Self contamination effects in the TAUVEX UV Telescope: Ground testing and computer simulation

The contamination effects due to outgassing from construction materials of the TAUVEX (Tel Aviv University UV Telescope) were evaluated using a combination of ground testing and computer simulations. Tests were performed from the material level of the system level including: (1) High sensitivity CVCM(10(exp -3 percent) measurements of critical materials. (2) Optical degradation measurements of samples specially contaminated by outgassing products at different contamination levels. (3) FTIR studies of chemical composition of outgassed products on above samples. (4) High resolution AFM studies of surface morphology of contaminated surfaces. The expected degradation of TAUVEX performance in mission was evaluated applying a computer simulation code using input parameters determined experimentally in the above tests. The results have served as guidelines for the proper selection of materials, cleanliness requirements, determination of the thermal conditions of the system and bakeout processes

Non-symmetric pinning of topological defects in living liquid crystals

Topological defects, such as vortices and disclinations, play a crucial role in spatiotemporal organization of equilibrium and non-equilibrium systems. The defect immobilization or pinning is a formidable challenge in the context of the out-of-equilibrium system, like a living liquid crystal, a suspension of swimming bacteria in lyotropic liquid crystal. Here we control the emerged topological defects in a living liquid crystal by arrays of 3D-printed microscopic obstacles (pillars). Our studies show that while −1/2 defects may be easily immobilized by the pillars, +1/2 defects remain motile. Due to attraction between oppositely charged defects, positive defects remain in the vicinity of pinned negative defects, and the diffusivity of positive defects is significantly reduced. Experimental findings are rationalized by computational modeling of living liquid crystals. Our results provide insight into the engineering of active systems via targeted immobilization of topological defects

Стабильные формирователи импульсного тока для питания светодиодов

Стабільні генератори світлових імпульсів нано- та субнаносекундної тривалості можуть знайти широке використання у експериментальній техніці, приладобудуванні. У якості джерела світла в таких генераторах перспективно використовувати стабільні, широкосмугові, швидкі світлодіоди на основі карбіду кремнію, що працюють у режимі електричного пробою. Існує необхідність створення стабільних формувачів імпульсного струму для їх живлення. Метою даної роботи є створення таких формувачів на основі лавинних та швидкісних польових транзисторів. Випробувана значна кількість типів та екземплярів недорогих епітаксіальних транзисторів, працюючих у лавинному режимі. Експериментально визначено оптимальний інтервал напруг пробою колекторного переходу. Досліджена часова нестабільність формувачів на лавинних транзисторах. Виявлені закономірності їх старіння. Відпрацьована схема формувача на польовому транзисторі, що працює під керівництвом лавинного, що дозволяє формувати субнаносекундні імпульси струму через світлодіоди амплітудою до 1 А.Stable generators of light impulses nano and subnanosecond duration can find a wide usage in an experimental technique, instrument-making. As a light source in such generators it is perspective to use stable, broadband, fast-acting light-emitting diodes on the basis of carbide of silicon, working in the mode of electric break-down. There is a necessity of creation of stable reshapers of pulse current for their power supply. A objective of this paper is creation of such reshapers on the basis of the avalanche and high-speed field-effect transistors. A lot of types and copies of inexpensive epitaxial transistors working in the avalanche mode is tested. The optimal interval of break-down voltages of collector junction is experimentally determined. Temporal instability of reshapers on avalanche transistors is investigated. Behaviour of their ageing(degradation) is studied. The circuit design of reshaper on the field transistor working by avalanche transistor control is developed. That allows to form the subnanosecond impulses of current through light-emitting diodes by amplitude to 1 A.Стабильные генераторы световых импульсов нано- и субнаносекундной длительности могут найти широкое применение в экспериментальной технике, приборостроении. В качестве источника света в таких генераторах перспективно использовать стабильные, широкополосные, быстродействующие светодиоды на основе карбида кремния, работающие в режиме электрического пробоя. Имеется необходимость создания стабильных формирователей импульсного тока для их питания. Целью данной работы является создание таких формирователей на основе лавинных и быстродействующих полевых транзисторов. Испытано значительное количество типов и экземпляров недорогих эпитаксиальных транзисторов, работающих в лавинном режиме. Экспериментально установлен оптимальный интервал напряжений пробоя коллекторного перехода. Исследована временная нестабильность формирователей на лавинных транзисторах. Выявлены закономерности их старения. Отработана схема формирователя на полевом транзисторе, работающем под управлением лавинного, которая позволяет формировать субнаносекундные импульсы тока через светодиоды амплитудой до 1 А

Graphical modeling of binary data using the LASSO: a simulation study

Background: Graphical models were identified as a promising new approach to modeling high-dimensional clinical data. They provided a probabilistic tool to display, analyze and visualize the net-like dependence structures by drawing a graph describing the conditional dependencies between the variables. Until now, the main focus of research was on building Gaussian graphical models for continuous multivariate data following a multivariate normal distribution. Satisfactory solutions for binary data were missing. We adapted the method of Meinshausen and Buhlmann to binary data and used the LASSO for logistic regression. Objective of this paper was to examine the performance of the Bolasso to the development of graphical models for high dimensional binary data. We hypothesized that the performance of Bolasso is superior to competing LASSO methods to identify graphical models. Methods: We analyzed the Bolasso to derive graphical models in comparison with other LASSO based method. Model performance was assessed in a simulation study with random data generated via symmetric local logistic regression models and Gibbs sampling. Main outcome variables were the Structural Hamming Distance and the Youden Index. We applied the results of the simulation study to a real-life data with functioning data of patients having head and neck cancer. Results: Bootstrap aggregating as incorporated in the Bolasso algorithm greatly improved the performance in higher sample sizes. The number of bootstraps did have minimal impact on performance. Bolasso performed reasonable well with a cutpoint of 0.90 and a small penalty term. Optimal prediction for Bolasso leads to very conservative models in comparison with AIC, BIC or cross-validated optimal penalty terms. Conclusions: Bootstrap aggregating may improve variable selection if the underlying selection process is not too unstable due to small sample size and if one is mainly interested in reducing the false discovery rate. We propose using the Bolasso for graphical modeling in large sample sizes

Simultaneous Analysis of All SNPs in Genome-Wide and Re-Sequencing Association Studies

Testing one SNP at a time does not fully realise the potential of genome-wide association studies to identify multiple causal variants, which is a plausible scenario for many complex diseases. We show that simultaneous analysis of the entire set of SNPs from a genome-wide study to identify the subset that best predicts disease outcome is now feasible, thanks to developments in stochastic search methods. We used a Bayesian-inspired penalised maximum likelihood approach in which every SNP can be considered for additive, dominant, and recessive contributions to disease risk. Posterior mode estimates were obtained for regression coefficients that were each assigned a prior with a sharp mode at zero. A non-zero coefficient estimate was interpreted as corresponding to a significant SNP. We investigated two prior distributions and show that the normal-exponential-gamma prior leads to improved SNP selection in comparison with single-SNP tests. We also derived an explicit approximation for type-I error that avoids the need to use permutation procedures. As well as genome-wide analyses, our method is well-suited to fine mapping with very dense SNP sets obtained from re-sequencing and/or imputation. It can accommodate quantitative as well as case-control phenotypes, covariate adjustment, and can be extended to search for interactions. Here, we demonstrate the power and empirical type-I error of our approach using simulated case-control data sets of up to 500 K SNPs, a real genome-wide data set of 300 K SNPs, and a sequence-based dataset, each of which can be analysed in a few hours on a desktop workstation

Quantile regression with group lasso for classification

Applications of regression models for binary response are very common and models specific to these problems are widely used. Quantile regression for binary response data has recently attracted attention and regularized quantile regression methods have been proposed for high dimensional problems. When the predictors have a natural group structure, such as in the case of categorical predictors converted into dummy variables, then a group lasso penalty is used in regularized methods. In this paper, we present a Bayesian Gibbs sampling procedure to estimate the parameters of a quantile regression model under a group lasso penalty for classification problems with a binary response. Simulated and real data show a good performance of the proposed method in comparison to mean-based approaches and to quantile-based approaches which do not exploit the group structure of the predictors

Rapid mechanochemical synthesis of amorphous alloys

A rapid method for preparing amorphous alloys has been developed utilizing a SPEX high energy ball mill. Parameters such as ball size, mass of balls and milling time, were systematically optimized to prepare amorphous alloys quickly. Amorphous alloys previously not thought possible to synthesize by means of SPEX milling were made amorphous by the new method. When applied to the preparation of Sn-Co-C and Si-Fe alloys, the method was found to produce fully amorphous alloys in a matter of hours instead of days or weeks of milling by traditional techniques. Electrochemical performance of the alloys produced by the rapid milling technique in Li cells was found to be the same or superior to alloys produced by the more time consuming methods, confirming their amorphous microstructure