Efficient DMFT-simulation of the Holstein-Hubbard Model

We present a method for solving impurity models with electron-phonon coupling, which treats the phonons efficiently and without approximations. The algorithm is applied to the Holstein-Hubbard model in the dynamical mean field approximation, where it allows access to strong interactions, very low temperatures and arbitrary fillings. We show that a renormalized Migdal-Eliashberg theory provides a reasonlable description of the phonon contribution to the electronic self energy in strongly doped systems, but fails if the quasiparticle energy becomes of order of the phonon frequency.Comment: Published versio

Aufbau und Test des Protonenmikroskops PRIOR

Die Untersuchung von Materie bei hoher Energiedichte (HEDM) im Labor stellt hohe Anforderungen an die Diagnostik, da diese Zustände meist nur für kurze Zeit erzeugt werden können und herkömmliche Diagnostikmethoden mit sichtbarem Licht oder Röntgenstrahlung aufgrund der hohen Dichte an ihre Grenzen kommen. Die in den 1990er Jahren am Los Alamos National Laboratory entwickelte Technik der Hochenergie–Protonenradiographie stellt eine vielversprechende Möglichkeit dar, diese Beschränkungen zu überwinden und die Dichte von HEDM mit hoher Zeit– und Ortsauflösung zu messen. Zu diesem Zweck wurde an der GSI das Protonenmikroskop PRIOR (Proton Radiography for FAIR) gebaut, das Proben nicht nur abbildet, sondern um einen Faktor 4.2 vergrößern kann und dabei Materie mit einer Flächendichte bis zu 20 g/cm2 durchdringen kann. Dabei wurde auf Anhieb eine Ortsauflösung von unter 30 µm und eine Zeitauflösung im Nanosekundenbereich erreicht. Diese Arbeit beschreibt Details zu Funktionsweise, Design und Aufbau des Protonenmikroskops, sowie erste Messungen und Simulationen zu essentiellen Komponenten wie Magnetlinsen, Kollimator und Szintillationsschirm. Für letzteren konnte gezeigt werden, dass der Einsatz geeigneter Plastikszintillatoren als Konverter eine Alternative zu den langsameren, aber strahlungsresistenteren Kristallszintillatoren ist, so dass sich eine Zeitauflösung im Bereich von 10 ns erreichen lässt. Desweiteren wurde eine genaue Charakterisierung der Eigenschaften des Systems vorgenommen, wie sie bei den ersten Experimenten im April 2014 gemessen wurden. Auch die Änderungen der Magnetfelder durch Strahlungsschäden wurde untersucht. Es wird außerdem einen Überblick über weiterführende Anwendungsmöglichkeiten gegeben. So wurden bereits erste Experimente an wamer dichter Materie durchgeführt, die mit einem Pulsed Power–Setup erzeugt wurde. Darüber hinaus wurden vielversprechende medizinische Anwendungen bei der Kombination von Protonenradiographie mit Strahlentherapie im Rahmen des PaNTERA–Projekts untersucht. Außerdem wird ein Ausblick auf die Möglichkeiten bei zukünftigen Experimenten an der neuen Beschleunigeranlage FAIR gegeben, wo dank höherer Strahlintensität und –energie noch bessere Ergebnisse zu erwarten sind. Insbesondere wurde der Frage nachgegangen, inwieweit Experimente mit LAPLAS–artige Targets (Laboratory for Planetary Science) mit Protonenradiographie untersucht werden können. Aus diesen lassen sich mit PRIOR wichtige Erkenntnisse über die Zustandsgleichungen von Materie treffen, wie sie in großen Gasplaneten vorkommt

A FS pn-CCD detector for low background applications such as CAST

In experiments with a very low event rate, background is always a factor to be considered and hopefully to be reduced to get a clear signal. An example of such an experiment is the CAST experiment which searches for axions, possible candidates for dark matter. This thesis gives an overview on the development of a new Framestore pn-CCD detector to fit the requirements of such experiments. General material composition and the working principle of a Graded-Z shielding concept for such a detector are explained. Exemplary simulations of such a shield and first test results for the detector’s vacuum and cooling systems are presented as well

Validation of Geant4-based Radioactive Decay Simulation

Radioactive decays are of concern in a wide variety of applications using Monte-Carlo simulations. In order to properly estimate the quality of such simulations, knowledge of the accuracy of the decay simulation is required. We present a validation of the original Geant4 Radioactive Decay Module, which uses a per-decay sampling approach, and of an extended package for Geant4-based simulation of radioactive decays, which, in addition to being able to use a refactored per-decay sampling, is capable of using a statistical sampling approach. The validation is based on measurements of calibration isotope sources using a high purity Germanium (HPGe) detector; no calibration of the simulation is performed. For the considered validation experiment equivalent simulation accuracy can be achieved with per-decay and statistical sampling

Radioactive Decays in Geant4

The simulation of radioactive decays is a common task in Monte-Carlo systems such as Geant4. Usually, a system either uses an approach focusing on the simulations of every individual decay or an approach which simulates a large number of decays with a focus on correct overall statistics. The radioactive decay package presented in this work permits, for the first time, the use of both methods within the same simulation framework - Geant4. The accuracy of the statistical approach in our new package, RDM-extended, and that of the existing Geant4 per-decay implementation (original RDM), which has also been refactored, are verified against the ENSDF database. The new verified package is beneficial for a wide range of experimental scenarios, as it enables researchers to choose the most appropriate approach for their Geant4-based application

Chronic viral infection promotes sustained Th1-derived immunoregulatory IL-10 via BLIMP-1

During the course of many chronic viral infections, the antiviral T cell response becomes attenuated through a process that is regulated in part by the host. While elevated expression of the immunosuppressive cytokine IL-10 is involved in the suppression of viral-specific T cell responses, the relevant cellular sources of IL-10, as well as the pathways responsible for IL-10 induction, remain unclear. In this study, we traced IL-10 production over the course of chronic lymphocytic choriomeningitis virus (LCMV) infection in an IL-10 reporter mouse line. Using this model, we demonstrated that virus-specific T cells with reduced inflammatory function, particularly Th1 cells, display elevated and sustained IL-10 expression during chronic LCMV infection. Furthermore, ablation of IL-10 from the T cell compartment partially restored T cell function and reduced viral loads in LCMV-infected animals. We found that viral persistence is needed for sustained IL-10 production by Th1 cells and that the transcription factor BLIMP-1 is required for IL-10 expression by Th1 cells. Restimulation of Th1 cells from LCMV-infected mice promoted BLIMP-1 and subsequent IL-10 expression, suggesting that constant antigen exposure likely induces the BLIMP-1/IL-10 pathway during chronic viral infection. Together, these data indicate that effector T cells self-limit their responsiveness during persistent viral infection via an IL-10-dependent negative feedback loop.This work was supported by an Australian NHMRC Overseas Biomedical Postdoctoral Fellowship (to I.A. Parish); a Yale School of Medicine Brown-Coxe Postdoctoral Fellowship (to I.A. Parish); the Alexander von Humboldt Foundation (SKA2010, to P.A. Lang); a CIHR grant (to P.S. Ohashi); and by the Howard Hughes Medical Institute and NIH grant RO1AI074699 (to S.M. Kaech). P.S. Ohashi holds a Canada Research Chair in Autoimmunity and Tumor immunity

HST/NICMOS Paschen-alpha Survey of the Galactic Center: Overview

We have recently carried out the first wide-field hydrogen Paschen-alpha line imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we present an overview of the observations, data reduction, preliminary results, and potential scientific implications, as well as a description of the rationale and design of the survey. We have produced mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. We detect a significant number of previously undetected stars with Paschen-alpha in emission. They are most likely massive stars with strong winds, as confirmed by our initial follow-up spectroscopic observations. About half of the newly detected massive stars are found outside the known clusters (Arches, Quintuplet, and Central). Many previously known diffuse thermal features are now resolved into arrays of intriguingly fine linear filaments indicating a profound role of magnetic fields in sculpting the gas. The bright spiral-like Paschen-alpha emission around Sgr A* is seen to be well confined within the known dusty torus. In the directions roughly perpendicular to it, we further detect faint, diffuse Paschen-alpha emission features, which, like earlier radio images, suggest an outflow from the structure. In addition, we detect various compact Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars at various evolutionary stages.Comment: accepted for publication in MNRAS; a version of higher resolution images may be found at http://www.astro.umass.edu/~wqd/papers/hst/paper1.pd