327 research outputs found
Magnetic dipole excitations in nuclei: elementary modes of nucleonic motion
The nucleus is one of the most multi-faceted many-body systems in the
universe. It exhibits a multitude of responses depending on the way one
'probes' it. With increasing technical advancements of beams at the various
accelerators and of detection systems the nucleus has, over and over again,
surprised us by expressing always new ways of 'organized' structures and layers
of complexity. Nuclear magnetism is one of those fascinating faces of the
atomic nucleus we discuss in the present review. We shall not just limit
ourselves to presenting the by now very large data set that has been obtained
in the last two decades using various probes, electromagnetic and hadronic
alike and that presents ample evidence for a low-lying orbital scissors mode
around 3 MeV, albeit fragmented over an energy interval of the order of 1.5
MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in
deformed nuclei, nor to the presently discovered evidence for low-lying
proton-neutron isovector quadrupole excitations in spherical nuclei. To the
contrary, we put the experimental evidence in the perspectives of understanding
the atomic nucleus and its various structures of well-organized modes of motion
and thus enlarge our discussion to more general fermion and bosonic many-body
systems.Comment: 59 pages, 59 figures, accepted for publication in Rev. Mod. Phys
Avoided level crossing statistics in open chaotic billiards
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 of 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 (TRS). 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 \textit{et al.} (Phys. Rev. E {\bf 73} (2006) 035201)
Gross, intermediate and fine structure of nuclear giant resonances: Evidence for doorway states
We review the phenomenon of fine structure of nuclear giant resonances and
its relation to different resonance decay mechanisms. Wavelet analysis of the
experimental spectra provides quantitative information on the fine structure in
terms of characteristic scales. A comparable analysis of resonance strength
distributions from microscopic approaches incorporating one or several of the
resonance decay mechanisms allows conclusions on the source of the fine
structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading
through the first step of the doorway mechanism, i.e.\ coupling between one
particle-one hole () and two particle-two hole () states is
identified as the relevant mechanism. In heavy nuclei it is dominated by
coupling to low-lying surface vibrations, while in lighter nuclei stochastic
coupling becomes increasingly important. The fine structure observed for the
isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation
of the strength (Landau damping), although some indications for the
relevance of the spreading width are also found.Comment: 15 pages, 16 figures. Submitted to Eur. Phys. J A, special issue
"Giant, pygmy, pairing resonances and related topics
Distribution of Scattering Matrix Elements in Quantum Chaotic Scattering
Scattering is an important phenomenon which is observed in systems ranging
from the micro- to macroscale. In the context of nuclear reaction theory the
Heidelberg approach was proposed and later demonstrated to be applicable to
many chaotic scattering systems. To model the universal properties,
stochasticity is introduced to the scattering matrix on the level of the
Hamiltonian by using random matrices. A long-standing problem was the
computation of the distribution of the off-diagonal scattering-matrix elements.
We report here an exact solution to this problem and present analytical results
for systems with preserved and with violated time-reversal invariance. Our
derivation is based on a new variant of the supersymmetry method. We also
validate our results with scattering data obtained from experiments with
microwave billiards.Comment: Published versio
Towards World of Warcraft as an Experiment Platform for Teams
We are interested in how virtual, synchronous teams organize to cope with tasks of different complexity. We follow an explorative approach to validate World of Warcraft as experiment platform for virtual teams. We explore which parts of the game are suitable for experiments, which phenomena can be studied in teams fighting in World of Warcraft and how data can be collected. We prototypically evaluate data from games to demonstrate the validity of our approach
Long-Term Stability of Polymer-Coated Surface Transverse Wave Sensors for the Detection of Organic Solvent Vapors
Arrays with polymer-coated acoustic sensors, such as surface acoustic wave (SAW) and surface transverse wave (STW) sensors, have successfully been applied for a variety of gas sensing applications. However, the stability of the sensors’ polymer coatings over a longer period of use has hardly been investigated. We used an array of eight STW resonator sensors coated with different polymers. This sensor array was used at semi-annual intervals for a three-year period to detect organic solvent vapors of three different chemical classes: a halogenated hydrocarbon (chloroform), an aliphatic hydrocarbon (octane), and an aromatic hydrocarbon (xylene). The sensor signals were evaluated with regard to absolute signal shifts and normalized signal shifts leading to signal patterns characteristic of the respective solvent vapors. No significant time-related changes of sensor signals or signal patterns were observed, i.e., the polymer coatings kept their performance during the course of the study. Therefore, the polymer-coated STW sensors proved to be robust devices which can be used for detecting organic solvent vapors both qualitatively and quantitatively for several years
Pair decay width of the Hoyle state and carbon production in stars
The pair decay width of the first excited 0⁺ state in ¹²C (the Hoyle state) is deduced from a novel analysis of the world data on inelastic electron scattering covering a wide momentum transfer range, thereby resolving previous discrepancies. The extracted value Γπ = (62.3 ± 2.0) μeV is independently confirmed by new data at low momentum transfers measured at the S-DALINAC and reduces the uncertainty of the literature values by more than a factor of three. A precise knowledge of Γπ is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process
Die Strömungsverhältnisse vor der Westküste Schleswig-Holsteins - Ergebnisse eines KFKI-Messprogramms
CRETA (Centrum fürreflektierte Textanalyse)– FachübergreifendeMethodenentwicklung in denDigital Humanities
This paper will present the concept of the newly established Stuttgart DH Center CRETA, which unites very different text-oriented disciplines such as literature, linguistics, history, political science, and philosophy, and which, on the other hand, not only applies methods and modeling techniques from machine learning, computational linguistics, and computer graphic visualization, but has begun to integrate them into a common DH methodology of deep reflective text analysis. Such a further development of the method inventory of the Digital Humanities is a long way and needs many participants. However, we can already illustrate aspects of the conception with case studies of scenarios from ongoing digital humanities projects, and it seems important to us to put the approach up for broad discussion
- …