Showing papers by "Achim Richter published in 2001"

Experimental observation of the topological structure of exceptional points.

Abstract: We report on a microwave cavity experiment where exceptional points (EPs), which are square root singularities of the eigenvalues as function of a complex interaction parameter, are encircled in the laboratory. The real and imaginary parts of an eigenvalue are given by the frequency and width of a resonance and the eigenvectors by the field distributions. Repulsion of eigenvalues--always associated with EPs--implies frequency anticrossing (crossing) whenever width crossing (anticrossing) is present. The eigenvalues and eigenvectors are interchanged while encircling an EP, but one of the eigenvectors undergoes a sign change which can be discerned in the field patterns.

First experimental test of a trace formula for billiard systems showing mixed dynamics.

Abstract: In general, trace formulas relate the density of states for a given quantum mechanical system to the properties of the periodic orbits of its classical counterpart. Here we report for the first time on a semiclassical description of microwave spectra taken from superconducting billiards of the Lima\ifmmode \mbox{\c{c}}\else \c{c}\fi{}on family showing mixed dynamics in terms of a generalized trace formula derived by Ullmo et al. [Phys. Rev. E 54, 136 (1996)]. This expression not only describes mixed-typed behavior but also the limiting cases of fully regular and fully chaotic systems and thus presents a continuous interpolation between the Berry-Tabor and Gutzwiller formulas.

The new photoactivation facility at the 4.3 MV Stuttgart DYNAMITRON: setup, performance, and first applications

Abstract: At the Stuttgart 4: 3M V DYNAMITRON accelerator a newbremsstrahlung irradiation facility w as installed for photoactivation experiments to complement the well-established arrangement for systematic photon-scattering studies taking advantage of the high photon fluxes achievable at this high-current accelerator. The population or depopulation of isomeric states in low-energy photon-induced reactions ðEg ¼ 0:5–4:0 MeV) can be studied with sensitivities far beyond previous limits. The setup of this facility, its performance, and the absolute calibrations are described in detail. First exemplary results for a test case, the reaction 115 In ðg;g 0 Þ 115m In, are presented and analyzed in combination with data from low-energy photon-scattering experiments ( 115 In ðg;g 0 Þ 115 In). The quality of the results demonstrates the

Wave dynamical chaos: an experimental approach in billiards

Abstract: Two experiments with super- and normal conducting microwave cavities which are used as an analog to two-dimensional, infinitely deep quantum potentials are presented. The eigenvalues of such a potential can be measured directly through the resonance frequencies of the cavity, while the eigenvectors can be determined by measuring the field distributions inside the cavity. In the case of open systems another information – the imaginary part of the eigenvalues – is observable by measuring the widths of the resonances. As examples for this experimental approach to quantum chaos the semiclassical reconstruction of length spectra of billiards with varying chaoticity based of various trace formulas proposed by Gutzwiller, by Ullmo, Grinberg and Tomsovic and by Berry and Tabor for systems with chaotic, mixed and regular dynamics, respectively, and the first observation of square root singularities – so called exceptional points recently proposed by Heiss – in the energy spectrum of coupled cavities are presented.

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

Abstract: As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 106 particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on 2H and 9Be targets at REX-ISOLDE energies of 2.2 MeV . A are confirmed.

Depopulation of the Jπ = 9- isomer in 180Ta to the Jπ = 1+ ground state by Coulomb excitation

Abstract: In-beam Coulomb excitation of the exotic odd-odd nucleus 180Ta has been studied by using a 136Xe beam and a setup consisting of five EUROBALL CLUSTER detectors and the Darmstadt-Heidelberg Crystal Ball array. Spectroscopic information on the extremely rare 180Ta is obtained from the comparison between an enriched (3.6±0.3% 180Ta) and a natural tantalum target. Possible evidence for a depopulation from the long-lived high-spin Jπ = 9- isomer to the short-lived Jπ = 1+ ground state is searched for by different methods. The decay of low-K bandheads, which are nanosecond isomers, towards the ground-state band can be demonstrated in delayed spectroscopy. A γγ coincidence analysis provides indications of K = 5 in-band transitions. Finally, when the Crystal Ball is used as an energy and γ multiplicity filter, signals of decay into the K = 0 band are observed.

Role of knockout contributions in giant resonance studies with (p,p'x) reactions

Abstract: The giant-resonance region in ${}^{40}\mathrm{Ca}$ and ${}^{48}\mathrm{Ca}$ has been studied using the reactions ${}^{48}\mathrm{Ca}{(p,p}^{\ensuremath{'}}{n}_{0}{)}^{47}\mathrm{Ca},$ ${}^{40}\mathrm{Ca}{(p,p}^{\ensuremath{'}}{p}_{0}{)}^{39}\mathrm{K},$ and ${}^{40}\mathrm{Ca}{(p,p}^{\ensuremath{'}}{\ensuremath{\alpha}}_{0}{)}^{36}\mathrm{Ar}$ for an incident proton energy ${E}_{p}=100 \mathrm{MeV}.$ Knockout cross sections for these reactions were calculated within the distorted wave impulse approximation formalism using parameters fixed from previous investigations. The results quantitatively account for the data at higher excitation energies while the knockout contributions underneath the main resonance strength are small.

Test of Trace Formulas for Spectra of Superconducting Microwave Billiards

Abstract: Experimental tests of various trace formulas, which in general relate the density of states for a given quantum mechanical system to the properties of the periodic orbits of its classical counterpart, for spectra of superconducting microwave billiards of varying chaoticity are reviewed by way of examples. For a two-dimensional Bunimovich stadium billiard the application of Gutzwiller's trace formula is shown to yield correctly locations and strengths of the peaks in the Fourier transformed quantum spectrum in terms of the shortest unstable classical periodic orbits. Furthermore, in two-dimensional billiards of the Limacon family the transition from regular to chaotic dynamics is studied in terms of a recently derived general trace formula by Ullmo, Grinberg and Tomsovic. Finally, some salient features of wave dynamical chaos in a fully chaotic three-dimensional Sinai microwave billiard are discussed. Here the reconstruction of the spectrum is not as straightforward as in the two-dimensional cases and a modified trace formula as suggested by Balian and Duplantier will have eventually to be applied.

Quantum chaos and wavedynamical chaos in two- and three-dimensional microwave billiards

Abstract: Examples of recent experiments with microwave resonators in two- and three-dimensions in which we study the quantum manifestation of classical chaos in systems with few degrees of freedom are presented. We show the application of random matrix theory and periodic orbit theory to different experimental systems, the spectral features of coupled billiards with varying strength and results on Anderson localization in a simple Bloch-like lattice.

Self energies of the pion and the delta isobar from the ^3He(e,e'pi^+)^3H reaction

Abstract: In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the $^3$He(e,e'$\pi^+)^3$H reaction have been measured in the excitation region of the $\Delta$ resonance to determine the longitudinal ($L$), transverse ($T$), and the $LT$ interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and $\Delta$-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the $\pi^+$ mass of $\Delta m_{\pi^+} = (-1.7^{+ 1.7}_{- 2.1})$ MeV/c$^2$ in the neutron-rich nuclear medium at a density of $\rho = (0.057^{+ 0.085}_{- 0.057})$ fm$^{-3}$. Our data are consistent with the $\Delta$ self energy determined from measurements of $\pi^0$ photoproduction from $^4$He and heavier nuclei.

RES-ISOLDE - Post-accelerated radioactive beams at CERN-ISOLDE

Abstract: The ISOLDE RIB-facility at CERN has today been producing a vast range of radioactive beams since more than 30 years. The low-energy beams of ISOLDE will be complemented by a post-accelerator, REX-ISOLDE, currently being assembled. In order to convert the pseudo-DC, singly-charged beam from the ISOLDE mass separators into a cooled and bunched beam at higher charge states, a novel scheme of trapping, cooling, and charge-state breeding has been devised, using a linear Penning trap and an Electron Beam Ion Source (EBIS). This allows for subsequent acceleration by a short, cost-effective LINAC consisting of an RFQ, an IH-structure and three seven-gap resonators, reaching 0.8–2.2 MeV/u. The installation of REX-ISOLDE is well underway and the first post-accelerated radioactive beams are expected to be obtained during late 2000.

Stern‐Gerlach‐Medaille: Billardspiel mit Mikrowellen: Die Analogie zwischen Schrödinger‐Gleichung und Helmholtz‐Gleichung ermöglicht Experimente zum Quantenchaos

Abstract: Bei der quantenmechanischen Beschreibung klassisch chaotischer Systeme findet man ein generisches Verhalten, das sich grundlegend von dem klassisch regularer Systeme unterscheidet. Dieses als Quantenchaos bekannte Phanomen lasst sich sowohl in echten Quantensystemen, wie z. B. Atomkernen, als auch in analogen Modellsystemen wie flachen Mikrowellenresonatoren beobachten. Daruber hinaus konnen die Methoden zur Behandlung des Quantenchaos auf das allgemeine wellenmechanische Chaos in Systemen erweitert werden, deren Verhalten durch eine vektorielle Wellengleichung beschrieben wird.

Quantumchaosandwavedynamicalchaosintwo-and three-dimensionalmicrowavebilliards (

Abstract: Examples of recent experiments with microwave resonators in two- and three-dimensions in which we study the quantum manifestation of classical chaos in systems with few degrees of freedom are presented. We show the application of random matrix theory and periodic orbit theory to dierent experimental systems, the spectral features of coupled billiards with varying strength and results on Anderson localization in a simple Bloch-like lattice. ?2001 Elsevier Science B.V. All rights reserved.