A parallel algorithm for the enumeration of benzenoid hydrocarbons

We present an improved parallel algorithm for the enumeration of fixed benzenoids B_h containing h hexagonal cells. We can thus extend the enumeration of B_h from the previous best h=35 up to h=50. Analysis of the associated generating function confirms to a very high degree of certainty that $B_h \sim A \kappa^h /h$ and we estimate that the growth constant $\kappa = 5.161930154(8)$ and the amplitude $A=0.2808499(1)$.Comment: 14 pages, 6 figure

The tropical shadow-vertex algorithm solves mean payoff games in polynomial time on average

We introduce an algorithm which solves mean payoff games in polynomial time on average, assuming the distribution of the games satisfies a flip invariance property on the set of actions associated with every state. The algorithm is a tropical analogue of the shadow-vertex simplex algorithm, which solves mean payoff games via linear feasibility problems over the tropical semiring $(\mathbb{R} \cup \{-\infty\}, \max, +)$. The key ingredient in our approach is that the shadow-vertex pivoting rule can be transferred to tropical polyhedra, and that its computation reduces to optimal assignment problems through Pl\"ucker relations.Comment: 17 pages, 7 figures, appears in 41st International Colloquium, ICALP 2014, Copenhagen, Denmark, July 8-11, 2014, Proceedings, Part

Progress in the Determination of 227Ac in Sea Water

227Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of 227Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of 227 Ac with MnO2, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial 225Ac isotope is counted, then after N100 days five daughters of 227Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15±5%, the chemical yield on average about 50%. The counting of five decay products of 227Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial 227Ac activity can be determined to better than 10% relative error for concentrations b10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of 227Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method

Die Anwendung natürlicher Radionuklide auf marine Fragestellungen in den Polargebieten

Die Polargebiete stellen Schlüsselgebiete im heutigen globalen Klimageschehen dar und bergen wertvolle Informationen über die Klimaentwicklung der jüngeren geologischen Vergangenheit (z.B. Sedimentkerne). Wichtige Fragen bezüglich des Zirkulationsgeschehens oder der Primärproduktion in den polaren Breiten werden jedoch nur ansatzweise verstanden.Die drei natürlich vorkommenden Zerfallsreihen (238U-206Pb, 235U-207Pb, und 232Th-208Pb) liefern mit ihren radioaktiven Zwischenprodukten wichtige Tracer für Prozeßstudien im marinen Milieu. Gemäß ihres geochemischen Verhaltens im Meerwasser lassen sich eher partikelreaktive (z.B. Thorium und Protactinium) und eher lösliche Isotope (z.B. Uran, Radium, Radon, Actinium) unterscheiden, die entsprechend im Sediment bzw. in der Wassersäule akkumulieren. Aktivitätsungleichgewichte innerhalb eines Mutter-Tochter-Paares einer Zerfallsreihe erlauben Rückschlüsse auf die zugrundeliegenden Prozesse wie z.B. laterale oder vertikale Mischung, Auftrieb von Tiefenwasser, Grundwassereinträge, Bestimmung der Exportproduktion aus der Deckschicht oder Partikelfluß ins Sediment. Die unterschiedlichen Halbwertszeiten (Tage bis Millionen Jahre) der Zerfallsprodukte ermöglichen die Untersuchung von Prozessen auf unterschiedlichen Zeitskalen.Anhand der Isotope 227Ac, 228Ra, 222Rn, 234Th und 230Th sollen mit Beipielen aus der europäischen Arktis und dem Atlantischen Sektor des Südozeans die vielfältigen Einsatzmöglichkeiten von natürlich vorkommenden Radionukliden für marine Fragestellungen illustriert werden

Radionuclides as tracers for particle flux and transport of water masses in the Atlantic sector of the Southern Ocean

The natural uranium decay series provide a suite of tracers to study transport processes in the ocean. We have used nuclides of the particle-reactive elements Th, Pa, Pb and Po for studies of particle flux in the Southern Ocean, whereas isotopes of the elements Ra and Ac served as tracers for the transport of water masses. Here we summarize the specific aspects of the behaviour of these nuclides in the Southern Ocean and give some examples of their application. We review the important influence of exchange between ocean basins by advection and upwelling on the long-lived nuclides. We show how the distribution of 234Th in surface waters across the ACC represents the export production, whereas in the benthic nepheloid layer this tracer is used to illustrate how the resuspension regime in the ACC is linked to the position of the oceanographic fronts

The influence of particle composition on Thorium scavenging in the NE Atlantic ocean (POMME experiment)

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