Refine
Year of publication
Document Type
- Article (2190)
- Preprint (1513)
- Doctoral Thesis (600)
- Conference Proceeding (249)
- diplomthesis (100)
- Bachelor Thesis (75)
- Master's Thesis (61)
- Contribution to a Periodical (46)
- Diploma Thesis (34)
- Book (33)
Keywords
- Kollisionen schwerer Ionen (47)
- heavy ion collisions (44)
- LHC (27)
- Quark-Gluon-Plasma (25)
- Heavy Ion Experiments (21)
- BESIII (20)
- e +-e − Experiments (19)
- equation of state (19)
- quark-gluon plasma (19)
- QCD (18)
Institute
- Physik (4996) (remove)
Proton emission in relativistic nuclear collisions is examined for events of low and high multiplicity, corresponding to large and small impact parameters. Peripheral reactions exhibit distributions of protons in agreement with spectator-participant decay modes. Central collisions of equal-size nuclei are dominated by the formation and decay of a fireball system. Central collisions of light projectiles with heavy targets exhibit an enhancement in sideward emission which is predicted by recent hydrodynamical calculations.
Inclusive energy spectra of protons, deuterons, and tritons were measured with a telescope of silicon and germanium detectors with a detection range for proton energies up to 200 MeV. Fifteen sets of data were taken using projectiles ranging from protons to 40Ar on targets from 27Al to 238U at bombarding energies from 240 MeV/nucleon to 2.1 GeV/nucleon. Particular attention was paid to the absolute normalization of the cross sections. For three previously reported reactions, He fragment cross sections have been corrected and are presented. To facilitate a comparison with theory the sum of nucleonic charges emitted as protons plus composite particles was estimated and is presented as a function of fragment energy per nucleon in the interval from 15 to 200 MeV/nucleon. For low-energy fragments at forward angles the protons account for only 25% of the nucleonic charges. The equal mass 40Ar plus Ca systems were examined in the center of mass. Here at 0.4 GeV/nucleon 40Ar plus Ca the proton spectra appear to be nearly isotropic in the center of mass over the region measured. Comparisons of some data with firestreak, cascade, and fluid dynamics models indicate a failure of the first and a fair agreement with the latter two. In addition, associated fast charged particle multiplicities (where the particles had energies larger than 25 MeV/nucleon) and azimuthal correlations were measured with an 80 counter array of plastic scintillators. It was found that the associated multiplicities were a smooth function of the total kinetic energy of the projectile. NUCLEAR REACTIONS U(20Ne,X), E / A=240 MeV/nucleon; U(40Ar,X), Ca(40Ar,X), U(20Ne,X), Au(20Ne,X), Ag(20Ne,X), Al(20Ne,X), U(4He,X), Al(4He,X), E / A=390 MeV/nucleon; U(40Ar,X), Ca(40Ar,X), U(20Ne,X), U(4He,X), U(p,X), E / A=1.04 GeV/nucleon; U(20Ne,X), E / A=2.1 GeV/nucleon; measured sigma (E, theta ), X=p,d,t.
Exclusive pi - and charged-particle production in collisions of Ar+KCl is studied at incident energies from 0.4 to 1.8 GeV/u. Complete disintegration of both nuclei is observed. The correlation between pi - and total charge multiplicity shows no islands of anomalous pion production. For constant numbers of proton participants the pi - multiplicity distributions are Poissons. For central collisions <n pi -> increases smoothly and to first order linearly with the c.m. energy. Disagreement with the firestreak model is found. Pacs numbers: 25.70.Hi, 24.10.Dp
Angular and energy distributions of fragments emitted from fast nucleus-nucleus collisions (Ne--> U at 250, 400, and 800 MeV/N) are calculated with use of nuclear fluid dynamics. A characteristic dependence of the energy spectra and angular distributions on the impact parameter is predicted. The preferential sideward emission of reaction fragments observed in the calculation for nearly central collisions seems to be supported by recent experimental data.
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.
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.
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.
We present a unified formulation of the interaction of electrons with the electromagnetic field in heavy ion collisions, based on quantized interacting fields. This reduces the effort in treating many-electron systems substantially, as compared with the usual S-matrix theory. Both formalisms are shown to be equivalent. The simplification achieved by our new approach is demonstrated in detail for the example of quasi-molecular radiation.
Lambda 's produced in central collisions of 40Ar+KC1 at 1.8-GeV/u incident energy were detected in a streamer chamber by their charged-particle decay. For central collisions with impact parameters b<2.4 fm the Lambda production cross section is 7.6±2.2 mb. A calculation in which Lambda production occurs in the early stage of the collision qualitatively reproduces the results but underestimates the transverse momenta. An average Lambda polarization of -0.10±0.05 is observed. PACS numbers: 25.70 Bc
The 16O ( gamma ,p0) reaction has been studied with linearly polarized bremsstrahlung photons in and below the giant E1 resonance. The parity of the absorbed radiation was determined from the observed azimuthal asymmetry of the emitted protons. Combined with unpolarized measurements the polarized results determine the proton decay amplitudes of the M1 resonance at Ex=16.2 MeV in 16O. The shape of the unpolarized 16O ( gamma ,p3) angular distribution in the giant E1 resonance was derived from the measured analyzing power. NUCLEAR REACTIONS 16O( gamma ,p), E=15-25 MeV; measured analyzing power theta =90° linearly polarized bremsstrahlung; 16O dipole levels deduced pi ; 16.2 MeV 1+ resonance deduced p0 decay amplitudes; 16O GEDR deduced p3 angular distribution.
Two-particle correlation data are presented for the reaction Ar (800 MeV/ nucleon) + Pb. The experimental results are analyzed in the nuclear fluid dynamical and in a linear cascade model. We demonstrate that the collective hydrodynamical correlations dominate the measured two-particle correlation function for the heavy system studied. We discuss the transition from the early stages of the reaction which are governed by few nucleon correlations, to the later stages with their macroscopic flow which can only be reached using heavy colliding systems. The sensitivity of the correlation data on the underlying compressional dissipative processes is analyzed.
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.
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.
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.
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.
The fluid dynamical model is used to study the reactions 20Ne+238U and 40Ar+40Ca at Elab=390 MeV/nucleon. The calculated double differential cross sections d²ð/dΩdE exhibit sidewards maxima in agreement with recent experimental data. The azimuthal dependence of the triple differential distributions, to be obtained from an event-by-event analysis of 4π; exclusive experiments, can yield deeper insight into the collision process: Jets of nuclear matter are predicted with a strongly impact-parameter-dependent thrust angle θjet(b). NUCLEAR REACTIONS Ar+Ca, Ne+U, Elab=393 MeV/nucleon, fluid dynamics with thermal breakup, double differential cross sections, azimuthal dependence of triple differential cross sections, event-by-event thrust analysis of 4π exclusive experiments.
We present an analysis of high energy heavy ion collisions at intermediate impact parameters, using a two-dimensional fluid-dynamical model including shear and bulk viscosity, heat conduction, a realistic treatment of the nuclear binding, and an analysis of the final thermal emission of free nucleons. We find large collective momentum transfer to projectile and target residues (the highly inelastic bounce-off effect) and explosion of the hot compressed shock zones formed during the impact. As the calculated azimuthal dependence of energy spectra and angular distributions of emitted nucleons depends strongly on the coefficients of viscosity and thermal conductivity, future exclusive measurements may allow for an experimental determination of these transport coefficients. The importance of 4π measurements with full azimuthal information is pointed out.
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.
The negative-pion multiplicity is measured for central collisions of 40Ar with KCl at eight energies from 0.36 to 1.8 GeV/nucleon and for 4He on KCl and 40Ar on BaI2 at 977 and 772 MeV/nucleon, respectively. A systematic discrepancy with a cascade-model calculation which fits proton- and pion-nucleus cross sections but omits potential-energy effects is used to derive the energy going into bulk compression of the system. A value of the incompressibility constant of K=240 MeV is extracted in a parabolic form of the nuclear-matter equation of state.
Pion production and charged-particle multiplicity selection in relativistic nuclear collisions
(1982)
Spectra of positive pions with energies of 15-95 MeV were measured for high energy proton, 4He, 20Ne, and 40Ar bombardments of targets of 27Al, 40Ca, 107,109Ag, 197Au, and 238U. A Si-Ge telescope was used to identify charged pions by dE / dx-E and, in addition, stopped pi + were tagged by the subsequent muon decay. In all, results for 14 target-projectile combinations are presented to study the dependence of pion emission patterns on the bombarding energy (from E / A=0.25 to 2.1 GeV) and on the target and the projectile masses. In addition, associated charged-particle multiplicities were measured in an 80-paddle array of plastic scintillators, and used to make impact parameter selections on the pion-inclusive data. NUCLEAR REACTIONS U(20Ne, pi +), E / A=250 MeV; U(40Ar, pi +), Ca(40Ar, pi +), U(20Ne, pi +), Au(20Ne, pi +), Ag(20Ne, pi +), Al(20Ne, pi +), U(4He, pi +), Al(4He, pi +). E / A=400 MeV; Ca(40Ar, pi +), U(20Ne, pi +), U(4He, pi +), U(p, pi +), E / A=1.05), GeV; U(20Ne, pi +), E / A=2.1 GeV; measured sigma (E, theta ), inclusive and selected on associated charged-particle multiplicity.
The parities of eleven J=1 levels in 208Pb were determined by nuclear resonance fluorescence scattering of linearly polarized photons. A new 1+ level at Ex=5.846 MeV with Gamma 02 / Gamma =1.2±0.4 eV was found. This level can probably be identified with the theoretically predicted isoscalar 1+ state in 208Pb. All other bound dipole states below 7 MeV with Gamma 02 / Gamma >1.5 eV have negative parity. The 1- assignment to the 4.842-MeV level is of special significance because of previous conflicting results about its parity.
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.
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 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.
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.
Ion formation from alkali halide solids caused by the irradiation of high power (some 108 W/ cm2) pulsed lasers is investigated by means of time-of-flight mass spectrometry (LAMMA®). It is shown that the ions are formed directly from the solid state, several uppermost atomic layers being involved; gas phase interactions are negligible. The ion formation rates, however, are in-compatible with the assumption of a quasiequilibrium phase transition, but should be explained in terms of non-adiabatic rate processes discussed in some detail. The light absorption of the transparent halide crystals is assumed to be initiated by multiphoton absorption - free electron production; the further energy transfer being maintained by rapid polaron-Joule-heating. The data are compatible with this model.
An event by event analysis is carried out for all charged particles observed in central collisions of 40Ar + KCl and 40Ar + Pb at 1.808 and 0.772 GeV/nucleon, respectively. Total transverse energy is used for impact parameter selection within the central trigger condition. The central Ar + KCl reaction exhibits a forward-backward oriented momentum flux. The flux distribution of the most central Ar + Pb events is approximately isotropic in the fireball center of mass.
Difficulties of the thermodynamical model approach to pion production in relativistic ion collisions
(1983)
Thermodynamical models with various forms of partial transparency of nuclear matter are considered. It is shown that the introduction of transparency, however, significantly improves agreement with pion data concerning multiplicities and transverse momenta leads to a serious discrepancy with average rapidities of pions. Qualitative arguments are given that difficulties of the thermodynamical approach can be overcome if one assumes hydrodynamical expansion in the first stage of nuclear interactions.
Transverse momenta and rapidities of Lambda 's produced in central nucleus-nucleus collisions at 4.5 GeV/c·u (C-C,...,O-Pb) were studied and compared with those from inelastic He-Li interactions at the same incident momentum. Polarization of the Lambda hyperons was found to be consistent with zero ( alpha P=-0.06=0.11 for Lambda 's from central collisions). An upper limit of the Lambda -bar / Lambda production ratio was estimated to be less than 4.5 x 10-3. The experiment was performed in a triggered streamer chamber.
11 262 keV 1+ state in 20Ne
(1983)
The excitation energy of the lowest 1+, T=1 state in 20Ne, which is important for parity nonconservation studies, has been determined in a photon scattering experiment to be 11 262.3 ± 1.9 keV. Values for the gamma -ray branching of this level to the ground state and to the first 2+ level in 20Ne are 84 ± 5% and 16 ± 5%, respectively. NUCLEAR REACTIONS 20Ne( gamma , gamma ), E gamma <18 MeV, bremsstrahlung; measured E gamma , gamma branching. Ne natural targets.
Studying Walecka's mean-field theory we find that one can reproduce the observed binding energy and density of nuclear matter within experimental precision in an area characterized by a line in the coupling-constant plane. A part of this line defines systems which exhibit a phase transition around Tc~200 MeV for zero baryon density. The rest corresponds to such systems where the phase transition is absent; in that case a peak appears in the specific heat around T~200 MeV. We interpret these results as indicating that the hadron phase of nuclear matter alone indicates the occurrence of an abrupt change in the bulk properties around ρV~0 and T~200 MeV.
Energy spectra for p, d, t, 3He, 4He, and 6He from the reaction 12C+197Au at 35 MeV/nucleon are presented. A common intermediate rapidity source is identified using a moving source fit to the spectra that yields cross sections which are compared to analogous data at other bombarding energies and to several different models. The excitation function of the composite to proton ratios is compared with quantum statistical, hydrodynamic, and thermal models.
We present a theoretical description of nuclear collisions which consists of a three-dimensional fluid-dynamical model, a chemical equilibrium breakup calculation for local light fragment (i.e., p, n, d, t, 3He, and 4He) production, and a final thermal evaporation of these particles. The light fragment cross sections and some properties of the heavy target residues are calculated for the asymmetric system Ne+U at 400 MeV/N. The results of the model calculations are compared with recent experimental data. Several observable signatures of the collective hydrodynamical processes are consistent with the present data. An event-by-event analysis of the flow patterns of the various clusters is proposed which can yield deeper insight into the collision dynamics.
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.
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.
The time dependent Hartree-Fock approximation is used to study the dynamical formation of long-lived superheavy nuclear complexes. The effects of long-range Coulomb polarization are treated in terms of a classical quadrupole polarization model. Our calculations show the existence of "resonantlike" structures over a narrow range of bombarding energies near the Coulomb barrier. Calculations of 238U + 238U are presented and the consequences of these results for supercritical positron emission are discussed. NUCLEAR REACTIONS 238U + 238U collisions as a function of bombarding energy, in the time-dependent Hartree-Fock approximation. Superheavy molecules and strongly damped collisions.
Proton spectra have been calculated for the reaction 12C(85 MeV/nucleon) + 197Au using a three-dimensional hydrodynamical model with viscosity and thermal conductivity and final thermal breakup. The theoretical results are compared to recent data. It is shown that the predicted flow effects are not observable as a result of the impact parameter averaging inherent in the inclusive proton spectra. In contrast, angular distributions of medium mass nuclei (A>3) in nearly central collisions can provide signatures for flow effects.
Measurement of complex fragments and clues to the entropy production from 42-137-MeV/nucleon Ar + Au
(1983)
Intermediate-rapidity fragments with A=1-14 emitted from 42-137-MeV/nucleon Ar + Au have been measured. Evidence is presented that these fragments arise from a common moving source. Entropy values are extracted from the mass distributions by use of quantum statistical and Hauser-Feshbach theories. The extracted entropy values of S/A≈2-2.4 are much smaller than the values expected from measured deuteron-to-proton ratios, but are still considerably higher than theoretically predicted values.
Analysis of Lambda and associative pion production in relativistic nucleus-nucleus collisions
(1984)
Pion and proton production are measured to investigate thermal equilibrium in central collisions of 40Ar+KCl at 1.8 GeV/nucleon. The bulk of the pion yield is isotropic in the c.m. system, with an apparent temperature of 58±3 MeV, much lower than the 118±2 MeV of the protons. It is shown that the low pion "temperature" can be explained by the decay kinematics of delta resonances in thermal equilibrium. A (5±1)% component in the pion spectrum is, however, found to have a temperature of 110±10 MeV. The effect on the spectra of possible contributions from collective radial flow is discussed.
Charged-particle exclusive data for Ar+Pb collisions at 0.772 GeV/u are analyzed in terms of collective variables for the event shapes in momentum space. Semicentral collisions lead to sidewards flow whereas nearly head-on collisions have spherical shapes in the c.m. frame, resulting from complete stopping of projectile motion. The hydrodynamical model predictions agree qualitatively with the data whereas the standard cascade model disagrees, lacking in stopping power and collective flow.
Kinetic energy flow in Nb(400 A MeV) + Nb: evidence for hydrodynamic compression of nuclear matter
(1984)
A kinetic-energy—flow analysis of multiplicity-selected collisions of 93Nb(Elab=400A MeV)+93Nb is performed on the basis of the nuclear fluid dynamical model. The effects of finite particle numbers on the flow tensor are explicitly taken into account. Strong sidewards peaks are predicted in dN/dcosθF, the distribution of event by event flow angles. This is in qualitative agreement with recent data from the "Plastic Ball" electronic detection system. Cascade simulations fail to reproduce the data.
Rapidity dependence of entropy production in proton- and nucleus-induced reactions on heavy nuclei
(1984)
The entropy of hot nuclear systems is deduced from the mass distribution of fragments emitted from high energy proton- and nucleus-induced reactions via a quantum statistical model. It is found that the entropy per baryon, S/A, of intermediate rapidity ("participant") fragments is higher than the entropy of target rapidity ("spectator") fragments. The spectator fragments exhibit S/A values of ≅ 1.8 independent of the projectile energy from 30 MeV/nucleon up to 350 GeV. This value of the entropy coincides with the entropy at which nuclear matter becomes unbound.
Microscopic calculations of collective flow probing the short-range nature of the nuclear force
(1984)
Collisions between two nuclei have been modeled by numerical solution of classical approximations to the equations of motion of the constituent nucleons. For the reaction Nb(400 MeV/u)+Nb, a correlated sidewards emission of nucleons is observed. This is attributed to the repulsive short-range component of the nucleon-nucleon potential. A strong dependence of the flow angle on the impact parameter is observed, in accord with recent experimental results.
Intranuclear cascade calculations and fluid dynamical predictions of the kinetic energy flow are compared for collisions of 40Ca + 40Ca and 238U + 238U. The aspect ratio, R13, as obtained from the global analysis, is independent of the bombarding energy for the intranuclear cascade model. Fluid dynamics, on the other hand, predicts a dramatic increase of R13 at medium energies Elab≲200 MeV/nucleon. In fact, R13(Elab) directly reflects the incompressibility of the nuclear matter and can be used to extract the nuclear equation of stat at high densities. Distortions of the flow tensor due to few nucleon scattering are analyzed. Possible procedures to remove this background from experimental data are discussed.
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.
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.
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.
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.
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.
The role of nonequilibrium and quantal effects in fast nucleus-nucleus collisions is studied via the Vlasov-Uehling-Uhlenbeck theory which includes the nuclear mean field dynamics, two-body collisions, and Pauli blocking. The intranuclear cascade model, where the dynamics is governed by independent NN collisions, and the Vlasov equation, where the nuclear mean field determines the collision dynamics, are also studied as reference cases. The Vlasov equation (no collision term) yields single particle distribution functions which–after the reaction–are only slightly modified in momentum space; even in central collisions, transparency is predicted. This is in agreement with the predictions of the quantal time-dependent Hartree-Fock method. In contrast, large momentum transfer is obtained when the Uehling-Uhlenbeck collision term is incorporated; then the final momentum distribution is nearly spherically symmetric in the center of mass and a well-equilibrated nuclear system is formed: the nuclei stop each other; the translational kinetic energy is transformed into randomized microscopic motion. The Vlasov-Uehling-Uhlenbeck theory is supplemented with a phase space coalescence model of fragment formation. Calculated proton spectra compare well with recent data for Ar(42, 92, and 137 MeV/nucleon) + Ca. Also the total yields of medium mass fragments are well reproduced in the present approach. The mean field dynamics without two-body collisions, on the other hand, exhibits forward peaked proton distributions, in contrast to the data. The cascade approach underpredicts the yields of low energy protons by more than an order of magnitude.
Nuclear collisions from 0.3 to 2 GeV/nucleon are studied in a microscopic theory based on Vlasov's self-consistent mean field and Uehling-Uhlenbeck's two-body collision term which respects the Pauli principle. The theory explains simultaneously the observed collective flow and the pion multiplicity and gives their dependence on the nuclear equation of state.
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.
A detailed study of pion production in inelastic and central nucleus-nucleus collisions was carried out using a 2 m streamer spectrometer. Nuclear targets mounted inside the streamer chamber were exposed to nuclear beams of 4.5 GeV/c/nucleon momentum. A systematic study of the influence of the central trigger on observed data is performed. The data on multiplicities, rapidities, transverse momenta, and emission angles of negative pions are presented for various pairs of colliding nuclei. Intercorrelations between various characteristics are studied and discussed. The results are compared with predictions of some theoretical models. It is shown that the main features of the pion production in nuclear collisions can be satisfactorily described by a model assuming independent nucleon-nucleon collisions with subsequent cascading process. However, the observed correlation between Lambda and pion characteristics seems to be unexplained by this picture.
Nuclear resonance fluorescence experiments with linearly polarized bremsstrahlung were performed to determine parities of strong dipole transitions in 40Ar. A total of 14 transitions—ten of them previously unknown—in the energy range from 4.7 to 10.2 MeV could be identified. From this experiment it is evident that the main dipole strength to bound states is due to E1 excitations. An upper limit of B(M1) [up arrow] <0.5 µN2 was found for individual magnetic dipole excitations in 40Ar in the energy region below neutron threshold.
Nuclear resonance fluorescence measurements with linearly polarized bremsstrahlung were performed to determine parities of bound dipole transitions in 206Pb. A new 1+ level at 5800 keV was found, which has almost the same strength as the isoscalar M1 transition in 208Pb. Twenty-four further dipole states in 206Pb below 7.6 MeV possess negative parity.
Im Jahre 1871 wurde durch den Naturwissenschaftlichen Verein Osnabrück (gegründet 1870) eine meteorologische Station eingerichtet. Sie hatte ihren Standort am Sommerhaus des damaligen Obergerichtsrats JOHANN-VOLLRATH KETTLER,Osnabrück, Ziegelstraße 7. KETTLER hat 1872 im 1. Jahresbericht des . Naturwissenschaftlichen Vereins über die "Entstehung, Einrichtung und die ersten Ergebnisse" berichtet. Dieser Bericht ist hier wiedergegeben, legt er uns dar, daß alle Messungen exakt und gewissenhaft durchgeführt wurden.
The novel momentum analysis technique introduced by Danielewicz and Odyniec can be used to detect and exhibit collective flow in the light system Ar(1800 MeV/nucleon) + KCl where the usual kinetic energy flow analysis fails. The microscopic Vlasov-Uehling-Uhlenbeck theory which includes the nuclear mean field, two-body collisions, and Pauli blocking is used to study this phenomenon. The resulting transverse momentum transfers turn out to be quite sensitive to the nuclear equation of state. From a comparison with experimental data, evidence is presented for a rather stiff nuclear equation of state. The cascade model is unable to describe the data.
Time dependent dirac equation with relativistic mean field dynamics applied to heavy ion scattering
(1986)
We treat the relativistic propagation of nucleons coupled to scalar- and vector-meson fields in a mean-field approximation. The time-dependent Dirac and mean-meson-field equations are solved numerically in three dimensions. Collisions of 16O(300, 600, and 1200 MeV/nucleon) + 16O are studied for various impact parameters. The results are compared to other recent theoretical approaches. The calculations predict spallation, large transverse-momentum transfer, and positive-angle sidewards flow, in qualitative agreement with the data in this energy regime.
We study the recent claim that the intranuclear cascade model exhibits collective sidewards flow. 4000 intranuclear cascade simulations of the reaction Nb(400 MeV/nucleon)+Nb are performed employing bound and unbound versions of the Cugnon cascade. We show that instability of the target and projectile nuclei in the unbound cascade produces substantial spurious sidewards flow angles, for spectators as well as for participants. Once the nuclear binding is included, the peak of the flow angle distributions for the participants alone is reduced from 35° to 17°. The theoretical ‘‘data’’ are subjected to the experimental multiplicity and efficiency cuts of the plastic ball 4π electronic spectrometer system. The flow angular distributions obtained from the bound cascade—with spectators and participants subjected to the plastic ball filter—are forward peaked, in contrast to the plastic ball data. We discuss the uncertainties encountered with the application of the experimental efficiency and multiplicity filter. The influence of the Pauli principle on the flow is also discussed. The lack of flow effects in the cascade model clearly reflects the absence of the nuclear compression energy that can cause substantially larger collective sidewards motion—there is too little intrinsic pressure built up in the cascade model.
The influence of fluctuations of the shape degree of freedom in collisions of deformed nuclei with energies between 0.8 and 2.1 GeV/nucleon is analyzed on the basis of an intranuclear cascade simulation for the strongly deformed systems 46Ti+ 46Ti and 166Er+ 166Er. While there is a considerable sensitivity of the global event variables to the orientation for polarized beams and targets, this dependence disappears in the average over all orientations for impact parameter selected and integrated events. The dependence of the nuclear stopping and thermalization on the size of the system under consideration and on the bombarding energy is also investigated.
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.
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.
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.
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 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.
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 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 analyze the phase structure of the nonlinear mean-field meson theory of baryonic matter (nucleons plus delta resonances). Depending on the choice of the coupling constants, we find three physically distinct phase transitions in this theory: a nucleonic liquid-gas transition in the low temperature, Tc<20 MeV, low density, ρ≃0.5ρ0, regime, a high-temperature (T≃150 MeV) finite density transition from a gas of massive hadrons to a nearly massless baryon, antibaryon plasma, and, third, a strong phase transition from the nucleonic fluid to a resonance-dominated ‘‘delta-matter’’ isomer at ρ>2ρ0 and Tc<50 MeV. All three phase transitions are of first order. It is shown that the occurrence of these different phase transitions depends critically on the coupling constants. Since the production of pions also depends strongly on the coupling constants, it is seen that the equation of state cannot be derived unambiguously from pion data.
The recent attempts to extract the temperature in the late stage of medium energy (20–60 MeV/nucleon) heavy ion collisions from the yields of γ- and particle-instable fragments are discussed. The quantum statistical model is employed to demonstrate that feeding from instable states distorts the yields used for the temperature determination severely. Some particle instable fragments are only moderately affected by feeding. These selected species can still be useful for determining the temperature. The breakup temperatures of the fragment conglomerate extracted with this method are T≃4–8 MeV, much smaller than the corresponding slope factors, which indicate T∼15 MeV.
We demonstrate that momentum-dependent nuclear interactions (MDI) have a large effect on the dynamics and on the observables of high-energy heavy-ion collisions: A soft potential with MDI suppresses pion and kaon yields much more strongly than a local hard potential and results in transverse momenta intermediate between soft and hard local potentials. The collective-flow angles and the deuteron-to-proton ratios are rather insensitive to the MDI. Only simultaneous measurements of these observables can give clues on the nuclear equation of state at densities of interest for supernova collapse and neutron-star stability.
The final states of central Ca + Ca and Nb + Nb collisions at 400 and 1050 MeV/nucleon and at 400 and 650 MeV/nucleon, respectively, are studied with two independently developed statistical models, namely the classical microcanonical model and the quantum-statistical grand canonical model. It is shown that these models are in agreement with each other for these systems. Furthermore, it is demonstrated that there is essentially a one-to-one relationship between the observed relative abundances of the light fragments p, d, t, 3He, and α and the entropy per nucleon, for breakup temperatures greater than 30 MeV. Entropy values of 3.5–4 are deduced from high-multiplicity selected fragment yield data.
We study the dynamics of high energy heavy ion collisions through the Vlasov-Uehling-Uhlenbeck approach. Equilibration is observed, for central collisions. It is shown that the produced entropy, the pion multiplicity, flow angle, and transverse momentum distributions saturate at the moment of maximum compression and temperature. The effects of the nuclear equation of state and the Pauli principle are investigated. For the flow angle distribution there is a 20 deg reduction of the peak flow angle due to the Pauli principle. A stiff equation of state results in a 10–20 deg increase over the soft equation of state at all energies. The transverse momentum at projectile rapidity exhibits a peak structure as a function of impact parameter b. A 40% difference between soft and hard equation of state is observed for the peak impact parameter, i.e., for intermediate multiplicities.
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
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.
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.
Semicentral Ar+KCl, La+La, and Ar+Pb collisions at 800 MeV/nucleon were studied using a streamer chamber. The results are analyzed in the framework of the transverse momentum analysis and in terms of the average sphericity matrix. A critical examination of the analysis procedures, both experimental and theoretical, is given. New procedures are described to account for overall momentum conservation in the reaction, and to correct for azimuthal variations in the detection efficiency. Average transverse momenta per nucleon in the reaction plane are presented for deuterons emitted in the forward hemisphere, as these provide the most reliable information. A Vlasov-Uehling-Uhlenbeck calculation with a stiff equation of state gives a good fit to the momenta in the Ar+Pb reaction. Flow effects parametrized further using the sphericity tensor are found stronger than in the cascade model and consistently weaker than predicted by hydrodynamics. Parameters from the sphericity tensor exhibit a larger variation as a function of multiplicity than do the average momenta per nucleon.
We demonstrate that strangeness separates in the Gibbs-phase coexistence between a baryon-rich quark-gluon plasma and hadron matter, even at T=0. For finite temperatures this is due to the associated production of kaons (containing s¯ quarks) in the hadron phase while s quarks remain in the deconfined phase. The s-s¯ separation results in a strong enhancement of the s-quark abundance in the quark phase. This mechanism is further supported by cooling and net strangeness enrichment due to the prefreezeout evaporation of pions and K+, K0, which carry away entropy and anti- strangeness from the system. Metastable droplets (i.e., stable as far as weak interactions are not regarded) of strange-quark matter (‘‘strangelets’’) can thus be formed during the phase transition. Such cool, compact, long-lived clusters could be experimentally observed by their unusually small Z/A ratio (≤0.1–0.3). Even if the strange-quark-matter phase is not stable under strong interactions, it should be observable by the delayed correlated emission of several hyperons. This would serve as a unique signature for the transient formation of a quark-gluon plasma.
We study effects of the mean field in hot compressed nuclear matter in the context of the Vlasov Uehling-Uhlenbeck theory. The expansion of a spherical distribution at different temperatures is studied along with collisions of Nb+Nb and Au+Au at lab energies from 50 to 1050 MeV/nucleon. In both the expansion and the actual heavy ion collision simulation, a transition behavior is seen only at the lowest temperature (T<10 MeV) or bombarding energy (E=50 MeV/nucleon), where the attractive part of the mean field is able to bind the expanding matter. At the lowest energy one thus sees the formation of a central residue, whereas at higher bombarding energies there is complete disintegration of the centrally colliding nuclei. The spectrum of emitted nucleons is found to be much hotter than the kinetic energy spectrum of the central emitting region. The extracted temperature slope parameters are in agreement with recent data.
Streamer chamber data for collisions of Ar + KCl and Ar + BaI2 at 1.2 GeV/nucleon are compared with microscopic model predictions based on the Vlasov-Uehling-Uhlenbeck equation, for various density-dependent nuclear equations of state. Multiplicity distributions and inclusive rapidity and transverse momentum spectra are in good agreement. Rapidity spectra show evidence of being useful in determining whether the model uses the correct cross sections for binary collisions in the nuclear medium, and whether momentum-dependent interactions are correctly incorporated. Sideward flow results do not favor the same nuclear stiffness parameter at all multiplicities.
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.
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 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.