Showing papers by "Achim Richter published in 2009"

Induced Violation of Time-Reversal Invariance in the Regime of Weakly Overlapping Resonances

Abstract: We measure the complex scattering amplitudes of a flat microwave cavity (a ``chaotic billiard''). Time-reversal ($\mathcal{T}$) invariance is partially broken by a magnetized ferrite placed within the cavity. We extend the random-matrix approach to $\mathcal{T}$ violation in scattering, determine the parameters from some properties of the scattering amplitudes, and successfully predict others. Our work constitutes the most precise test of the random-matrix theoretical approach to $\mathcal{T}$ violation so far available.

Measurement of the longitudinal spin transfer to Λ and \bar{\varLambda} hyperons in polarised muon DIS

Abstract: The longitudinal polarisation transfer from muons to Λ and $\bar{\varLambda}$ hyperons, $D_{LL}^{\varLambda(\bar{\varLambda})}$ , has been studied in deep-inelastic scattering off an unpolarised isoscalar target at the COMPASS experiment at CERN. The spin transfers to Λ and $\bar{\varLambda}$ produced in the current fragmentation region exhibit different behaviours as a function of x and x F . The measured x and x F dependences of D are compatible with zero, while $D_{LL}^{\bar{\varLambda}}$ tends to increase with x F , reaching values of 0.4–0.5. The resulting average values are D = −0.012±0.047±0.024 and $D_{LL}^{\bar{\varLambda}}$ = 0.249±0.056±0.049. These results are discussed in the frame of recent model calculations.

Experimental test of a two-dimensional approximation for dielectric microcavities

Abstract: Open dielectric resonators of different shapes are widely used for the manufacture of microlasers. A precise determination of their resonance frequencies and widths is crucial for their design. Most microlasers have a flat cylindrical geometry, and a two-dimensional approximation, the so-called method of the effective index of refraction, is commonly employed for numerical calculations. Our aim has been an experimental test of the precision and applicability of a model based on this approximation. We performed very thorough and accurate measurements of the resonance frequencies and widths of two passive circular dielectric microwave resonators and found significant deviations from the model predictions. From this we conclude that the model generally fails in the quantitative description of three-dimensional dielectric resonators.

Global investigation of the fine structure of the isoscalar giant quadrupole resonance

Abstract: Fine structure in the region of the isoscalar giant quadrupole resonance (ISGQR) in $^{58}\mathrm{Ni}$, $^{89}\mathrm{Y}$, $^{90}\mathrm{Zr}$, $^{120}\mathrm{Sn}$, $^{166}\mathrm{Er}$, and $^{208}\mathrm{Pb}$ has been observed in high-energy-resolution ($\ensuremath{\Delta}{E}_{1/2}\ensuremath{\simeq}35\text{\ensuremath{-}}50$ keV) inelastic proton scattering measurements at ${E}_{0}=200$ MeV at iThemba LABS. Calculations of the corresponding quadrupole excitation strength functions performed within models based on the random-phase approximation (RPA) reveal similar fine structure when the mixing of one-particle one-hole states with two-particle two-hole states is taken into account. A detailed comparison of the experimental data is made with results from the quasiparticle-phonon model (QPM) and the extended time-dependent Hartree-Fock (ETDHF) method. For $^{208}\mathrm{Pb}$, additional theoretical results from second RPA and the extended theory of finite Fermi systems (ETFFS) are discussed. A continuous wavelet analysis of the experimental and the calculated spectra is used to extract dominant scales characterizing the fine structure. Although the calculations agree with qualitative features of these scales, considerable differences are found between the model and experimental results and amongst different models. Within the framework of the QPM and ETDHF calculations it is possible to decompose the model spaces into subspaces approximately corresponding to different damping mechanisms. It is demonstrated that characteristic scales mainly arise from the collective coupling of the ISGQR to low-energy surface vibrations.

Avoided-level-crossing statistics in open chaotic billiards.

Abstract: We investigate a two-level model with a large number of open decay channels in order to describe avoided level crossing statistics in open chaotic billiards. This model allows us to describe the fundamental changes in the probability distribution of the avoided level crossings compared with the closed case. Explicit expressions are derived for systems with preserved and broken time-reversal symmetry. We find that the decay process induces a modification at small spacings of the probability distribution of the avoided level crossings due to an attraction of the resonances. The theoretical predictions are in complete agreement with the recent experimental results of Dietz [Phys. Rev. E 73, 035201 (2006)].

Nonperiodic echoes from quantum mushroom-billiard hats.

Abstract: Nonperiodic tunable quantum echoes have been observed in experiments with an open microwave billiard whose geometry under certain conditions provides Fibonacci-like sequences of classical delay times. These sequences combined with the reflection at the opening induced by the wave character of the experiment and the size of the opening allow to shape quantum pulses. The pulses are obtained by response of an integrable scattering system.

Complete dipole response in 208Pb from high-resolution polarized proton scattering at 0°

Abstract: At the Research Center for Nuclear Physics, Osaka, Japan, the 208Pb(p,p´) reaction was measured at Ep=295 MeV and scattering angles Θlab= 0° - 10°. A high energy resolution of the order of ΔE/E ≈ 8x10^-5 was achieved, corresponding to ΔE=25-30 keV (FWHM). Cross sections were extracted by a multipole decomposition analysis of the angular distributions. Dominant contributions at very forward angles originate from E1 excitation due to Coulomb projectile-target interaction and spin M1 transitions caused by the spin-isospin part of the proton-nucleus interaction. A separation of these contributions was performed with two independent methods, viz. a multipole decomposition of the angular distributions and utilizing polarization transfer observables. Excellent agreement between both techniques is achieved within errors bars. The B(E1) strength distribution was extracted in the energy range between 5 and 20 MeV. Below the neutron separation energy (Sn = 7.367 MeV) it shows excellent agreement with available (γ,γ´) data and in the region of the giant dipole resonance with photoabsorption experiments. The shape of the angular distributions indicates a structural change of E1 strength below and above 8.2 MeV. The centroid energy and summed B(E1) strength of the PDR are extracted and amount to Ec=7.43(2) MeV and ΣB(E1)=1.54(16) e2fm2, respectively. Previously unobserved strength is found in the region above neutron threshold up to 8.2 MeV. The deduced E1 polarizability in the energy range from 5 to 19 MeV is αD = 18.7(13)fm3/e2. Averaging over all available data a highly precise value of αD = 18.9(5) fm3/e2 can be extracted. As the strong correlations predicted by microscopical models, this puts important constraints on the neutron skin thickness in 208Pb and the density dependence of the symmetry energy. The fine structure of the giant dipole resonance was analyzed with wavelet methods. Characteristic scales at E=100 keV, 340 keV, 520 keV, 1 MeV, and 2.1 MeV can be found. A comparison with microscopic calculations including the coupling to 2p-2h states suggests Landau damping as the dominant mechanism contributing to the decay width. Level densities of 1- states were extracted with a fluctuation analysis in the giant dipole resonance region. All phenomenological and microscopic models fail to describe the level densities in 208Pb, except for a version of the back-shifted Fermi gas model allowing for additional phenomenological parameters in local mass regions.

The baseband low level rf control for the s-dalinac: a flexible solution for other frequencies?*

Abstract: The low level RF system for the Superconducting DArmstadt electron LINear ACcelerator S-DALINAC developed 20 years ago and operating, converts the 3 GHz signals down to the base band and not to an intermediate frequency. While designing the new, digital RF control system this concept was kept: the RF module does the I/Q and amplitude modulation/ demodulation while the main (LF-) board, housing an FPGA analyses and processes the signals. Recently, the flexibility of this concept was realized: By replacing the modulator/ demodulators on the RF module, cavities operating at frequencies other than the S-DALINAC 3 GHz can be controlled with only minor modifications: A 6 GHz version, needed for a harmonic bunching system at the S-DALINAC and a 324 MHz solution to be used on a room temperature cavity at GSI will be presented. This paper will also review the concept of the digital low level RF control loops in more detail an report an the results gained during first operation on a superconducting cavity.

Friedel oscillations in microwave billiards

Abstract: Friedel oscillations of electron densities near step edges have an analog in microwave billiards. A random plane-wave model, normally only appropriate for the eigenfunctions of a purely chaotic system, can be applied and is tested for non-purely-chaotic dynamical systems with measurements on pseudointegrable and mixed dynamics geometries. It is found that the oscillations in the pseudointegrable microwave cavity match the random plane-wave modeling. Separating the chaotic from the regular states for the mixed system requires incorporating an appropriate phase-space projection into the modeling in multiple ways for good agreement with experiment.

Excitation energy and strength of the pygmy dipole resonance in stable tin isotopes

Abstract: The $^{112,120}$Sn$(\gamma,\gamma')$ reactions have been studied at the S-DALINAC. Electric dipole (E1) strength distributions have been determined including contributions from unresolved strength extracted by a fluctuation analysis. Together with available data on $^{116,124}$Sn, an experimental systematics of the pygmy dipole resonance (PDR) in stable even-mass tin isotopes is established. The PDR centroid excitation energies and summed strengths are in reasonable agreement with quasiparticle-phonon model calculations based on a nonrelativistic description of the mean field but disagree with relativistic quasiparticle random-phase approximation predictions.

Beam Based Alignment Simulations and Measurements at the S-DALINAC

Abstract: Operational Experience at the Darmstadt superconducting linac (S-DALINAC) showed unexpected effects on beam dynamics and beam quality. So operators could observe transverse beam deflections by changing phases of the SRF-Cavities. Furthermore there has been occurred a growth of normalized tranverse emittance by a factor of 2. The beam current in the S-DALINAC does not exceed 60 μA so space-charge effects could be eliminated to be the reason for the observations. In this work the effect of misalignment of the SRF-Cavities in the linac has been examined using beam-dynamic simulations with the tracking code GPT and measurements on the electron beam of the S-DALINAC. By measuring the transverse deflection of the beam by changes of the phases of the SRF-Cavities and comparing results with GPT-simulations a misalignment of the 5-cell capture cavity and first 20-cell cavity of several mm in both transverse directions could be found. This misalignment can explain transverse deflections as well as emittance growth. A correction of misalignment has been carried out using the described results. First measurements showed no more emittance growth and less beam-deflections by SRF-Cavities.

Electron scattering on the Hoyle state and carbon production in stars

Abstract: High‐resolution inelastic electron scattering experiments were performed at the S‐DALINAC for a precise determination of the partial pair width Γπ of the second Jπ = 0+ state, the so‐called Hoyle state, in 12C. Results for the monopole matrix element (directly related to Γπ) from a nearly model‐independent analysis based on an extrapolation of low‐q data to zero momentum transfer are presented. Additionally, a Fourier‐Bessel analysis of the transition form factor is discussed. The combined result of both methods leads to a pair width Γπ62.2(10) μeV.

Development of a FPGA based RF Control System for the S-DALINAC

Abstract: The Superconducting DArmstadt electron LINear ACcelerator S-DALINAC has a maximum energy of 130 MeV and beam currents of up to 60 µA. To reach this energy conveniently in cw superconducting cavities with a high Q at a frequency of 3 GHz are used. In order to achieve a minimum energy spread, the amplitude and phase of the cavities have to be controlled strictly in order to compensate the impact of microphonic perturbations. The existing analogue RF control system based on a self exited loop, converts the 3 GHz signals down to the base band. This concept will also be followed by the new digital system currently under design. It is based on a FPGA in the low frequency part, giving a great flexibility in the control algorithm and providing additional diagnostics. The low level RF system is controlled via CAN bus. We will report on the design concept, the status and the latest results measured with a prototype.