Auto-ionization of positrons in heavy ion collisions

Autoionization of positrons occurs as a fundamentally new process of quantum electrodynamics, if empty 1s- or 2p _1/2? etc. electronic shells obtain binding energies larger than 2m _e c ². This effect should be experimentally observable in the scattering of very heavy ions (Z≧80) on each other since in such collisions superheavy electronic molecules are formed (superheavy quasi-molecules). The scattering mechanism and the distribution of autoionization positrons are discussed. The adiabaticity of the heavy ion collision is studied and the electron-positron pair production background to the ionization problem is estimated. Analytic solutions are obtained for 1/r-potentials for the caseZα≧1. The phase shifts of negative energy solutions in the case of cutoff Coulomb potentials reveal the accuracy of the autoionization formalism.     

Pion and proton emission in interactions induced by relativistic heavy nuclei withZ≧12

The emission of pions and protons in interactions between heavy nuclei from the cosmic radiation (12≦Z≦26) and photoemulsion nuclei has been studied. The angular and energy distributions of target protons are similar to corresponding distributions in proton-nucleus interactions for emission angles >30°. In heavy ion interactions a forward peak of high energy target protons (E>200 MeV)is observed. The energy spectrum of target protons for angles <30° is remarkably flat for heavy ion interactions. For increasing disintegration of the target nucleus the number of produced pions per emitted recoil target proton is almost constant in heavy ion interactions, while it decreases rapidly in proton-nucleus interactions. The logtgΘ distributions of pions have small standard deviations even in interactions with a large target disintegration.     

γ-γ Physics with peripheral relativistic heavy ion collisions

The high flux of equivalent photons present in relativistic heavy ion collisions of two chargesZ ₁ andZ ₂ gives rise to the collision of two equivalent photons. The cross-sections for various processes are directly related to the correspondingγ- γ cross-sections. As compared toγ- γ physics being studied at e⁺ e^? colliders, we find that high energy states will not be so easily accessible at the existing facilities, however, the enhancement factor (Z ₁ Z ₂)² in the expression for the cross section will provide very large photon fluxes for lower energies.     

Spontaneous vacuum decay of supercritical nuclear composites

We show that in deep inelastic heavy ion collisions withZ ₁+Z ₂> 173 the spontaneous decay of the neutral vacuum by emission of positrons may be isolated from other competing positron producing processes. From the details of the spontaneous positron spectrum information about the nature (lifetime, shape and angular momentum) of the composite nuclear system may be derived.     

Study of the 2p 1/2 σ excitation probability in collision systems withZ 1+Z 2>150 using line width analysis

The 90°-line width ofK x-ray lines induced in heavy ion collisions is mainly determined by the kinematical broadening. Its conversion into mean impact parameters together with total excitation cross section allows to obtain mean excitation probabilities. Within the energy range (1.4–5.9) MeV/u mean excitation probabilities for 2p _1/2 σ-excitation are deduced. Collision systems in theZ-range (150–180) are investigated. A very poor agreement between experiment and theoretical predictions has been found. The excitation probabilities are larger than predicted and are nearly independent of the atomic numberZ of the united atom. The collision energy dependence is much weaker than predicted. Even a decrease of the mean excitation probabilities at high collision energies has been found.     

Hyperfine quenching of polarized two-electron ions in an external magnetic field

The hyperfine quenching mechanism of metastable states in polarized heliumlike heavy ions is considered. The lifetime dependence of these states on the ion polarization in an external magnetic field is established. This dependence is presented for the ²³P₀ state of the europium (Z=63) ion and is proposed as a method for the measurement of the ion polarization in experiments for the search of parity violating effects.     

‘Slow’ residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesA _Res>A _tar,Z _Res>Z _tar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions²⁰Ne+²⁰⁸Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/ν _CN?0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ?23% (at E/A=8.6 MeV/u) and ?8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.     

Measurements ofK-shell ionization with132Xe and208Pb projectiles in the energy range of 1.4–5.9 MeV/u

Projectile and targetK-shell ionization cross sections induced by 3.6-, 4.7-, and 5.9 MeV/u¹³²Xe ions and 1.4-, 3.6-, 4.7-, and 5.9 MeV/u²⁰⁸Pb ions from the UNILAC in thin solid targets between C and U are measured. The cross sections are discussed in terms of the molecular model of innershell vacancy production in heavy ion-atom collisions. The sharing of 2p _1/2σ vacancies between theK shells of the two collision partners in these very heavy ion-atom collisions is found to deviate from the Meyerhof-Demkov formula forR≦10^?2. The measured ionization cross sections are compared with theoretical calculations for 1sσ and 2p _1/2σ excitation cross sections. AZ _UA=Z₁+Z ₂ dependence is found independent ofZ ₁/Z ₂. Outer-shell vacancy configurations measured in these close encounters are reported.     

Open problems in formation and decay of composite systems in heavy ion reactions

New highly exclusive experiments in the field of formation and decay of composite systems in heavy ion reactions are presented. Dynamical effects are reviewed in the light of recent works on the role of the N/Z asymmetry between projectile and target. The possibility of extracting directly from the experimental data the emission barrier of alpha particles emitted from highly excited nuclei is discussed. Finally, the first experimental evidence of double giant resonance excitation in fusion-evaporation reaction is presented.     

Highest spin reached in the superdeformed well in the A≈190 mass region

The high efficiency of the new-generation of γ-arrays as EUROGAM or GAMMASPHERE makes reachable the fission limit in angular momentum of the nuclei populated by means of heavy ion reactions in the A ≈ 190 mass region. This is established by the strong correlation between the fissility parameters Z ²/A and the spin values of the state of highest energy observed in the yrast superdeformed bands.     

Cold fission as heavy ion emission

The last version of the analytical superasymmetric fission model is applied to study cold fission processes. Strong shell effects are present either in one or both fission fragments. A smooth behaviour is observed when the proton or the neutron numbers are changed by four units. IncreasingZ andN, in the transuranium region, a sharp transition from asymmetry with a large peak-to-valley ratio to symmetry atZ=100 and/orN=164 is obtained. The transition toward asymmetry at higherZ andN is much smoother. The most probable cold fission light fragments from²³⁴U,²³⁶U,²³⁹Np and²⁴⁰Pu are¹⁰⁰Zr,¹⁰⁴Mo,¹⁰⁶Mo and¹⁰⁶Mo, respectively, in good agreement with experimental data. The unified treatment of alpha decay, heavy ion radioactivities and cold fission is illustrated for²³⁴U — the first nucleus in which all three groups have been already observed.     

The eigenspectrum of the relativistic two-center Hamiltonian in the diatomic basis of bound hydrogenlike Dirac-spinors

During heavy ion collisions below the Coulomb barrier adiabatic molecular orbitals are formed for the inner electrons. For systems with total changes exceending Z=Z₁ + Z₂ 60 relativistic effects are important and we, therefore, use the two-center Dirac Hamiltonian to describe the dynamics. We investigate the method to approximate these molecular orbitals by an expansion in a di-atomic basis of Dirac wave functions and determine the binding energies by a variation of the expansion coefficients. Deviations from adiabaticity are taken into account to first order in the relative velocityv/c 0.1) of the nuclei which is considerably smaller than the electron velocity.     

On the chemical potential of atomic ions

The following relation between the chemical potential μ(Z, N) of an atomic ion with nuclear charge Z and N electrons and their partial derivatives (?μ/?N)_Z and (?μ/?Z)_N has been derived by March within the context of the Thomas-Fermi approximation $\text{Z(?μ/?Z)}{}_{\mathrm{N}}\text{+N(?μ/?N)}{}_{\mathrm{Z}}\text{=}\text{3}\text{4}\text{μ}$. We have performed a numerical test which shows that the same relation is also approximately fulfilled in other density functional models more sophisticated than the Thomas-Fermi model.     

Out-of-plane correlations of α-particles and heavy ions in the reaction of 373 MeV32S with gold

Out-of-plane correlations between α-particles and projectile-like heavy ions have been measured for the system³²S+¹⁹⁷Au at E_LAB=373 MeV. The angular widths of the correlations range from 24° to 40° (FWHM) depending on theZ of the heavy ion. In the framework of a model calculation these widths are shown to be very sensitive to the outof-plane momentum of the α-particle at emission. It is concluded that only little heating of the reaction partners occurs prior to α-particle emission.     

Study of high energetic electrons emitted during heavy ion collisions

Double differential cross sectionsd ² σ/dΩdE were measured for high energetic electrons emitted in heavy ion collisions. Electrons were detected in the energy range of 60 keV-500 keV for various target projectile combinations 66≦Z _u =Z _t +Z _p ≦145 and projectile velocities between 7 % and 10 % of the speed of light. Clear evidence was found that these electrons stem from the united atom formed during the collision. Slope and height of the spectra are discussed with respect to the momentum distribution of strongly bound states (i.e. theL-shell) at momenta far above the mean value. In addition for the systems S, Ni, Br→Pb electrons were detected in coincidence withK x-rays of Pb. By this method the contribution of theK-shell of the combined system to the total spectrum could be separated. Binding energies of theK-shell were estimated by a slope comparison between the coincident and single spectrum. The resulting values are close to the united atom limit.     

Mass and kinetic energy distribution in cold fission of233U,235U and239Pu induced by thermal neutrons

The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of²³³U,²³⁵U and²³⁹Pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragmentsZ=50–52,N=80–82 and light fragmentsZ=40–42,N=60–64.     

The Lead Radius Experiment PREX

The Lead Radius Experiment PREX will run in Spring of 2010. The experiment measures the parity-violating asymmetry in the elastic scattering of polarized electrons from a lead nucleus at an energy of 1.05 GeV and a scattering angle of 5°. The Z ₀ boson couples mainly to neutrons, and provides a clean measurement of R _n with a projected experimental precision of ±1%. The measurement is a fundamental test of nuclear theory and pins down the density-dependence of the symmetry energy of neutron rich nuclear matter which has impacts on neutron star structure, heavy ion collisions, and atomic parity violation experiments. Recent developments in the experiment are described.     

Isospin aspects in nuclear reactions involving Ca beams at 25 MeV/nucleon

Isospin dependence of dynamical and thermodynamical properties observed in reactions ⁴⁰Ca+ ^40,48Ca and ⁴⁰Ca + ⁴⁶Ti at 25 MeV/nucleon has been studied. We used the CHIMERA multi-detector array. Strong isospin effects are seen in the isotopic distributions of light nuclei and in the competition between different reaction mechanisms in semi-central collisions. We will show also preliminary results obtained in nuclear collision ⁴⁸Ca + ⁴⁸Ca at 25MeV/nucleon, having very high N/Z value in the entrance channel (N/Z = 1.4). The enhancement of evaporation residue production confirms the strong role played by the N/Z degree of freedom in nuclear dynamics.     

Experimental observation of excited state muon transfer in mixtures of hydrogen isotopes

Molecular dynamic (MD) computer simulations are used to investigate the stopping of heavy ions in strongly coupled electron plasmas. Our results show, that in this regime collisions between the electrons as well as non-linear screening effects yield at low ion velocities a dependence of the stopping power on the ion chargeZ which scales like Z^1.43 instead of the usual Z² ln(const/Z)-scaling for weak coupling. This is connected with an enhanced local density of electrons around a highly charged, slow ion.Supported by the Bundesministerium für Bildung und Forschung (BMBF), the Gesellschaft für Schwerionenforschung Darmstadt (GSI) and the European Community.     

Low energy muonic hydrogen reactions with heavy atoms

Average s-wave cross-sections appropriate for liquid hydrogen temperatures are calculated with the aid of an in-coming wave boundary condition for the muon transfer process μ^-p + Z → (μ^-Z)¹ + p on heavy atoms. The surface-correlated nature of the heavy atom X-rays is emphasized.