arXiv subjects

Thanh Nguyen

Publications and source records attributed to Thanh Nguyen.

97 records · Page 6Linked to original sources

arXiv2017

Characterization of Three High Efficiency and Blue Sensitive Silicon Photomultipliers

We report about the optical and electrical characterization of three high efficiency and blue sensitive Silicon photomultipliers from FBK, Hamamatsu, and SensL. Key features of the tested devices when operated at 90% breakdown probability are peak photon detection efficiencies between 40% and 55%, temperature dependencies of gain and PDE that are less than 1%/$^{\circ}$C, dark rates of $\sim$50\,kHz/mm$^{2}$ at room temperature, afterpulsing of about 2%, and direct optical crosstalk between 6% and 20%. The characteristics of all three devices impressively demonstrate how the Silicon-photomultiplier technology has improved over the past ten years. It is further demonstrated how the voltage and temperature characteristics of a number of quantities can be parameterized on the basis of physical models. The models provide a deeper understanding of the device characteristics over a wide bias and temperature range. They also serve as examples how producers could provide the characteristics of their SiPMs to users. A standardized parameterization of SiPMs would enable users to find the optimal SiPM for their application and the operating point of SiPMs without having to perform measurements thus significantly reducing design and development cycles.

physics.ins-det↗

arXiv2016

Photon Detection Efficiency Measurements of the VERITAS Cherenkov Telescope Photomultipliers after four Years of Operation

The photon detection efficiency of two sets of R10560-100-20 superbialkali photomultiplier tubes from Hamamatsu were measured between 200 nm and 750 nm to quantify a possible degradation of the photocathode sensitivity after four years of operation in the cameras of the VERITAS Cherenkov telescopes. A sample of 20 photomultiplier tubes, which was removed from the telescopes was compared with a sample of 20 spare photomultiplier tubes, which had been kept in storage. It is found that the average photocathode sensitivity marginally increased below 300 nm and dropped by 10% to 30% above 500 nm. The average photocathode sensitivity folded with the Cherenkov spectrum emitted by particles in air showers, however, reveals a consistent detection yield of 18.9+/-0.2% and 19.1+/-0.2% for the sample removed from the telescope and the spare sample, respectively.

astro-ph.IM↗

arXiv2015

Updated Results from VERITAS on the Crab Pulsar

The Crab pulsar and plerion are some of the brightest and best studied non-thermal astrophysical sources. The recent discovery of pulsed gamma-ray emission above 100 gigaelectronvolts (GeV) from the Crab pulsar with VERITAS (the Very Energetic Radiation Imaging Telescope Array System) challenges commonly accepted pulsar emission models and puts the gamma-ray emission region far out in the magnetosphere - close to or even beyond the light cylinder. We present updated VERITAS results from the analysis of a data set that is twice as large as the original data set published in 2011. The results are discussed in the context of discriminating between different models put forward to explain gamma-ray emission mechanisms and acceleration regions within the Crab pulsar's magnetosphere.

astro-ph.HE↗

arXiv2015

The Cost of Free Spectrum

There has been growing interest in increasing the amount of radio spectrum available for unlicensed broad-band wireless access. That includes "prime" spectrum at lower frequencies, which is also suitable for wide area coverage by licensed cellular providers. While additional unlicensed spectrum would allow for market expansion, it could influence competition among providers and increase congestion (interference) among consumers of wireless services. We study the value (social welfare and consumer surplus) obtained by adding unlicensed spectrum to an existing allocation of licensed spectrum among incumbent service providers. We assume a population of customers who choose a provider based on the minimum delivered price, given by the weighted sum of the price of the service and a congestion cost, which depends on the number of subscribers in a band. We consider models in which this weighting is uniform across the customer population and where the weighting is either high or low, reflecting different sensitivities to latency. For the models considered, we find that the social welfare depends on the amount of additional unlicensed spectrum, and can actually decrease over a significant range of unlicensed bandwidths. Furthermore, with nonuniform weighting, introducing unlicensed spectrum can also reduce consumer welfare.

cs.GT↗

arXiv2013

Searching and Bargaining with Middlemen

We study decentralized markets with the presence of middlemen, modeled by a non-cooperative bargaining game in trading networks. Our goal is to investigate how the network structure of the market and the role of middlemen influence the market's efficiency and fairness. We introduce the concept of limit stationary equilibrium in a general trading network and use it to analyze how competition among middlemen is influenced by the network structure, how endogenous delay emerges in trade and how surplus is shared between producers and consumers.

cs.GT↗

arXiv2011

Multilingual Schema Matching for Wikipedia Infoboxes

Recent research has taken advantage of Wikipedia's multilingualism as a resource for cross-language information retrieval and machine translation, as well as proposed techniques for enriching its cross-language structure. The availability of documents in multiple languages also opens up new opportunities for querying structured Wikipedia content, and in particular, to enable answers that straddle different languages. As a step towards supporting such queries, in this paper, we propose a method for identifying mappings between attributes from infoboxes that come from pages in different languages. Our approach finds mappings in a completely automated fashion. Because it does not require training data, it is scalable: not only can it be used to find mappings between many language pairs, but it is also effective for languages that are under-represented and lack sufficient training samples. Another important benefit of our approach is that it does not depend on syntactic similarity between attribute names, and thus, it can be applied to language pairs that have distinct morphologies. We have performed an extensive experimental evaluation using a corpus consisting of pages in Portuguese, Vietnamese, and English. The results show that not only does our approach obtain high precision and recall, but it also outperforms state-of-the-art techniques. We also present a case study which demonstrates that the multilingual mappings we derive lead to substantial improvements in answer quality and coverage for structured queries over Wikipedia content.

cs.DB↗

arXiv2003

10 um wavefront spatial filtering: first results with chalcogenide fibers

Wavefront cleaning by single-mode fibers has proved to be efficient in optical-infrared interferometry to improve calibration quality. For instance, the FLUOR instrument has demonstrated the capability of fluoride glass single-mode fibers in this respect in the K and L bands. New interferometric instruments developped for the mid-infrared require the same capability for the 8-12 um range. We have initiated a program to develop single-mode fibers in the prospect of the VLTI mid-infrared instrument MIDI and of the ESA/DARWIN and NASA/TPF missions that require excellent wavefront quality. In order to characterize the performances of chalcogenide fibers we are developping, we have set up an experiment to measure the far-field pattern radiated at 10 um. In this paper, we report the first and promising results obtained with this new component.

astro-ph↗