Showing papers by "Achim Richter published in 2006"

Fine structure of the Gamow-Teller resonance in 90Nb and level density of 1+ states.

Abstract: The fine structure of the Gamow-Teller resonance in a medium-heavy nucleus is observed for the first time in a high-resolution $^{90}\mathrm{Zr}(^{3}\mathrm{He},t)^{90}\mathrm{Nb}$ experiment at the Research Center for Nuclear Physics, Osaka. Using a novel wavelet analysis technique, it is possible to extract characteristic energy scales and to quantify their relative importance for the generation of the fine structure. This method combined with the selectivity of the reaction permits an extraction of the level density of ${1}^{+}$ states in $^{90}\mathrm{Nb}$.

First experimental observation of superscars in a pseudointegrable barrier billiard.

Abstract: With a perturbation body technique intensity distributions of the electric field strength in a flat microwave billiard with a barrier inside up to mode numbers as large as about 700 were measured. A method for the reconstruction of the amplitudes and phases of the electric field strength from those intensity distributions has been developed. Recently predicted superscars have been identified experimentally and - using the well known analogy between the electric field strength and the quantum mechanical wave function in a two-dimensional microwave billiard - their properties determined.

Gamow-Teller Strengths in the A = 14 Multiplet: A Challenge to the Shell Model

Abstract: A new experimental approach to the famous problem of the anomalously slow Gamow-Teller (GT) transitions in the {beta} decay of the A = 14 multiplet is presented. The GT strength distributions to excited states in {sup 14}C and {sup 14}O was studied in high-resolution (d,{sup 2}He) and ({sup 3}He,t) charge-exchange reactions on {sup 14}N. No-core shell-model (NCSM) calculations capable of reproducing the suppression of the {beta} decays predict a selective excitation of J{sup {pi}} = 2{sup +} states. The experimental confirmation represents a validation of the assumptions about the underlying structure of the {sup 14}N ground state wave function. However, the fragmentation of the GT strength over three 2{sup +} final states remains a fundamental issue not explained by the present NCSM using a 6 {h_bar}{omega} model space, suggesting possibly the need to include cluster structure in these light nuclei in a consistent way.

Spectral statistics in an open parametric billiard system

Abstract: Laboratoire de Physique Th´eorique et Mod`eles Statistiques,Bˆatiment 100, Universit´e de Paris-Sud, 91405 Orsay Cedex, France(Dated: February 6, 2008)We present experimental results on the eigenfrequency statistics of a superconducting, chaoticmicrowave billiard containing a rotatable obstacle. Deviations of the spectral fluctuations from pre-dictions based on Gaussian orthogonal ensembles of random matrices are found. They are explainedby treating the billiard as an open scattering system in which microwave power is coupled in andout via antennas. To study the interaction of the quantum (or wave) system with its environmenta highly sensitive parametric correlator is used.

Nonperiodic echoes from mushroom billiard hats.

Abstract: Mushroom billiards have the remarkable property to show one or more clear cut integrable islands in one or several chaotic seas, without any fractal boundaries. The islands correspond to orbits confined to the hats of the mushrooms, which they share with the chaotic orbits. It is thus interesting to ask how long a chaotic orbit will remain in the hat before returning to the stem. This question is equivalent to the inquiry about delay times for scattering from the hat of the mushroom into an opening where the stem should be. For fixed angular momentum we find that no more than three different delay times are possible. This induces striking nonperiodic structures in the delay times that may be of importance for mesoscopic devices and should be accessible to microwave experiments.

Strength Distributions and Symmetry Breaking in Coupled Microwave Billiards

Abstract: Flat microwave cavities can be used to experimentally simulate quantum mechanical systems. By coupling two such cavities, we study the equivalent to symmetry breaking in quantum mechanics. As the coupling is tunable, we can measure resonance strength distributions as a function of the symmetry breaking. We analyze the data by employing a qualitative model based on random matrix theory and show that the results derived from the strength distribution are consistent with those previously obtained from spectral statistics.

A silicon microstrip detector in a magnetic spectrometer for high-resolution electron scattering experiments at the S-DALINAC ?

Abstract: A silicon microstrip detector was developed as focal plane detector of the 169.7 ° magic angle double-focussing spectrometer at the superconducting Darmstadt electron linear accelerator (S-DALINAC). It allows experiments with minimum ionizing electrons at data rates up to 100 kHz, utilizing the maximum resolution of the spectrometer achievable in dispersion-matching mode.

Strength distributions and symmetry breaking in coupled microwave billiards

Abstract: Flat microwave cavities can be used to experimentally simulate quantum mechanical systems. By coupling two such cavities, we study the equivalent to the symmetry breaking in quantum mechanics. As the coupling is tunable, we can measure resonance strength distributions as a function of the symmetry breaking. We analyze the data employing a qualitative model based on Random Matrix Theory (RMT) and show that the results derived from the strength distribution are consistent with those previously obtained from spectral statistics.

Recent Simulation Results of the Polarized Electron Injector (SPIN) of the S-DALINAC

Abstract: Recent research and development for a polarized electron source (SPIN) for the recirculating superconducting electron linear accelerator S-DALINAC will be presented. The polarized electron beam will be produced by photoemission from an InAlGaAs/GaAs superlattice cathode and will be accelerated to 100 kV electrostatically. The results of the beam dynamics simulation will be shown in detail. The start phase space of the electron bunch at the gun exit has been approximated. The transverse focusing system consists of very short quadrupoles. Further main components of the new injector are a Wien filter, a Mott polarimeter, a chopper-prebuncher system (based on devices used at the Mainz Mikrotron MAMI), and diverse beam diagnostic tools. For the approximation of the start phase space MAFIA and for the beam dynamic simulation V-Code is used.

On some spectral properties of billiards and nuclei—similarities and differences

Abstract: Generic and non-generic features of billiards and nuclei which show up in certain spectral properties are discussed by way of selected examples. Firstly, the short- and long-range correlations of levels belonging to the magnetic dipole Scissors Mode in heavy deformed nuclei at an excitation energy of about 3 MeV prove that this mode is indeed caused by an ordered or regular collective motion. Secondly, the fine structure distribution of the so-called electric Pygmy Dipole Resonance around 6–7 MeV excitation energy seems to indicate a situation where the spectral properties are governed by mixed dynamics, i.e. by regular and chaotic features. However, in nuclei quantitative conclusions are always severely hampered by missing levels due to limited experimental resolution and detector efficiency. Thirdly, it is shown that this situation can be largely overcome by studying spectral properties in superconducting microwave billiards considered as nuclear analogues. As an example resonance strength distributions in billiards of mixed and fully chaotic dynamics are considered. Finally, it is demonstrated how symmetry breaking effects in nuclei—e.g. isospin symmetry breaking—can be studied through those resonance strength distributions by modelling the nuclear problem with coupled billiards.

Numerical simulation and optimization of a 3-ghz chopper/prebuncher system for the s-dalinac*

Abstract: A new source of polarized electrons with energy of 100 keV is presently being developed at the superconducting Darmstadt electron linear accelerator S-DALINAC for future nuclear- and radiation-physics experiments. The pulsed electron beam emitted by the photocathode will be cut to 50 ps by a chopper operated at 3 GHz, and further bunch compression down to 5 ps will be achieved by a two-stage prebuncher section. The chopper-prebuncher system is based on similar devices used at the Mainz Mikrotron (MAMI) where the accelerator frequency is slightly smaller (2.45 GHz). For the chopper, a cylindrical resonator operating at TM110 mode is selected to deflect the electron beam onto an ellipse, i.e., both horizontally and vertically. This is simply achieved by particular slits on both ends of the resonator. The prebunching system consists of two cavities. For increasing the longitudinal capture efficiency, the first cavity will be operated at the fundamental accelerator frequency of the S-DALINAC of 3 GHz, and the second cavity at 6 GHz. The cavities are designed to work at the TM010 mode and TM020 mode for the fundamental and first harmonic, respectively.

Database extension for the beam dynamics simulation tool v-code ∗

Abstract: Beam dynamics simulations for long structures can be performed effectively if the involved beam line elements (BLE) do not interfere with each other. This condition permits to concentrateona singleBLE whileswitching onlyto the immediately following without keeping all nearby parts in memory. Such a procedure automatically avoids the necessity to build up a complete field description in advance. In addition, a time consuming search for contributing elements can be excluded. Nevertheless, some devices require for a careful handling to meet those favorable properties. A typical example is given by a compact quadrupole triplet with individual magnetic fields reaching far into neighboring yokes. The beneficial assumption can still be maintained in a global sense if the whole physicaldeviceis treated as a single software unit with non-interferingfields to adjacent structures. A database which was assembled previously has to be extended according to the given requirements.

Shell-model test of the rotational-model relation between static quadrupole moments Q(2+1), B(E2)'s, and orbital M1 transitions

Abstract: In this work, we examine critically the relation between orbital magnetic dipole (scissors mode) strength and quadrupole deformation properties. Assuming a simple $K=0$ ground-state band in an even-even nucleus, the quantities $Q({2}_{1}^{+})$ (i.e., the static quadrupole moment) and $B(E2){}_{{0}_{1}\ensuremath{\rightarrow}{2}_{1}}$ both are described by a single parameter---the intrinsic quadrupole moment ${Q}_{0}$. In the shell model, we can operationally define ${Q}_{0}(\mathrm{static})$ and ${Q}_{0}(\mathit{BE}2)$ and see if they are the same. Following a brief excursion to the sd shell, we perform calculations in the fp shell. The nuclei we consider ($^{44,46,48}\mathrm{Ti}$ and $^{48,50}\mathrm{Cr}$) are far from being perfect rotors, but we find that the calculated ratios ${Q}_{0}(\mathrm{static})/{Q}_{0}(\mathit{BE}2)$ with an FPD6 interaction are often very large (very close to unity) and far from the simple vibrational limit of zero. The experimental ratios for $^{46}\mathrm{Ti}$ and $^{48}\mathrm{Ti}$ are somewhat smaller ($~0.75$), but the $^{50}\mathrm{Cr}$ value is larger, exceeding unity. We also discuss the quadrupole collectivity of orbital magnetic dipole transitions. We find that the large orbital $B(M1)$ strength in $^{44}\mathrm{Ti}$ relative to $^{46}\mathrm{Ti}$ and $^{48}\mathrm{Ti}$ cannot be explained by simple deformation arguments.

Magnetic Dipole Strength and the T = 0 Proton–Neutron Residual Interaction

Abstract: The impact of the isoscalar proton–neutron ( p n ) residual interaction on magnetic dipole strength distributions is investigated for the examples 46,48 Ti. Shell-model calculations with different interactions are performed, where either the full set of T = 0 two-body matrix elements (TBME) from the FPD6 effective interaction, a set of constant TBME, or no TBME at all are considered. The comparison with experimental data from high-resolution electron scattering reveals clear experimental signatures of the T = 0 p n interaction part.

Methods to reduce the electron beam energy spread at the s-dalinac*

Abstract: The S-DALINAC is a recirculating superconducting electron linac operating at 3 GHz. The accelerator delivers a cw beam with energies up to 130 MeV to serve electron scattering experiments where highest momentum resolutions, typ. below 10 -4 and excellent beam qualities are required. Current activities aim to reduce the energy spread of the beam of the accelerator by two methods: Long term drifts, mainly a result of temperature drifts, will be corrected by a feedback system which measures the energy variation of the extracted beam continuously using rf monitors. By means of time-of-flight analysis in a modified beamline a correction signal can be generated as a feedback for the rf control of the accelerating cavities. This system was set-up recently and first results will be reported. Furthermore, the influence of short term fluctuations, e.g. triggered by microphonics, on the electron energy can significantly be reduced utilizing the inherent stability of a microtron if the synchronous phase and longitudinal dispersion are chosen properly. The concept, particle simulations and the experimental verification will be shown as well as necessary modifications to the recirculation scheme to use it in an all-day operation.

Analysis of the surface resistance of the superconducting accelerating cavities of the S-DALINAC

Abstract: The quality factors of the 20 cell, 3 GHz niobium cavities of the S-DALINAC are smaller by almost a factor of three compared to their design value. Combined with the limited cryogenic power of the helium refrigerator in use, this restricts the maximum electron energy of the S-DALINAC in continuous wave operation. Extensive studies to elaborate the origin of this degradation are presented. A 120 ∘ C baking combined with appropriate diagnostic tools for an attempt to improve the Q values has been applied to nine of the 11 cavities. The impact of this treatment on the BCS and residual resistance was evaluated and both were improved. Additionally, quality factor measurements of all modes of the TM 010 passband allowed to derive a spatial distribution of the mean surface resistance. The RF losses in the end cells are systematically higher compared to the inner cells. The studies presented in this article allowed to develop a model describing the Q degradation of the resonators and thereby identifying possible prospects to cure this limitation.

Longitudinal beam dynamic simulation of s-dalinac polarized injector ∗

Abstract: In future, a polarized gun will extend the experimental possibilities of the superconducting recirculating linear electron accelerator S-DALINAC. Therefore a new injector is being designed where a new 100 keV polarized source SPIN will be addded to the present unpolarized thermionic source. A polarization degree of 80 %, a mean current of 60 μA and a 3 GHz cw structure are required. All features of the new source will be tested and measured at a seperate beam line. The longitudinal beam dynamics of the injector are studied. The electron bunch length behind the gun is about 50 ps. The electrons have to be bunched to 5 ps for capturing the electrons to the main linac. Therefore a chopper/prebuncher system based on the devices used at MAMI (Mainz Microton) is being designed. The system consists of a harmonic chopper cavity, a collimator, a fundamental and a first harmonic prebuncher. Recent simulation results will be presented here.

sd-pf shell model studies of M1 strength in Ar isotopes

Abstract: The M1 strength distributions in the 36,38Ar isotopes calculated in configurational space of two major N=2 and N=3 harmonic oscillator shells are discussed. It is shown that considerable breaking of 40Ca core is required to reproduce total M1 sum rule and fragmentation of the M1 strength for 38 Ar.

Nature of One‐ and Two‐Phonon Mixed Symmetry States in 92Zr and 94Mo from High‐Resolution Electron and Proton Scattering

Abstract: High‐resolution inelastic electron (performed at the S‐DALINAC) and proton (performed at iThemba LABS) scattering experiments on 92Zr and 94Mo with emphasis on E2 transitions are presented The measured form factors and angular distributions provide a measure for the F‐spin purity, respectively the isovector nature, of the proposed one‐phonon mixed symmetry states and furthermore provide a sensitive test of a possible two‐phonon character of excited 2+ states.