Coupling highly excited nuclei to the atomic shell in dense astrophysical plasmas

In dense astrophysical plasmas, neutron capture populates highly excited nuclear states close to the neutron threshold. The impact of additional low-energy nuclear excitations via coupling to the atomic shell on the ability of the so-formed compound nucleus to retain the captured neutron is investigated. We focus on the mechanism of nuclear excitation by electron capture in plasmas characterized by electron fluxes typical for the slow neutron capture process of stellar nucleosynthesis. The small effect of this further excitation on the neutron capture and gamma decay sequence relevant for nucleosynthesis is quantified and compared to the corresponding effect of an additional low-energy photoexcitation step.Comment: 11 pages, 10 figures; v2 minor modifications to match the published version, results unchange

Nuclear Excitation by Electron Capture in Stellar Environments

In the resonant process of nuclear excitation by electron capture (NEEC), a free electron recombines into the atomic shell of an ion with the simultaneous excitation of the nucleus. This process is expected to be efficient in populating excited nuclear states in stellar plasmas of high electron density. In this work, we study the possible interplay of NEEC with nuclear excitation by neutron capture as well as associated gamma-decay and neutron emission in the context of nucleosynthesis. Neutron capture followed by beta-decay of the thus formed neutron-rich daughter isotopes constitutes the basic reaction leading to the synthesis of heavy isotopes. For the first time the impact of NEEC taking place prior to the decay of the high-energy state formed by neutron capture is investigated. We show that an additional nuclear excitation of the order of magnitude of 10 keV can cause substantial changes to the net decay rates of the excited nucleus making neutron re-emission predominant. As a consequence, the production of the daughter isotopes would be signi_cantly damped. This first estimate motivates further studies on the impact of NEEC on neutron capture nucleosynthesis

VEGF-expressing mesenchymal stem cells for improved angiogenesis in regenerative medicine : a bone tissue engineering approach

Rapid vascularization of tissue-engineered grafts is a major bottleneck in the development of regenerative medicine approaches. In order to overcome this limitation, we aimed to develop a bone tissue engineering strategy combining cell therapy with pro-angiogenic gene therapy. Vascular Endothelial Growth Factor (VEGF) is the master regulator of physiological vascular growth and is commonly used as a therapeutic transgene for the induction of angiogenesis. However, uncontrolled and high levels of VEGF expression can lead to aberrant vascular growth. To achieve controlled expression in vivo, a high-throughput flow cytometry-based method has previously been developed in our group. Linking the VEGF cDNA to a cell-surface marker (a truncated version of CD8a) in a bicistronic construct enabled the rapid purification of genetically modified myoblasts secreting a desired VEGF level, using FACS sorting based on the intensity of CD8 expression in each cell. Controlled VEGF expression in skeletal muscle, achieved by implantation of these FACS-purified myoblast populations, induced only normal, stable and functional vascular networks and avoided any aberrant angiogenesis. The aims of this thesis were to adapt this method to human adipose tissue- and bone marrow-derived mesenchymal stromal/stem cells (ASC and BMSC), and to apply these in a bone tissue engineering approach to increase the vascularization potential of osteogenic grafts. As MSC gradually loose their regenerative potential during in vitro expansion, we first optimized our genetic engineering method for MSC, so as to enable high transduction efficiency and FACS-purification with minimal in vitro manipulation. Chapter 2 describes the generation of an optimized protocol allowing routine transduction efficiencies of > 90% of primary human ASC and BMSC already during the first plating, as well as flow cytometry purification of transduced cells at the time of the first passage. In addition we demonstrated that it was possible to FACS-purify specific sub-populations of transduced MSC homogeneously producing desired VEGF doses. Neither retroviral vector transduction, FACS-purification, nor the expression of the transgenes VEGF and CD8 impaired MSC proliferation and in vitro differentiation potential. Transgene expression was not lost during in vitro differentiation. In Chapter 3, proof-of-principle was obtained by applying this platform to a bone tissue engineering approach. Human BMSC, transduced and rapidly FACS-purified to eliminate non-expressing cells, were seeded onto hydroxyapatite granules to generate non-critically sized constructs, and were implanted subcutaneously in nude rats. In vivo vascularization potential was significantly increased in VEGF-expressing BMSC. Although VEGF expression was heterogeneous, no aberrant angiogenesis was observed. Indeed, orderly vascular beds were induced, with flow-conducting arterioles feeding into extensive capillary networks, where metabolic exchanges can take place efficiently. The improvement in vascularization was not diminished by extensive in vitro expansion of the transduced BMSC up to 35 population doublings, showing that genetic modification conferred a stable angiogenic potential. As expected, these expanded BMSC lost their osteogenic potential. However, their sustained capacity to induce vascularization could be useful in other applications, where effective expansion of the vascular bed is required, but not progenitor differentiation, such as in cell-based approaches for therapeutic angiogenesis in peripheral or coronary artery diseases. By minimizing cell expansion, both naïve and control transduced MSC generated abundant bone tissue in vivo. However, VEGF over-expression specifically caused a strong reduction in bone formation. This correlated with an increased recruitment of TRAP-positive osteoclasts specifically in VEGF-expressing constructs. These data suggest that VEGF over-expression might impair bone formation by disrupting the balance between bone formation and resorption towards excessive degradation. To fully understand the underlying mechanism, further experiments will be needed. The method described in chapter 2 provides a general platform to generate populations of genetically modified MSC, expressing specific levels of a therapeutic transgene, already at the time of the first passage. Therefore, it has the potential to be applied in other fields of regenerative medicine, beyond bone tissue engineering. We briefly describe two recently initiated projects, based on the results described in this thesis, which aim at either promoting or inhibiting angiogenesis in order to improve cardiac function after myocardial infarction, or cartilage tissue formation, respectively

Comparing Anomaly-Based Network Intrusion Detection Approaches Under Practical Aspects

While many of the currently used network intrusion detection systems (NIDS) employ signature-based approaches, there is an increasing research interest in the examination of anomaly-based detection methods, which seem to be more suited for recognizing zero-day attacks. Nevertheless, requirements for their practical deployment, as well as objective and reproducible evaluation methods, are hereby often neglected. The following thesis defines aspects that are crucial for a practical evaluation of anomaly-based NIDS, such as the focus on modern attack types, the restriction to one-class classification methods, the exclusion of known attacks from the training phase, a low false detection rate, and consideration of the runtime efficiency. Based on those principles, a framework dedicated to developing, testing and evaluating models for the detection of network anomalies is proposed. It is applied to two datasets featuring modern traffic, namely the UNSW-NB15 and the CIC-IDS-2017 datasets, in order to compare and evaluate commonly-used network intrusion detection methods. The implemented approaches include, among others, a highly configurable network flow generator, a payload analyser, a one-hot encoder, a one-class support vector machine, and an autoencoder. The results show a significant difference between the two chosen datasets: While for the UNSW-NB15 dataset several reasonably well performing model combinations for both the autoencoder and the one-class SVM can be found, most of them yield unsatisfying results when the CIC-IDS-2017 dataset is used.Obwohl viele der derzeit genutzten Systeme zur Erkennung von Netzwerkangriffen (engl. NIDS) signaturbasierte Ansätze verwenden, gibt es ein wachsendes Forschungsinteresse an der Untersuchung von anomaliebasierten Erkennungsmethoden, welche zur Identifikation von Zero-Day-Angriffen geeigneter erscheinen. Gleichwohl werden hierbei Bedingungen für deren praktischen Einsatz oft vernachlässigt, ebenso wie objektive und reproduzierbare Evaluationsmethoden. Die folgende Arbeit definiert Aspekte, die für eine praxisorientierte Evaluation unabdingbar sind. Dazu zählen ein Schwerpunkt auf modernen Angriffstypen, die Beschränkung auf One-Class Classification Methoden, der Ausschluss von bereits bekannten Angriffen aus dem Trainingsdatensatz, niedrige Falscherkennungsraten sowie die Berücksichtigung der Laufzeiteffizienz. Basierend auf diesen Prinzipien wird ein Rahmenkonzept vorgeschlagen, das für das Entwickeln, Testen und Evaluieren von Modellen zur Erkennung von Netzwerkanomalien bestimmt ist. Dieses wird auf zwei Datensätze mit modernem Netzwerkverkehr, namentlich auf den UNSW-NB15 und den CIC-IDS- 2017 Datensatz, angewendet, um häufig genutzte NIDS-Methoden zu vergleichen und zu evaluieren. Die für diese Arbeit implementierten Ansätze beinhalten, neben anderen, einen weit konfigurierbaren Netzwerkflussgenerator, einen Nutzdatenanalysierer, einen One-Hot-Encoder, eine One-Class Support Vector Machine sowie einen Autoencoder. Die Resultate zeigen einen großen Unterschied zwischen den beiden ausgewählten Datensätzen: Während für den UNSW-NB15 Datensatz verschiedene angemessen gut funktionierende Modellkombinationen, sowohl für den Autoencoder als auch für die One-Class SVM, gefunden werden können, bringen diese für den CIC-IDS-2017 Datensatz meist unbefriedigende Ergebnisse

Stochastic Improvement of Cyclic Railway Timetables

Real-time railway operations are subject to stochastic disturbances. However, a railway timetable is a deterministic plan. Thus a timetable should be designed in such a way that it can cope with the stochastic disturbances as well as possible. For that purpose, a timetable usually contains time supplements in several process times and buffer times between pairs of consecutive trains. This paper describes a Stochastic Optimization Model that can be used to allocate the time supplements and the buffer times in a given timetable in such a way that the timetable becomes maximally robust against stochastic disturbances. The Stochastic Optimization Model was tested on several instances of NS Reizigers, the main operator of passenger trains in the Netherlands. Moreover, a timetable that was computed by the model was operated in practice in a timetable experiment on the so-called “Zaanlijnâ€. The results show that the average delays of trains can often be reduced significantly by applying relatively small modifications to a given timetable.Railway Timetabling;Stochastic Optimization;Robustness

Rule-based detection of access to education and training in Germany

As a result of transformation processes, the German labor market is highly dependent on vocational training, retraining and continuing education. To match training seekers and offers, we present a novel approach towards the automated detection of access to education and training in German training offers and advertisements. We will in particular focus on (a) general school and education degrees and schoolleaving certificates, (b) professional experience, (c) a previous apprenticeship and (d) a list of skills provided by the German Federal Employment Agency. This novel approach combines several methods: First, we provide a mapping of synonyms in education combining different qualifications and adding deprecated terms. Second, we provide a rule-based matching to identify the need for professional experience or apprenticeship. However, not all access requirements can be matched due to incompatible data schemata or non-standardizes requirements, e.g initial tests or interviews. While we can identify several shortcomings, the presented approach offers promising results for two data sets: training and re-training advertisements