130 research outputs found
Fluctuation properties of strength functions associated with giant resonances
We performed fluctuation analysis by means of the local scaling dimension for
the strength function of the isoscalar (IS) and the isovector (IV) giant
quadrupole resonances (GQR) in Ca, where the strength functions are
obtained by the shell model calculation within up to the 2p2h configurations.
It is found that at small energy scale, fluctuation of the strength function
almost obeys the Gaussian orthogonal ensemble (GOE) random matrix theory limit.
On the other hand, we found a deviation from the GOE limit at the intermediate
energy scale about 1.7MeV for the IS and at 0.9MeV for the IV. The results
imply that different types of fluctuations coexist at different energy scales.
Detailed analysis strongly suggests that GOE fluctuation at small energy scale
is due to the complicated nature of 2p2h states and that fluctuation at the
intermediate energy scale is associated with the spreading width of the
Tamm-Dancoff 1p1h states.Comment: 14 pages including 13figure
Electron recombination with multicharged ions via chaotic many-electron states
We show that a dense spectrum of chaotic multiply-excited eigenstates can
play a major role in collision processes involving many-electron multicharged
ions. A statistical theory based on chaotic properties of the eigenstates
enables one to obtain relevant energy-averaged cross sections in terms of sums
over single-electron orbitals. Our calculation of the low-energy electron
recombination of Au shows that the resonant process is 200 times more
intense than direct radiative recombination, which explains the recent
experimental results of Hoffknecht {\em et al.} [J. Phys. B {\bf 31}, 2415
(1998)].Comment: 9 pages, including 1 figure, REVTe
Scaling Analysis of Fluctuating Strength Function
We propose a new method to analyze fluctuations in the strength function
phenomena in highly excited nuclei. Extending the method of multifractal
analysis to the cases where the strength fluctuations do not obey power scaling
laws, we introduce a new measure of fluctuation, called the local scaling
dimension, which characterizes scaling behavior of the strength fluctuation as
a function of energy bin width subdividing the strength function. We discuss
properties of the new measure by applying it to a model system which simulates
the doorway damping mechanism of giant resonances. It is found that the local
scaling dimension characterizes well fluctuations and their energy scales of
fine structures in the strength function associated with the damped collective
motions.Comment: 22 pages with 9 figures; submitted to Phys. Rev.
Dielectronic Recombination of Ground-State and Metastable Li+ Ions
Dielectronic recombination has been investigated for Delta-n = 1 resonances
of ground-state Li+(1s^2) and for Delta-n = 0 resonances of metastable Li+(1s2s
^3S). The ground-state spectrum shows three prominent transitions between 53
and 64 eV, while the metastable spectrum exhibits many transitions with
energies < 3.2 eV. Reasonably good agreement of R-matrix, LS coupling
calculations with the measured recombination rate coefficient is obtained. The
time dependence of the recombination rate yields a radiative lifetime of 52.2
+- 5.0 s for the 2 ^3S level of Li+.Comment: Submitted to Phys. Rev. A; REVTeX, 4 pages, 3 figure
Interference effects in the photorecombination of argonlike Sc3+ ions: Storage-ring experiment and theory
Absolute total electron-ion recombination rate coefficients of argonlike
Sc3+(3s2 3p6) ions have been measured for relative energies between electrons
and ions ranging from 0 to 45 eV. This energy range comprises all dielectronic
recombination resonances attached to 3p -> 3d and 3p -> 4s excitations. A broad
resonance with an experimental width of 0.89 +- 0.07 eV due to the 3p5 3d2 2F
intermediate state is found at 12.31 +- 0.03 eV with a small experimental
evidence for an asymmetric line shape. From R-Matrix and perturbative
calculations we infer that the asymmetric line shape may not only be due to
quantum mechanical interference between direct and resonant recombination
channels as predicted by Gorczyca et al. [Phys. Rev. A 56, 4742 (1997)], but
may partly also be due to the interaction with an adjacent overlapping DR
resonance of the same symmetry. The overall agreement between theory and
experiment is poor. Differences between our experimental and our theoretical
resonance positions are as large as 1.4 eV. This illustrates the difficulty to
accurately describe the structure of an atomic system with an open 3d-shell
with state-of-the-art theoretical methods. Furthermore, we find that a
relativistic theoretical treatment of the system under study is mandatory since
the existence of experimentally observed strong 3p5 3d2 2D and 3p5 3d 4s 2D
resonances can only be explained when calculations beyond LS-coupling are
carried out.Comment: 11 pages, 7 figures, 3 tables, Phys. Rev. A (in print), see also:
http://www.strz.uni-giessen.de/~k
Multiscale fluctuations in nuclear response
The nuclear collective response is investigated in the framework of a doorway
picture in which the spreading width of the collective motion is described as a
coupling to more and more complex configurations. It is shown that this
coupling induces fluctuations of the observed strength. In the case of a
hierarchy of overlapping decay channels, we observe Ericson fluctuations at
different scales. Methods for extracting these scales and the related lifetimes
are discussed. Finally, we show that the coupling of different states at one
level of complexity to some common decay channels at the next level, may
produce interference-like patterns in the nuclear response. This quantum effect
leads to a new type of fluctuations with a typical width related to the level
spacing.Comment: 34 Latex pages including 6 figures (submitted to Phys. Rev. C
Super-Radiant Dynamics, Doorways, and Resonances in Nuclei and Other Open Mesoscopic Systems
The phenomenon of super-radiance (Dicke effect, coherent spontaneous
radiation by a gas of atoms coupled through the common radiation field) is well
known in quantum optics. The review discusses similar physics that emerges in
open and marginally stable quantum many-body systems. In the presence of open
decay channels, the intrinsic states are coupled through the continuum. At
sufficiently strong continuum coupling, the spectrum of resonances undergoes
the restructuring with segregation of very broad super-radiant states and
trapping of remaining long-lived compound states. The appropriate formalism
describing this phenomenon is based on the Feshbach projection method and
effective non-Hermitian Hamiltonian. A broader generalization is related to the
idea of doorway states connecting quantum states of different structure. The
method is explained in detail and the examples of applications are given to
nuclear, atomic and particle physics. The interrelation of the collective
dynamics through continuum and possible intrinsic many-body chaos is studied,
including universal mesoscopic conductance fluctuations. The theory serves as a
natural framework for general description of a quantum signal transmission
through an open mesoscopic system.Comment: 85 pages, 10 figure
Comparative validation of single-shot optical techniques for laparoscopic 3-D surface reconstruction
Intra-operative imaging techniques for obtaining the shape and morphology of soft-tissue surfaces in vivo are a key enabling technology for advanced surgical systems. Different optical techniques for 3-D surface reconstruction in laparoscopy have been proposed, however, so far no quantitative and comparative validation has been performed. Furthermore, robustness of the methods to clinically important factors like smoke or bleeding has not yet been assessed. To address these issues, we have formed a joint international initiative with the aim of validating different state-of-the-art passive and active reconstruction methods in a comparative manner. In this comprehensive in vitro study, we investigated reconstruction accuracy using different organs with various shape and texture and also tested reconstruction robustness with respect to a number of factors like the pose of the endoscope as well as the amount of blood or smoke present in the scene. The study suggests complementary advantages of the different techniques with respect to accuracy, robustness, point density, hardware complexity and computation time. While reconstruction accuracy under ideal conditions was generally high, robustness is a remaining issue to be addressed. Future work should include sensor fusion and in vivo validation studies in a specific clinical context. To trigger further research in surface reconstruction, stereoscopic data of the study will be made publically available at www.open-CAS.com upon publication of the paper
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