Showing papers in "European Physical Journal A in 1983"

Laser-enhanced electron-ion capture and antihydrogen formation

Abstract: The electron-ion capture rate for low electron energies is calculated for various electron velocity distributions. Capture rates for electron-ion recombination stimulated by irradiation with light are evaluated. The results are applied to electron cooling and to positron-antiproton recombination to form antihydrogen. It is shown that laser-induced capture is a powerful method to study the electron cooling process and to maximize the antihydrogen rate. With this technique a pulsed antihydrogen beam of selectable energy and well collimated with an intensity of a few atoms per second can be anticipated.

Potential pockets in the 238 U+ 238 U system and their possible consequences

Abstract: Within the double-folding model the separation, shape, and orientation dependence of the interaction potential is studied for two heavy ions. An effective nucleon-nucleon interaction (M3Y) derived fromG-matrix elements and based upon the Reid soft-core potential is used. Deformed Fermi-type matter densities with static quadrupole and hexadecapole deformations were utilized. The model is applied to the238U+238U system and shows dramatic dependence on the deformations and orientations.

Positrons from supercritical fields of giant nuclear systems

Abstract: During collisions of heavy nuclei with a combined chargeZ≳160 the electronic 1s-state is deeply bound due to the strong Coulomb field, forZ≧173 it even enters as a resonance the lower continuum of the Dirac-Hamiltonian. In pure Rutherford scattering no qualitative indication for the filling of a dynamically createdK-hole by the spontaneous positron creation process is predicted, but the study of heavy-ion collisions with nuclear time delay due to the attractive nuclear force promises clear signatures for the decay of the vacuum. Emphasis is laid also on the quantitative influence of the electron-electron interaction and ofE0-transitions in the giant nuclear system on positron emission, the latter treated in a classical approximation. We compare our results with recent experimental data of two different groups at GSI, Darmstadt.

The nucleon axial form factor and the pion-nucleon vertex function

Abstract: The nucleon axial current and related form factors are investigated in a model of relativistic quarks confined by a scalar potential of the formM(r)=c rn, with special emphasis on center-of-mass corrections and pionic effects. Pionic contributions to the axial form factorGA(q2) from afπ∂μφ term with constantfπ are demonstrated to vanish. The pion-nucleon form factorGπNN(q2) is derived and turns out to be longer ranged thanGA(q2). The induced pseudo-scalar form factorGp(q2) is shown to be connected toGπNN(q2) by the standard PCAC relation, contributions from the quark core toGp(q2) being negligibly small.

Observation of proximity- and non-equilibrium effects in ternary heavy ion reactions

Abstract: Kinematically complete experiments have been performed on the two- and three-body exit channels in the reactions84Kr+166Er and129Xe+122Sn at 125 MeV/u Three-body events occur with an unusually high probability They arise from a fast two-step mechanism where a sequential fission-like process follows a deep inelastic collision with preferentially very large energy losses Strong Coulomb proximity effects are observed in the three-body final state which, treated quantitatively in Coulomb trajectory calculations, establish a time-scale of 1·10−21 s between the consecutive scission acts The angular distribution of fission fragments is consistent with an orientation of the fission axis approximately collinear with the axis of the first scission, and the mass distribution of the fission is asymmetric with the heavier mass emitted preferentially opposite to the direction of the third particle The high fission probability, the short time-scale, the near collinear orientation and the fission mass asymmetry together present consistent evidence for a new phenomenon of non-equilibrium fission

On the interpretation of heavy-ion sub-barrier fusion data

Abstract: We study the well-known deviation of measured fusion cross-sections below the barrier from predictions of one-dimensional tunnelling models. A simple parametrisation of measured excitation functions is discussed which allows to identify a smooth general trend depending only on gross nuclear properties. The parametrisation is substantiated by quantum mechanically exact calculations in a two-dimensional tunnelling model. Various possibilities for the geometrical interpretation of the second degree of freedom are discussed. Neck formation is identified as the most probable degree of freedom effecting the overall behavior of sub-barrier fusion.

On the Sensitivity of the Inclusive Distributions in Parton Jets to the Coherence Effects in QCD Gluon Cascades

Abstract: The KNO multiplicity distributions and properties of two-particle correlations in QCD jets are considered in the framework of the perturbative approach accounting for effects of the coherence in the soft gluon bremsstrahlung.

Two-component exciton model

Abstract: The exciton model of nuclear reactions with the distinguishability of protons and neutrons explicitly included is discussed. The Pauli master equation and transition rates in this two-component formulation are given. Both the simple considerations and actual calculations show that the two-component model exhibits new features and modifies conclusions of the one-component formulation. The ratio of the proton to neutron preequilibrium emissions in proton induced reactions decreases twice to three times when compared with the usual one-component model.

Precise determination of the171Yb+ ground state Hyperfine separation

Abstract: We performed a microwave-optical double resonance experiment on the ground state of171Yb+ ions. About 105 particles were confined in a r.f. quadrupole trap for periods of several hours in the presence of He buffer gas. Hyperfine pumping by a pulsed dye laser was followed by microwave transitions, which we observed via changes in the ionic fluorescence intensity. The ground state hyperfine splitting has been determined togD W=12642812124.2±1.4 Hz. The ultimate line width obtained in this experiment was 33 mHz, corresponding to a lineQ of 3.8·1011. The final error ofgD W is mainly determined by the accuracy of the available frequency reference.

Neutron scattering length of lithium and boron and their isotopes

Abstract: Coherent neutron scattering lengths and free cross sections were measured in order to determine the spin dependent scattering lengths of the isotopes of Li and B.

Treatment of the positron-hydrogen system in hyperspherical coordinates

Abstract: The proton-electron-positron system has been treated within the frame of hyperspherical coordinates. Adiabatic potentials and coupling matrix elements which depend on the electron-positron mean-square radius have been obtained forS-states. From these data we have calculated elastic positron-hydrogen scattering phase shifts. No stable ground state and no resonance below the threshold for positronium formation in the ground state has been found. Our investigation however predicts a resonance slightly below the threshold forH(n=2). Its total binding energy obtained in this work isE 0≃−0.128506 a.u.

Activation measurement of the3He(4He,γ)7Be reaction

Abstract: The energy integrated absolute cross section of the3He(4He,γ)7Be reaction has been determined by an activation measurement, using the branching of the7Be 53.44d β-decay to the 478 keV state in7Li and its subsequentγ-decay. Depending on the branching ratio used (10.4% or 15.4%) we obtain a zero-energy nuclear cross section factor ofS(0)=0.56±0.03 keV·barn orS(0)=0.38±0.03 keV·barn, respectively.

Rotational properties of shell-model states in fp-shell nuclei

Abstract: The relation between the microscopic shell model and the collective rotor model is investigated. As the first step an extensive shell-model calculation is performed on about twentyfp-shell nuclei with massA=52–60. It turns out that, with the model space and the effective interaction chosen, the shell model is well able to reproduce the experimental data in this mass region. As the second step the shell-model wavefunctions are used to calculate energies, electromagnetic moments and transition rates of states with spin up toJ=16. As the third step the observables calculated with the shell model are used to investigate whether these microscopic results can be reproduced by a simple rotational model. About twenty pure axially symmetric rotor bands, generated by the shell model, could be localized. Their properties are presented and discussed.

The beta-decay of95Rb and97Rb

Abstract: Theβ-decay of 377 ms95Rb and 168ms97Rb has been thoroughly investigated by means ofγ-ray, conversion electron andβ-delayed neutron spectroscopy. More than 97% of theβ-decay to particle-bound states has been placed in level schemes of95Sr and97Sr. High-resolution neutron spectroscopic studies have allowed to extend the knowledge of the excitation spectra up to 9 MeV, respectively 10 MeV. The density of neutron-unbound levels in95Sr and97Sr has been derived from peak stripping analyses of the neutron spectra. Beta-strength functions (S β(E)) have been investigated in detail. They indicate that Gamow-Tellerβ-decay of both precursors is dominated by nuclear structure. The discovered pronounced resonances and the variation in the shape ofS β(E) are compared with predictions from the gross theory ofβ-decay and shell model calculations in the random phase approximation (RPA).

Identification of the proton drip line

Abstract: A group of proton emitters of Au, Ir, Re, Ta has been identified by means of mass values derived from the experimental data. The proton drip line has been determined. It is shown that on the boundary of the proton stability the values of proton pairing energies increase by about 50% as compared with the isotone nuclei near the betastability line.

Experimental investigation of the reaction12C(γ, π0)X in the photon energy range between threshold and 450 MeV

Abstract: The photoproduction of neutral pions on12C has been investigated between threshold and 450 MeV photon energy using the tagged photon beam at the Bonn 500 MeV synchrotron and a large acceptance detector. Theπ0 detector consists of cylindrical wire chambers covering a solid angle of nearly 4π. Using a scintillation counter as an active12C target an attempt was made to separate coherent and incoherentπ0 photoproduction. The measured total and differential cross sections are compared to corresponding theoretical calculations.

The reflection of light swift particles from heavy solid targets

Abstract: The reflection of light particles bombarding heavy targets has been investigated with the single-collision model, which assumes straight line trajectories between the surface and the large angle scattering event upon incidence and exit of the swift particle. This model allows analytical integration, under some approximations, to yield energy and angular distributions of reflected particles as well as particle and energy reflection coefficients in terms of analytical formulae. The results are compared with data from the Monte Carlo computer program TRIM for H and He incident on Au. This comparison shows that the model can be applied for reduced energies larger thane=5, and that it is well suited for any higher energy up to relativistic energies, in contrast to previous theoretical or empirical formulae.

Alpha decay of new neutron deficient gold, mercury, and thallium isotopes

Abstract: In fusion evaporation reactions of a92Mo beam with targets of neutron deficient Rb — Mo isotopes very neutron deficient isotopes of elements between Au and Po have been produced. The new isotopes173, 174Au,175, 176Hg, and179Tl were identified by alpha spectroscopy. The mass excess value of176Hg could be linked to known values of theN−Z=16 chain. The location of the new isotopes with respect to the proton drip line is discussed. A new high energyα transition of (7.20±0.02) MeV andT 1/2=(1.4±0.5) ms has been found in the reaction92Mo+89Y→181Tl* at an excitation energy of 37 MeV. It is tentatively assigned to isotopes produced in 2-particle evaporation channels.

High resolution laser spectroscopy of low-lying p-states in Sr I and Ba I

Abstract: Hyperfine structure and isotope shifts of the second excitedP states of Sr I and Ba I have been measured. In addition, the Sr I first resonance line has been studied.

The new neutron-rich isotope183Lu

Abstract: The new neutron-rich isotope183Lu was produced in multinucleon transfer reactions by irradiatingnatW/Ta targets with 11.7 MeV/u136Xe ions, and identified by mass separation and decay spectroscopy. The measured half-life of183Lu is 58±4 s. The properties of the excited states of183Hf are discussed.

Ionization of hydrogen and deuterium atoms in thermal energy collisions with state-selected Ne(3 s 3 P 2 , 3 P 0 ) metastable atoms

Abstract: High resolution energy spectra of electrons and ions resulting from thermal energy collisions of hydrogen and deuterium atoms with state-selected metastable Ne(Ne(3s 3 P 2,3 P 0) atoms are reported. The electron spectra for Ne(3 P 2)+H(D) are very broad: The high energy part due to formation of NeH+ (NeD+) bound states (associative ionization), amounts to about 30% of the ionizing events, whereas the dominant part of the spectrum including a prominent low-energy peak is due to Penning ionization out of a strongly-attractive entrance potential curve. Comparison of the spectra with quantum mechanical fit calculations yields fairly accurate information on this potential, in particular its well depthD e [Ne(3 P 2)−H,D]= 2.0(1) eV. The spectra for Ne(3 P 0)+H, D are comparatively narrow with much lower cross sections than the one for the Ne(3 P 2) state. The corresponding entrance channel is a weakly bound van der Waals molecule with a well depth below 0.1 eV. A perturbation calculation of the Ne(3s)+H(1s) potential energy curves at large distances explains the observed difference between the Ne(3 P 2)+H(D) and Ne(3 P 0)+H(D) systems. Symmetry arguments are given that the major contribution to the Ne(3 P 2)+H(D) spectra is due to the2 Σ potential.

Direct proton decay of 0.56-s147Tm and search for this decay mode among very neutron-deficient isotopes with 53≦Z≦67

Abstract: The earlier preliminary assignment of a 1,055±6 keV proton line to direct proton decay of147Tm is supported by cross bombardment measurements and by a negative result from a positron-proton coincidence experiment. The half-life was remeasured to be 0.56 ±0.04s. For two types of thermal ion sources, overall efficiencies were estimated for on-line mass separation of known short-lived isotopes of promethium, europium, terbium, and holmium. Direct proton decay was searched for among very neutron-deficient isotopes of these elements, and of iodine and caesium. No evidence for direct proton decay was found. Based on estimated overall efficiencies, on calculated cross-sections, and on predictions from the gross-theory ofβ decay, half-life limits for direct proton decay were deduced.

Coupling of two quasiparticles to coherent cores in the Pt isotopes

Abstract: A model for coupling the motion of particles to that of a quadrupole-collective core is proposed. The quadrupole vibrations and rotations of the core are described by angular momentum projected coherent states. The spherical shell model particles interact among themselves by pairing and surface delta interactions. The particles are coupled to the core through a multipole-multipole interaction. The method is applied to several even mass isotopes of Pt. The agreement with experimental data of the excitation energies, gyromagnetic factors andE2 probabilities is very good.

γ-Spectroscopy of 89 215 Ac126 and proton pairing inN=126 isotones

Abstract: Theγ-decay of 5 excited states along the yrast line has been observed in215Ac by inbeamγ-spectroscopy. Lifetimes andg-factors of 3 isomeric levels have been measured. The data show that pairing dominates the structure ofN=126 isotones.

Group-theoretical classification of angular momentum coherences in hydrogen

Abstract: A new classification scheme of angular momentum coherences based on the dynamical symmetry group O4 of hydrogen is proposed. The existence of a specific constant of motion, the Runge-Lenz vector, enables an extension of the Fano-Macek approach to mixed-l coherences observed in inelastic atomic collision processes. Using aSU(2) ×SU(2) pseudospin formulation a correspondence between the completen-shell density matrix and the expectation values of multipoles of the angular momentum and the Runge-Lenz vector is established. Explicit expressions are given forn=2 andn=3.

Band structure of even-even selenium isotopes in the proton-neutron interacting Boson model

Abstract: Available systematic IBM calculations [1-6] for Krypton and Strontium isotopes have been extended to Selenium. The analysis in terms of the IBM is complicated by the interplay of collective and noncollective degrees of freedom. However, satisfactory agreement has been obtained forN≦42.

The single proton nucleus133Sb

Abstract: Theβ-decay of133Sn, populating levels in the single proton valence nucleus133Sb, has been studied. The observed energy levels andβ-transition probabilities are discussed in terms of the shell model.

Polarizability effects in electronic and muonic atoms

Abstract: TheS state polarizability shifts are derived from the virtual forward Compton scattering in the unretarded dipole approximation. In the non-relativistic limit ω N /2m≪1, the shift is proportional to the photonuclear sum rule σ−3/2, while in the relativistic limit ω N /2m≫1 it is proportional to a logarithmically weighted σ−2 sum rule. In both cases, the characteristic momentum transfer is (2mω N )1/2. The non-locality from the intermediate lepton propagation removes the divergence typical of the static limit. Explicit formulas for the shifts are given for both the relativistic and non-relativistic limits.

Products of 3≦Z≦9 from long-lived intermediates in the reactions 336 MeV40Ar+Ag,154Sm and Au

Abstract: Angular and energy distributions have been measured for products of 3≦Z≦9 from reactions of 336MeV40Ar+107,9Ag/154Sm/197Au. The c.m. differential cross sections and mean energies increase as one moves from ≈100deg. to ≈170deg. similar to those for fission or evaporation-like emission. Most characteristics mimic those observed for4He, but the cross sections are smaller by a factor of as ≈1/300 for each reaction studied. A semiclassical analysis has been used to estimate the effective barriers to emission and root-mean-square spins of the emitters.

Natural Radiative Lifetimes In the 2s1/2 and 2d5/2,3/2 Sequences of Aluminum

Abstract: Natural radiative lifetimes have been measured for the 3s2ns2S1/2 (n≦10) and 3s2nd2D5/2,3/2 (n≦11) sequences of aluminum using pulsed laser excitation of an atomic beam. The investigated states were populated from the ground configuration using UV pulses of wavelength down to 210 nm produced by a Nd: YAG- or an excimer-pumped dye-laser system. TheS-state lifetimes increase regularily with increasing principal quantum number but this is not the case for theD sequence.