in a dynamical classical model. In particular the influence of nuclear forces on the cross sections is studied. Since they are counteracting the Coulomb force, they diminish the cross sections for Coulomb fission significantly and shift the Coulomb barrier towards lower energies.
The theory of collective correlations in nuclei is formulated for giant resonances interacting with surface vibrations. The giant dipole states are treated in the particle-hole framework, while the surface vibrations are described by the collective model. Consequently, this treatment of nuclear structure goes beyond both the common particle-hole model (including its various improvements which take ground-state correlations into account) and the pure collective model. The interaction between giant resonances and surface degrees of freedom as known from the dynamic collective theory is formulated in the particle-hole language. Therefore, the theory contains the particle-hole structures and the most important "collective intermediate" structures of giant resonances. Detailed calculations are performed for 12C, 28Si, and 60Ni. A good detailed agreement between theory and experiment is obtained for all these nuclei, although only 60Ni is in the region where one would expect the theory to work well (50< A <110).
The influence of the Coulomb and nuclear forces on the Coulomb barrier in heavy-ion reactions is studied in a dynamical classical model. It is shown that the fusion barrier is smaller than the conventional Coulomb barrier of two underformed nuclei. The model yields a dynamical picture of the excitation mechanism of surface vibrations and giant resonances. It is suggested that-due to nuclear forces-the excitation of the octupole mode is strongly enhanced over the excitation of the quadrupole mode in experiments at the Coulomb barrier.
Continuum structure of Ca40
(1967)
The total S1- matrix of Ca40 has been calculated for excitation energies between 11 and 28 MeV. As typical results, the (γ, p0) and the total absorption cross sections are shown and compared with experiments. It is shown that the proper treatment of the one-particle, one-hole shell-model continuum accounts for most of the observed structures.
Using the eigenchannel reaction theory we performed coupled-channel calculations for Si28 and computed the differential cross section for Al27(p, γ0)Si28 over the energy range 6 MeV<Ep <16 MeV. The obtained angular distributions are nearly constant over the whole energy range and agree with the experiment in that they are almost isotropic. Thus, it seems that in this framework we can give a natural explanation for the peculiar behavior of the Al27(p, γ0)Si28 cross section.
The theory of Raman scattering is extended to include electric-quadrupole radiation. The results obtained are used to compute the elastic and Raman scattering cross sections of heavy deformed nuclei. The dipole and quadrupole resonances are described by a previously developed theory which includes surface vibrations and rotations. The computed cross sections are compared with experimental data for all those nuclei where both absorption and scattering cross sections are available. Some discrepances still exist in certain details; however, the over-all agreement between theory and experiment is very good.
In a collective treatment the energies of the giant resonances are given by the boundary conditions at the nuclear surface, which is subject to vibration in spherical nuclei. The general form of the coupling between these two collective motions is given by angular-momentum and parity conservation. The coupling constants are completely determined within the hydrodynamical model. In the present treatment the influence of the surface vibrations on the total photon-absorption cross section is calculated. It turns out that in most of the spherical nuclei this interaction leads to a pronounced structure in the cross section. The agreement with the experiments in medium-heavy nuclei is striking; many of the experimental characteristics are reproduced by the present calculations. In some nuclei, however, there seem to be indications of single-particle excitations which are not yet contained in this work.
The modes and frequencies of the giant quadrupole resonance of heavy deformed nuclei have been calculated. The quadrupole operator is computed and the absorption cross section is derived. The quadrupole sum rule is discussed, and the relevant oscillator strengths have been evaluated for various orientations of the nucleus. The giant quadrupole resonances have energies between 20 and 25 MeV. The total absorption cross section is about 20% of the giant dipole absorption cross section. Of particular interest is the occurrence of the quadrupole mode which is sensitive to the nuclear radius in a direction of approximately θ=(1/4)π from the symmetry axis. This may give information on the details of the nuclear shape.
A method is proposed by which the eigenstates and the eigenvalues of the S matrix, i.e., the eigenchannels, can be directly computed from the nuclear problem, for example, from the shell model. The calculation of all cross sections, viz., partial and total cross sections, is then exceedingly simple. The characteristics of the eigenchannels are described and the relation with other reaction theories is briefly discussed.