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

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

The Economic Lot-Sizing Problem with an Emission Constraint

We consider a generalisation of the lot-sizing problem that includes an emission constraint. Besides the usual financial costs, there are emissions associated with production, keeping inventory and setting up the production process. Because the constraint on the emissions can be seen as a constraint on an alternative cost function, there is also a clear link with bi-objective optimisation. We show that lot-sizing with an emission constraint is NP-hard and propose several solution methods. First, we present a Lagrangian heuristic to provide a feasible solution and lower bound for the problem. For costs and emissions for which the zero inventory property is satisfied, we give a pseudo-polynomial algorithm, which can also be used to identify the complete Pareto frontier of the bi-objectiv

Economic lot-sizing with remanufacturing: complexity and efficient formulations

Within the framework of reverse logistics, the classic economic lot-sizing problem has been extended with a remanufacturing option. In this extended problem, known quantities of used products are returned from customers in each period. These returned products can be remanufactured, so that they are as good as new. Customer demand can then be fulfilled both from newly produced and remanufactured items. In each period, we can choose to set up a process to remanufacture returned products or produce new items. These processes can have separate or joint set-up costs. In this paper, we show that both variants are NP-hard. Furthermore, we propose and compare several alternative MIP formulations of both problems. Because ‘natural’ lot-sizing formulations provide weak lower bounds, we propose tighter formulations, namely shortest path formulations, a partial shortest path formulation and an adaptation of the (l, S, WW)-inequalities for the classic problem with Wagner-Whitin costs. We test their efficiency on a large number of test data sets and find that, for both problem variants, a (partial) shortest path type formulation performs better than the natural formulation, in terms of both the LP relaxation and MIP computation times. Moreover, this improvement can be substantial

Qualifikations- und Berufsfeldprojektionen bis 2030: In der Arbeitszeit steckt noch eine Menge Potenzial

Wie sich Angebot und Nachfrage auf dem Arbeitsmarkt bis zum Jahr 2030 entwickeln könnten, haben das IAB und das BIBB (Bundesinstitut für Berufsbildung) in einer gemeinsamen Projektion modelliert. Sie zeigt, wie viele Personen mit unterschiedlicher Qualifikation und in zwölf Berufshauptfeldern voraussichtlich ihre Arbeit anbieten bzw. nachgefragt werden. Um ein noch differenzierteres Zukunftsbild zu erhalten, werden diese 'Kopf'-Zahlen hier erstmals aus der 'Stunden'-Perspektive analysiert. Das führt zu einem etwas entspannteren Szenario für den künftigen Arbeitsmarkt, weil die Arbeitszeitbetrachtung weitere ungenutzte Potenziale erkennen lässt

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 “Zaanlijn”. The results show that the average delays of trains can often be reduced significantly by applying relatively small modifications to a given timetable