Superheavy element chemistry at GSI – status and perspectives

Superheavy elements have been synthesized and chemically characterized one-atom-at-a-time up to element 108. Presently, the quest for identification and investigation of element 112 is one of the hottest topics in this field. The transactinide elements 104 to 108 are members of group 4 to 8 of the periodic table and element 112 belongs into group 12. For some of these elements detailed chemical properties have been revealed which show stunning deviations from simple extrapolations within their respective group while others exhibit great similarities with their lighter homologues. All presently known chemical properties of seaborgium (Sg, element 106) — the heaviest element whose behavior was investigated in aqueous solution — and hassium (Hs, element 108) were obtained in experiments performed at the GSI in large international collaborations. Recently, the highly efficient and very clean separation of Hs was applied for nuclear studies of various Hs nuclides investigating their cross section and their nuclear decay properties in the region of the N=162 neutron shell. To overcome certain limitations of the presently used on-line chemical separations the new TransActinide Separator and Chemistry Apparatus (TASCA) — with a gas-filled recoil separator as a front-end tool — was designed and built at the GSI in a collaborative effort. Presently in its commissioning phase, TASCA shall be a key instrument for a big leap into quantitatively and qualitatively new experiments in the region of superheavy elements.     

A primer for electroweak induced low-energy nuclear reactions

Under special circumstances, electromagnetic and weak interactions can induce low-energy nuclear reactions to occur with observable rates for a variety of processes. A common element in all these applications is that the electromagnetic energy stored in many relatively slow-moving electrons can — under appropriate circumstances — be collectively transferred into fewer, much faster electrons with energies sufficient for the latter to combine with protons (or deuterons, if present) to produce neutrons via weak interactions. The produced neutrons can then initiate low-energy nuclear reactions through further nuclear transmutations. The aim of this paper is to extend and enlarge upon various examples analysed previously, present order of magnitude estimates for each and to illuminate a common unifying theme amongst all of them.     

Organic thin-film transistors of pentacene films fabricated from a supersonic molecular beam source

Top-contact organic thin-film transistors (OTFTs) of pentacene have been fabricated on bare SiO₂ and SiO₂ modified with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The pentacene films were deposited from a supersonic molecular beam source with kinetic energy of incident molecules ranging from 1.5 to 6.7 eV. The field-effect mobility of OTFTs was found to increase systematically with increasing kinetic energy of the molecular beam. The improvements are more important on HMDS- and OTS-treated surfaces than on bare SiO₂. Tapping mode atomic force microscopy images reveal that pentacene thin films deposited at high kinetic energy form with significantly larger grains—independent of surface treatment—than films deposited using low-energy beams.     

Fission of highly excited fragments from collisions of 750 A.MeV 238U-ions on 208Pb

Fragments of relativistic 750 A.MeV U-projectiles were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and ΔE-ToF techniques. For elements between Ge and Sb, measurements of isotopic yield distributions and velocities revealed three processes: fragmentation, low-energy fission, and high-energy fission. The last of these regimes is presently reported. First and second moments of distributions of mass numbers, atomic numbers and velocities of the corresponding fragments allowed us to identify ¹⁰¹ ₄₃Tc₅₆ as the most probable fragment of a high energy symmetric fission reaction. Moreover, we could deduce a hypothetical mean fissioning fragmentation product ²⁰⁸Rn and its highly excited pre-fragmentation parent ²²⁷Ra produced in a primary abrasion reaction at an excitation energy of about 290 MeV. Received: 26 January 1998 / Revised version: 16 March 1998     

A New High-intensity, Low-momentum Muon Beam for the Generation of Low-energy Muons at PSI

At the Paul Scherrer Institute (PSI, Villigen, Switzerland) a new high-intensity muon beam line with momentum p < 40 MeV/c is currently being commissioned. The beam line is especially designed to serve the needs of the low-energy, polarized positive muon source (LE-μ⁺) and LE-μ SR spectrometer at PSI. The beam line replaces the existing μ E4 muon decay channel. A large acceptance is accomplished by installing two solenoidal magnetic lenses close to the muon production target E that is hit by the 590-MeV PSI proton beam. The muons are then transported by standard large aperture quadrupoles and bending magnets to the experiment. Several slit systems and an electrostatic separator allow the control of beam shape, momentum spread, and to reduce the background due to beam positrons or electrons. Particle intensities of up to 3.5 × 10⁸ μ⁺/s and 10⁷ μ⁻/s are expected at 28 MeV/c beam momentum and 1.8 mA proton beam current. This will translate into a LE-μ⁺ rate of 7,000/s being available at the LE-μ SR spectrometer, thus achieving μ⁺ fluxes, that are comparable to standard μ SR facilities.     

Cosmic rays in the heliosphere

Summary This is a rapporteur paper for the XXIV International Cosmic-Ray Conference covering topics of anomalous cosmic rays, shock acceleration, modulation and transport theory, cosmic-ray gradients, corotating interaction regions, coronal-mass ejections, and solar neutrinos. Among the highlights of the meeting are —conclusive proof that most anomalous cosmic rays are singly charged,—undisputed detection of anomalous cosmic-ray hydrogen,—discovery of unexpectedly large anisotropies of pickup ions,—observation of pronounced solar rotational modulation of cosmic-ray fluxes to the highest heliolatitudes (∼80°) probed by the Ulysses spacecraft, and—new measurements of modulation effects sensitive to particle charge sign. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Setting the Quantum Integrand of M-Theory

In anomaly-free quantum field theories the integrand in the bosonic functional integral—the exponential of the effective action after integrating out fermions—is often defined only up to a phase without an additional choice. We term this choice ``setting the quantum integrand'. In the low-energy approximation to M-theory the E₈-model for the C-field allows us to set the quantum integrand using geometric index theory. We derive mathematical results of independent interest about pfaffians of Dirac operators in 8k+3 dimensions, both on closed manifolds and manifolds with boundary. These theorems are used to set the quantum integrand of M-theory for closed manifolds and for compact manifolds with either temporal (global) or spatial (local) boundary conditions. In particular, we show that M-theory makes sense on arbitrary 11-manifolds with spatial boundary, generalizing the construction of heterotic M-theory on cylinders. The work of D.F. is supported in part by NSF grant DMS-0305505. The work of G.M. is supported in part by DOE grant DE-FG02-96ER40949     

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 618–622 (25 April 1997)     

The FIGARO II experiment: a general outline of the mission and the principal scientific results

Summary The FIGARO II (French Italian Gamma-Ray Observatory) experiment has been launched successfully three times: in July 1986 from Milo (Trapani), in November 1988 from Charleville (Australia) and in July 1990 again from Milo. In the first flight the observational program was limited to the Crab pulsar PSR0531+21 only because of a telemetry failure: the high sensitivity of FIGARO II allowed an accurate study of the pulse shape as well as a phase-resolved spectroscopy. It was also possible to evaluate the dispersion measure of the Crab pulsar at the flight date from the time delay between gamma-ray and radio pulses. The major results of the second flight were a stringent upper limit to the low-energy gamma-ray flux from PSR 0833-45 (Vela pulsar)—well below the detection claimed by the UCR group —and the observation of a strong emission in the 0.511 MeV annihilation line from the inner region of our Galaxy. The data acquired in the third flight are still under analysis, but preliminary results suggest changes in the shape of the pulse profile. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

The Girardeau's fermion-boson procedure in the light of the composite-boson many-body theory

We reconsider the procedure developed for atoms a few decades ago by Girardeau, in the light of the composite-boson many-body theory we recently proposed. The Girardeau's procedure makes use of a so called “unitary Fock-Tani operator” which in an exact way transforms one composite bound atom into one bosonic “ideal” atom. When used to transform the Hamiltonian of interacting atoms, this operator generates an extremely complex set of effective scatterings between ideal bosonic atoms and free fermions which makes the transformed Hamiltonian impossible to write explicitly, in this way forcing to some truncation. The scatterings restricted to the ideal-atom subspace are shown to read rather simply in terms of the two elementary scatterings of the composite-boson many-body theory, namely, the energy-like direct interaction scatterings — which describe fermion interactions without fermion exchange — and the dimensionless Pauli scatterings — which describe fermion exchanges without fermion interaction. We here show that, due to a fundamental difference in the scalar products of elementary and composite bosons, the Hamiltonian expectation value for N ground state atoms obtained by staying in the ideal-atom subspace and working with boson operators only, differ from the exact ones even for N = 2 and a mapping to the ideal-atom subspace performed, as advocated, from the fully antisymmetrical atomic state, i.e., the state which obeys the so-called “subsidiary condition”. This shows that, within this Girardeau's procedure too, we cannot completely forget the underlying fermionic components of the particles if we want to correctly describe their interactions.     

Shiva diagrams for composite-boson many-body effects: how they work

The purpose of this paper is to show how the diagrammatic expansion in fermion exchanges of scalar products of N-composite-boson (“coboson”) states can be obtained in a practical way. The hard algebra on which this expansion is based, will be given in an independent publication. Due to the composite nature of the particles, the scalar products of N-coboson states do not reduce to a set of Kronecker symbols, as for elementary bosons, but contain subtle exchange terms between two or more cobosons. These terms originate from Pauli exclusion between the fermionic components of the particles. While our many-body theory for composite bosons leads to write these scalar products as complicated sums of products of “Pauli scatterings” between two cobosons, they in fact correspond to fermion exchanges between any number P of quantum particles, with 2 ≤P≤N. These P-body exchanges are nicely represented by the so-called “Shiva diagrams”, which are topologically different from Feynman diagrams, due to the intrinsic many-body nature of the Pauli exclusion from which they originate. These Shiva diagrams in fact constitute the novel part of our composite-exciton many-body theory which was up to now missing to get its full diagrammatic representation. Using them, we can now “see” through diagrams the physics of any quantity in which enters N interacting excitons — or more generally N composite bosons —, with fermion exchanges included in an exact — and transparent — way.     

Network robustness to targeted attacks. The interplay of expansibility and degree distribution

We study the property of certain complex networks of being both sparse and highly connected, which is known as “good expansion” (GE). A network has GE properties if every subset S of nodes (up to 50% of the nodes) has a neighborhood that is larger than some “expansion factor” φ multiplied by the number of nodes in S. Using a graph spectral method we introduce here a new parameter measuring the good expansion character of a network. By means of this parameter we are able to classify 51 real-world complex networks — technological, biological, informational, biological and social — as GENs or non-GENs. Combining GE properties and node degree distribution (DD) we classify these complex networks in four different groups, which have different resilience to intentional attacks against their nodes. The simultaneous existence of GE properties and uniform degree distribution contribute significantly to the robustness in complex networks. These features appear solely in 14% of the 51 real-world networks studied here. At the other extreme we find that ∼40% of all networks are very vulnerable to targeted attacks. They lack GE properties, display skewed DD — exponential or power-law — and their topologies are changed more dramatically by targeted attacks directed at bottlenecks than by the removal of network hubs.     

Tailoring the wetting response of silicon surfaces via fs laser structuring

Control over the wettability of solids and manufacturing of functional surfaces with special hydrophobic and self-cleaning properties has aroused great interest because of its significance for a vast range of applications in daily life, industry and agriculture. We report here a simple method for preparing stable superhydrophobic surfaces by irradiating silicon (Si) wafers with femtosecond (fs) laser pulses and subsequently coating them with chloroalkylsilane monolayers. It is possible, by varying the laser pulse fluence on the surface, to achieve control of the wetting properties through a systematic and reproducible variation of roughness at micro- and nano-scale which mimics both the topology of the “model” superhydrophobic surface—the natural lotus leaf—, as well as its wetting response. Water droplets can move along these irradiated superhydrophobic surfaces, under the action of small gravitational forces, and experience subsequent immobilization, induced by surface tension gradients. These results demonstrate the potential of manipulating liquid motion through selective laser patterning.     

An alternative approach for microlithography light source

A hybrid master oscillator–power amplifier (hybrid MOPA) scheme is proposed as a microlithography light source. The seed pulses are generated in the visible spectral range—where the necessary spectral purity is more easily achieved—and after frequency conversion are amplified in an excimer amplifier. The new concept enables us to decrease the bandwidth considerably, approaching the theoretical limit posed by the uncertainty relation. The feasibility of the new approach is demonstrated by a dye/excimer MOPA system, generating deep ultraviolet DUV (248-nm) pulses of 0.2-pm bandwidth.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

Pressure variation of reentrant transition temperature in liquid crystals

High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T_AN exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (γ, λ_eff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling λ_eff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through γ and λ_eff which justifies the experimental pressure dependence in the reentrant transition temperature . The pressure dependence of γ and λ_eff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.     

Accurate calculation of the scattering length for the cooling of hydrogen atoms by lithium atoms

An improved ab initio calculation has been performed for the potential for the LiH a ³Σ⁺ state, using two very large basis sets. The Basis Set Superposition Error (BSSE) correction has been determined for both basis sets and the non-Born-Oppenheimer correction estimated to be negligible. The best potential is approximately 10% deeper than the previous estimate. Vibrational energies and scattering lengths have been calculated for ^6,7LiH(D) with both potentials, with and without the BSSE correction, and also with an estimated potential expected to bracket the true potential. The ⁷LiH scattering length is estimated to be (45 ± 4)a₀ and hence the low-energy cross-section in the best a ³Σ⁺ potential is about half that calculated previously. Enhanced cooling by ⁷Li of trapped H atoms remains feasible. Received 30 April 2001     

Interfaces in perovskite heterostructures

Recent advances in film synthesis have made it possible to investigate the properties of well-controlled interfaces in perovskite metal-oxides. A review of published experimental data and computational results indicate that so far most interfaces that have been analyzed in ferroelectric materials—while necessary to impose large lattice strain on the polar material—contribute little to the ferroelectricity and may instead be detrimental to the desired properties. In contrast, a very different situation arises at interfaces that show changes in the electronic configuration as a consequence of a compositional discontinuity. Data is shown for LaMnO₃/SrTiO₃ superlattices as an example of electronic effects that produce enhanced properties, further illustrating the richness of interfacial properties that can be obtained at interfaces (as shown in numerous published results for different but related interfaces).     

Measurement of the interaction cross-section and related topics

Recent experimental results concerning interaction cross-sections ( σ_I) are reviewed. The σ_I values were measured by a transmission method using the fragment separator at GSI. The σ_I values for B, C, N, O and F isotopes and the recently measured σ_I for Ar are presented. As related topics, an analysis by the recently developed Glauber model for a few-body system is introduced. By using this analysis, the effective density distributions for light neutron-rich nuclei can be deduced. The recently shown magic number N = 16 near to the neutron drip line is also discussed. Received: 1 May 2001 / Accepted: 4 December 2001