Dissociation of O2 at Al(111): The Role of Spin Selection Rules

A most basic and puzzling enigma in surface science is the description of the dissociative adsorption of O2 at the (111) surface of Al. Already for the sticking curve alone, the disagreement between experiment and results of state-of-the-art first-principles calculations can hardly be more dramatic. In this paper we show that this is caused by hitherto unaccounted spin selection rules, which give rise to a highly non-adiabatic behavior in the O2/Al(111) interaction. We also discuss problems caused by the insufficient accuracy of present-day exchange-correlation functionals.Comment: 4 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Validation of the burnup code MOTIVE with respect to fuel assembly decay heat data

The burn-up code MOTIVE is a 3D code for fuel assembly inventory determination developed at GRS in recent years. It modularly couples an external Monte Carlo neutron transport code to the in-house inventory code VENTINA. In the present publication, we report on the validation of MOTIVE with respect to full-assembly decay heat measurements of light water reactor fuel. For this purpose, measurements on pressurized water reactor and boiling water reactor fuel assemblies from different facilities have been analyzed with MOTIVE. The calculated decay heat values are compared to the measured data in terms of absolute and relative deviations. These results are discussed and compared to other published validation analyses. Moreover, the observed deviations between measurements and calculations are analyzed further by taking into account the results of the validation of nuclide inventory determination with MOTIVE. The influence of possible biases of calculated nuclide densities important to decay heat at the given decay times are investigated and discussed

Non-Adiabatic Potential-Energy Surfaces by Constrained Density-Functional Theory

Non-adiabatic effects play an important role in many chemical processes. In order to study the underlying non-adiabatic potential-energy surfaces (PESs), we present a locally-constrained density-functional theory approach, which enables us to confine electrons to sub-spaces of the Hilbert space, e.g. to selected atoms or groups of atoms. This allows to calculate non-adiabatic PESs for defined charge and spin states of the chosen subsystems. The capability of the method is demonstrated by calculating non-adiabatic PESs for the scattering of a sodium and a chlorine atom, for the interaction of a chlorine molecule with a small metal cluster, and for the dissociation of an oxygen molecule at the Al(111) surface.Comment: 11 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface

The measured low initial sticking probability of oxygen molecules at the Al(111) surface that had puzzled the field for many years was recently explained in a non-adiabatic picture invoking spin-selection rules [J. Behler et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to conserve the initial spin-triplet character of the free O2 molecule during the molecule's approach to the surface. A new locally-constrained density-functional theory approach gave access to the corresponding potential-energy surface (PES) seen by such an impinging spin-triplet molecule and indicated barriers to dissociation which reduce the sticking probability. Here, we further substantiate this non-adiabatic picture by providing a detailed account of the employed approach. Building on the previous work, we focus in particular on inaccuracies in present-day exchange-correlation functionals. Our analysis shows that small quantitative differences in the spin-triplet constrained PES obtained with different gradient-corrected functionals have a noticeable effect on the lowest kinetic energy part of the resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Нейтронно-физический расчет ядерного реактора типа УГР

Объектом исследования является ядерный реактор мощностью 1550МВт с топливом UO2 и обогащением 2,5 % и с теплоносителем углекислый газ. Цель работы – реализация нейтронно-физического расчёта реактора, состоящего в физическом обосновании конструкции и определении совокупности физических параметров, удовлетворяющих поставленным требованиям. В процессе исследования проводились расчеты нейтронно-физических характеристик реактора, на основании которых были получены таблицы и построены графики, а так же начерчен чертеж; произведен расчет финансовой составляющей работы, описаны внешние факторы, влиявшие на выполнение работы. В результате исследования произведён нейтронно-физический расчет данного реактора, в частности оценены размеры активной зоны, рассчитаны коэффициенты размножения «холодного» и «горячего» реакторов, произведен расчет реактора на конец кампании.The object of research is nuclear reactor power 1550МВт fuel and UO2 enriched to 2.5 % with the coolant carbon dioxide. Purpose – implementation of neutron-physical calculation of the reactor consisting in a physical rationale for the design and definition of a set of physical parameters that meet the requirements. In the process of investigation the calculations of neutron-physical characteristics of the reactor on the basis of which was obtained tables and graphs, and blueprint drawing. the calculation of the financial component of the operation described external factors affecting performance of the work. The study produced neutron-physical calculation of the reactor, in particular the estimated size of the active zone, coefficients were calculated propagation "cold" and "hot" reactors, calculation of reactor at the end of the campaign

Potential of Airborne LiDAR Derived Vegetation Structure for the Prediction of Animal Species Richness at Mount Kilimanjaro

The monitoring of species and functional diversity is of increasing relevance for the development of strategies for the conservation and management of biodiversity. Therefore, reliable estimates of the performance of monitoring techniques across taxa become important. Using a unique dataset, this study investigates the potential of airborne LiDAR-derived variables characterizing vegetation structure as predictors for animal species richness at the southern slopes of Mount Kilimanjaro. To disentangle the structural LiDAR information from co-factors related to elevational vegetation zones, LiDAR-based models were compared to the predictive power of elevation models. 17 taxa and 4 feeding guilds were modeled and the standardized study design allowed for a comparison across the assemblages. Results show that most taxa (14) and feeding guilds (3) can be predicted best by elevation with normalized RMSE values but only for three of those taxa and two of those feeding guilds the difference to other models is significant. Generally, modeling performances between different models vary only slightly for each assemblage. For the remaining, structural information at most showed little additional contribution to the performance. In summary, LiDAR observations can be used for animal species prediction. However, the effort and cost of aerial surveys are not always in proportion with the prediction quality, especially when the species distribution follows zonal patterns, and elevation information yields similar results

Roadmap on machine learning in electronic structure

In recent years, we have been witnessing a paradigm shift in computational materials science. In fact, traditional methods, mostly developed in the second half of the XXth century, are being complemented, extended, and sometimes even completely replaced by faster, simpler, and often more accurate approaches. The new approaches, that we collectively label by machine learning, have their origins in the fields of informatics and artificial intelligence, but are making rapid inroads in all other branches of science. With this in mind, this Roadmap article, consisting of multiple contributions from experts across the field, discusses the use of machine learning in materials science, and share perspectives on current and future challenges in problems as diverse as the prediction of materials properties, the construction of force-fields, the development of exchange correlation functionals for density-functional theory, the solution of the many-body problem, and more. In spite of the already numerous and exciting success stories, we are just at the beginning of a long path that will reshape materials science for the many challenges of the XXIth century

Messung der Zerfallsasymmetrien der radiativen Hyperonzerfälle Ξ0→Δγ\Xi^0 \to \Delta\gamma und Ξ0→Σ0γ\Xi^0 \to \Sigma^0\gamma mit dem NA48/1-Experiment

The radiative decay of a hyperon into a light hyperon and a photon allows to study the structure of the electroweak interaction of hadrons. For this purpose, the decay asymmetry is an appropriate observable. It describes the distribution of the daughter hyperon with respect to the polarization $vec{P}$ of the mother hyperon by $dN / d cos(Theta) propto 1 + alpha |vec{P}| cos(Theta)$, where $Theta$ is the angle between $vec{P}$ and the momentum of the daughter hyperon. The radiative decay $Xi^0 to Lambda gamma$ is of particular interest since all calculations at quark level predict a positive decay asymmetry whereas two existing measurements result in a negative value of $alpha_{Lambda gamma} = -0.73 pm 0.17$. The goal of the analysis presented here was to verify these results and to improve the accuracy of the decay asymmetry measurement. In addition, the decay asymmetry of the similar decay $Xi^0 to Sigma^0 gamma$ was measured, and the well-known decay $Xi^0 to Lambda pi^0$ was used to test the analysis strategy. During the data taking period in 2002, the NA48/1 experiment at CERN was searching for rare $K_S$ and hyperon decays. The collected data represents the world's largest sample of $Xi^0$ decays. From this sample, about 52,000 $Xi^0 to Lambda gamma$ decays, 15,000 $Xi^0 to Sigma^0 gamma$ decays and 4 mill. $Xi^0 to Lambda pi^0$ decays with small background were extracted as well as the corresponding $antiXi$ decays. The available $antiXi$ samples amount about one tenth of the $Xi^0$ samples. The measurement of the decay asymmetries was based on the comparison between data and a detailed Monte Carlo simulation, giving the following results: $alpha_{Lambda gamma} = -0.701 +- 0.019_{stat} +- 0.064_{sys}$, n$alpha_{Sigma^0 gamma} = -0.683 +- 0.032_{stat} +- 0.077_{sys}$, $alpha_{Lambda pi^0} = -0.439 +- 0.002_{stat} +- 0.056_{sys}$, $alpha_{antiLambda gamma} = 0.772 +- 0.064_{stat} +- 0.066_{sys}$, $alpha_{antiSigma^0 gamma} = 0.811 +- 0.103_{stat} +- 0.135_{sys}$, $alpha_{antiLambda pi^0} = 0.451 +- 0.005_{stat} +- 0.057_{sys}$. The uncertainty on the $Xi^0 to Lambda gamma$ decay asymmetry could be reduced by a factor 3, confirming the negative value of this asymmetry and, as a result, the disagreement with the predictions of the calculations at quark level. The $antiXi$ asymmetries have been measured for the first time, allowing to set limits on possible CP violation in $Xi^0$ decays

Messung der Zerfallsasymmetrien der radiativen Hyperonzerfälle Xi 0 Lambda gamma und Xi 0 Sigma 0 gamma mit dem NA48/1-Experiment

Der radiative Zerfall eines Hyperons in ein leichteres Hyperon und ein Photon erlaubt eine Untersuchung der Struktur der elektroschwachen Wechselwirkung von Hadronen. Dazu wird die Zerfallsasymmetrie $alpha$ betrachtet. Sie beschreibt die Verteilung des Tochterhyperons bezüglich der Polarisation $vec{P}$ des Mutterhyperons mit $dN / d cos(Theta) propto 1 + alpha |vec{P}| cos(Theta)$, wobei $Theta$ der Winkel zwischen $vec{P}$ und dem Impuls des Tochterhyperons ist. Von besonderem Interesse ist der radiative Zerfall $Xi^0 to Lambda gamma$, für den alle Rechnungen auf Quarkniveau eine positive Asymmetrie vorhersagen, wohingegen bisher eine negative Asymmetrie von $alpha_{Lambda gamma} = -0,73 +- 0,17$ gemessen wurde. Ziel dieser Arbeit war es, die bisherigen Messungen zu überprüfen und die Asymmetrie mit einer deutlich höheren Präzision zu bestimmen. Ferner wurden die Zerfallsasymmetrie des radiativen Zerfalls $Xi^0 to Sigma^0 gamma$ ermittelt und zum Test der angewandten Analysemethode der gut bekannte Zerfall $Xi^0 to Lambda pi^0$ herangezogen. Während der Datennahme im Jahr 2002 zeichnete das NA48/1-Experiment am CERN gezielt seltene $K_S$- und Hyperonzerfälle auf. Damit konnte der weltweit größte Datensatz an $Xi^0$-Zerfällen gewonnen werden, aus dem etwa 52.000 $Xi^0 to Lambda gamma$-Zerfälle, 15.000 $Xi^0 to Sigma^0 gamma$-Zerfälle und 4 Mill. $Xi^0 to Lambda pi^0$-Zerfälle mit nur geringem Untergrund extrahiert wurden. Ebenso wurden die entsprechenden $antiXi$-Zerfälle mit etwa einem Zehntel der obigen Ereigniszahlen registriert. Die Bestimmung der Zerfallsasymmetrien erfolgte durch den Vergleich der gemessene Daten mit einer detaillierten Detektorsimulation und führte zu den folgenden Resultaten dieser Arbeit: $alpha_{Lambda gamma} = -0,701 +- 0,019_{stat} +- 0,064_{sys}$, $alpha_{Sigma^0 gamma} = -0,683 +- 0,032_{stat} +- 0,077_{sys}$, $alpha_{Lambda pi^0} = -0,439 +- 0,002_{stat} +- 0,056_{sys}$, $alpha_{antiLambda gamma} = 0,772 +- 0,064_{stat} +- 0,066_{sys}$, $alpha_{antiSigma^0 gamma} = 0,811 +- 0,103_{stat} +- 0,135_{sys}$, $alpha_{antiLambda pi^0} = 0,451 +- 0,005_{stat} +- 0,057_{sys}$. Somit konnte die Unsicherheit der $Xi^0 to Lambda gamma$-Zerfallsasymmetrie auf etwa ein Drittel reduziert werden. Ihr negatives Vorzeichen und damit der Widerspruch zu den Vorhersagen der Quarkmodellrechnungen ist so zweifelsfrei bestätigt. Mit den zum ersten Mal gemessenen $antiXi$-Asymmetrien konnten zusätzlich Grenzen auf eine mögliche CP-Verletzung in den $Xi^0$-Zerfällen, die $alpha_{Xi^0} neq -alpha_{antiXi}$ zur Folge hätte, bestimmt werden.The radiative decay of a hyperon into a light hyperon and a photon allows to study the structure of the electroweak interaction of hadrons. For this purpose, the decay asymmetry is an appropriate observable. It describes the distribution of the daughter hyperon with respect to the polarization $vec{P}$ of the mother hyperon by $dN / d cos(Theta) propto 1 + alpha |vec{P}| cos(Theta)$, where $Theta$ is the angle between $vec{P}$ and the momentum of the daughter hyperon. The radiative decay $Xi^0 to Lambda gamma$ is of particular interest since all calculations at quark level predict a positive decay asymmetry whereas two existing measurements result in a negative value of $alpha_{Lambda gamma} = -0.73 pm 0.17$. The goal of the analysis presented here was to verify these results and to improve the accuracy of the decay asymmetry measurement. In addition, the decay asymmetry of the similar decay $Xi^0 to Sigma^0 gamma$ was measured, and the well-known decay $Xi^0 to Lambda pi^0$ was used to test the analysis strategy. During the data taking period in 2002, the NA48/1 experiment at CERN was searching for rare $K_S$ and hyperon decays. The collected data represents the world's largest sample of $Xi^0$ decays. From this sample, about 52,000 $Xi^0 to Lambda gamma$ decays, 15,000 $Xi^0 to Sigma^0 gamma$ decays and 4 mill. $Xi^0 to Lambda pi^0$ decays with small background were extracted as well as the corresponding $antiXi$ decays. The available $antiXi$ samples amount about one tenth of the $Xi^0$ samples. The measurement of the decay asymmetries was based on the comparison between data and a detailed Monte Carlo simulation, giving the following results: $alpha_{Lambda gamma} = -0.701 +- 0.019_{stat} +- 0.064_{sys}$, n$alpha_{Sigma^0 gamma} = -0.683 +- 0.032_{stat} +- 0.077_{sys}$, $alpha_{Lambda pi^0} = -0.439 +- 0.002_{stat} +- 0.056_{sys}$, $alpha_{antiLambda gamma} = 0.772 +- 0.064_{stat} +- 0.066_{sys}$, $alpha_{antiSigma^0 gamma} = 0.811 +- 0.103_{stat} +- 0.135_{sys}$, $alpha_{antiLambda pi^0} = 0.451 +- 0.005_{stat} +- 0.057_{sys}$. The uncertainty on the $Xi^0 to Lambda gamma$ decay asymmetry could be reduced by a factor 3, confirming the negative value of this asymmetry and, as a result, the disagreement with the predictions of the calculations at quark level. The $antiXi$ asymmetries have been measured for the first time, allowing to set limits on possible CP violation in $Xi^0$ decays