362 research outputs found
The Reaction 7Li(pi+,pi-)7B and its Implications for 7B
The reaction 7Li(pi+,pi-)7B has been measured at incident pion energies of
30-90 MeV. 7Li constitutes the lightest target nucleus, where the pionic charge
exchange may proceed as a binary reaction to a discrete final state. Like in
the Delta-resonance region the observed cross sections are much smaller than
expected from the systematics found for heavier nuclei. In analogy to the
neutron halo case of 11Li this cross section suppression is interpreted as
evidence for a proton halo in the particle-unstable nucleus 7B.Comment: 4 pages, 4 figure
Search for Narrow NNpi Resonances in Exclusive p p -> p p pi+ pi- Measurements
Narrow structures in the range of a few MeV have been searched for in p p pi+
and p p pi- invariant mass spectra obtained from exclusive measurements of the
p p -> p p pi+ pi- reaction at Tp = 725, 750 and 775 MeV using the PROMICE/WASA
detector at CELSIUS. The selected reaction is particularily well suited for the
search for NN and / or N Delta decoupled dibaryon resonances. Except for a
possible fluctuation at 2087 MeV/c^2 in Mpppi- no narrow structures could be
identified neither in Mpppi+ nor in Mpppi- on the 3 sigma level of statistical
significance, giving an upper limit (95% C.L.) for dibaryon production in this
reaction of sigma < 20 nb for 2020 MeV/c^2 < m(dibaryon) < 2085 MeV/c^2Comment: 3 pages, 4 figure
Collision damping in the pi 3He -> d'N reaction near the threshold
We present a simple quantum mechanical model exploiting the optical potential
approach for the description of collision damping in the reaction pi 3He -> d'N
near the threshold, which recently has been measured at TRIUMF. The influence
of the open d'N -> NNN channel is taken into account. It leads to a suppression
factor of about ten in the d' survival probability. Applications of the method
to other reactions are outlined.Comment: RevTeX4, 14 pages, 3 Postscript figures, uses epsfig.sty, to appear
in Phys.Rev.
Piezo-Polymer-Composite Unimorph Actuators for Active Cancellation of Flow Instabilities Across Airfoils
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.This article presents a smart device for active cancellation of flow instabilities. An array of two piezo unimorph actuators fabricated in piezo-polymer-composite technology is combined with a thin silicone membrane to mimic a movable wall with a closed surface. By locally displacing the thin membrane, a surface wave is generated that interferes with naturally occurring flow instabilities within the boundary layer of an airfoil. Using flow sensors and an intelligent control enables a destructive interference and therefore, an attenuation of natural flow instabilities. This leads to a delay of transition. The boundary layer remains laminar which means drag is reduced. Within the next pages, the setup of the device with actuators, membrane, sensors, and control is introduced. The main focus of this article is on actuator design, modeling, and implementation for wind tunnel experiments. Results of actuator characterization are presented. The non-linear behavior of the piezoactuator (harmonic distortions and impact of high electric fields) is investigated in detail. This study concludes with the results obtained in wind tunnel experiments which prove the functionality of the presented approach. A maximal attenuation of natural occurring flow instabilities of 80% is achieved.DFG, SPP 1207, Strömungsbeeinflussung in der Natur und Techni
User Grouping and Power Allocation in NOMA Systems : A Reinforcement Learning-Based Solution
Author's accepted manuscript.Available from 05/09/2021.acceptedVersio
Bodenmüdigkeit und Stickstoffdynamik im Wurzelraum von Apfel im Erwerbsanbau
Die Nachbaukrankheit im Apfelanbau (Apple Replant Disease, ARD) besitzt eine hohe ökologische und ökonomische Relevanz, da der Nachbau oft über einen Zeitraum von 30 Jahren betroffen ist. Aus diesem Grund wird im Rahmen des BonaRes-Programms der Ordiamur-Verbund zu diesem Thema gefördert. Die Betrachtung von Nährstoffkreisläufen im Boden, insbesondere der Prozesse in der Rhizosphäre, ist zum Verständnis und zur Überwindung dieser Krankheit unabdingbar. Wir betrachten hier den N-Kreislauf in einem Topfexperiment mit 15N-Düngerapplikation, sowie auf der Pflanze-Bodenskala in einer Obstplantage (Meckenheim, Deutschland). Das 15N-Düngeexperiment wurde für ARD-Böden, jungfräuliche und behandelte Böden durchgeführt. Es gibt erste Hinweise darauf, dass Stickstoff im Gesamtbodenmaterial von ARD-Böden angereichert und immobilisiert wird. In der mikroskaligen Betrachtung (Gradienten von der Wurzel in den Boden) konnten wir erste Effekte von ARD-Behandlung auf den C- und N-Kreislauf durch Veränderung der Stabilisotopenverhältnisse beobachten. In Zukunft wird der Grad der ARD und der veränderten C- und N-Kreisläufe mit räumlichen Bodeneigenschaftenkarten korreliert.
Hinweis der Autoren: Die Zuordnung zu einem Thema ist variabel- ggf. passt auch Kommission IV; K IV u. K I. Zu begrüßen wäre die Einordnung gemeinsam mit anderen ORDIAMUR-Projektpartnern (AG Vetterlein, AG Guggenberger
Modelling of amorphous polymer surfaces in computer simulation
We study surface effects in amorphous polymer systems by means of computer
simulation. In the framework of molecular dynamics, we present two different
methods to prepare such surfaces. {\em Free} surfaces are stabilized solely by
van--der--Waals interactions whereas {\em confined} surfaces emerge in the
presence of repelling plates. The two models are compared in various computer
simulations. For free surfaces, we analyze the migration of end--monomers to
the surface. The buildup of density and pressure profiles from zero to their
bulk values depends on the surface preparation method. In the case of confined
surfaces, we find density and pressure oszillations next to the repelling
plates. We investigate the influence of surfaces on the coordination number, on
the orientation of single bonds, and on polymer end--to--end vectors.
Furthermore, different statistical methods to determine location and width of
the surface region for systems of various chain lengths are discussed and
applied. We introduce a ``height function'' and show that this method allows to
determine average surface profiles only by scanning the outermost layer of
monomers.Comment: 23 pages, 12 figure
Nonequilibrium dynamics: a renormalized computation scheme
We present a regularized and renormalized version of the one-loop nonlinear
relaxation equations that determine the non-equilibrium time evolution of a
classical (constant) field coupled to its quantum fluctuations. We obtain a
computational method in which the evaluation of divergent fluctuation integrals
and the evaluation of the exact finite parts are cleanly separated so as to
allow for a wide freedom in the choice of regularization and renormalization
schemes. We use dimensional regularization here. Within the same formalism we
analyze also the regularization and renormalization of the energy-momentum
tensor. The energy density serves to monitor the reliability of our numerical
computation. The method is applied to the simple case of a scalar phi^4 theory;
the results are similar to the ones found previously by other groups.Comment: 15 pages, 9 postscript figures, revtex; version published in Phys.
Rev, with minor corrections; improves the first version of 1996 by including
the discussion of energy momentum tenso
Nonequilibrium dynamics: preheating in the SU(2) Higgs model
The term `preheating' has been introduced recently to denote the process in
which energy is transferred from a classical inflaton field into fluctuating
field (particle) degrees of freedom without generating yet a real thermal
ensemble. The models considered up to now include, besides the inflaton field,
scalar or fermionic fluctuations. On the other hand the typical ingredient of
an inflationary scenario is a nonabelian spontaneously broken gauge theory. So
the formalism should also be developed to include gauge field fluctuations
excited by the inflaton or Higgs field. We have chosen here, as the simplest
nonabelian example, the SU(2) Higgs model. We consider the model at temperature
zero. From the technical point of view we generalize an analytical and
numerical renormalized formalism developed by us recently to coupled channnel
systems. We use the 't Hooft-Feynman gauge and dimensional regularization. We
present some numerical results but reserve a more exhaustive discussion of
solutions within the paramter space of two couplings and the initial value of
the Higgs field to a future publication.Comment: 30 pages, 10 figures in enhanced postscript, 2 unreadable figures
made accessibl
Study of the p p -> p p pi+ pi- Reaction in the Low-Energy Tail of the Roper Resonance
Exclusive measurements of the p p -> p p pi+ pi- reaction have been carried
out at Tp = 775 MeV at CELSIUS using the PROMICE/WASA setup. Together with data
obtained at lower energy they point to a dominance of the Roper excitation in
this process. From the observed interference of its decay routes N* -> N sigma
and N* -> Delta pi -> N sigma their energy-dependent relative branching ratio
is determined
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