Pier Scour Prediction in Non-Uniform Gravel Beds

YesPier scour has been extensively studied in laboratory experiments. However, scour depth relationships based on data at the laboratory scale often yield unacceptable results when extended to field conditions. In this study, non-uniform gravel bed laboratory and field datasets with gravel of median size ranging from 2.7 to 14.25 mm were considered to predict the maximum equilibrium scour depth at cylindrical piers. Specifically, a total of 217 datasets were collected: 132 from literature sources and 85 in this study using new experiments at the laboratory scale, which constitute a novel contribution provided by this paper. From the analysis of data, it was observed that Melville and Coleman’s equation performs well in the case of laboratory datasets, while it tends to overestimate field measurements. Guo’s and Kim et al.’s relationships showed good agreements only for laboratory datasets with finer non-uniform sediments: deviations in predicting the maximum scour depth with non-uniform gravel beds were found to be significantly greater than those for non-uniform sand and fine gravel beds. Consequently, new K-factors for the Melville and Coleman’s equation were proposed in this study for non-uniform gravel-bed streams using a curve-fitting method. The results revealed good agreements between observations and predictions, where this might be an attractive advancement in overcoming scale effects. Moreover, a sensitivity analysis was performed to identify the most sensitive K-factors

The ANTARES Optical Beacon System

ANTARES is a neutrino telescope being deployed in the Mediterranean Sea. It consists of a three dimensional array of photomultiplier tubes that can detect the Cherenkov light induced by charged particles produced in the interactions of neutrinos with the surrounding medium. High angular resolution can be achieved, in particular when a muon is produced, provided that the Cherenkov photons are detected with sufficient timing precision. Considerations of the intrinsic time uncertainties stemming from the transit time spread in the photomultiplier tubes and the mechanism of transmission of light in sea water lead to the conclusion that a relative time accuracy of the order of 0.5 ns is desirable. Accordingly, different time calibration systems have been developed for the ANTARES telescope. In this article, a system based on Optical Beacons, a set of external and well-controlled pulsed light sources located throughout the detector, is described. This calibration system takes into account the optical properties of sea water, which is used as the detection volume of the ANTARES telescope. The design, tests, construction and first results of the two types of beacons, LED and laser-based, are presented.Comment: 21 pages, 18 figures, submitted to Nucl. Instr. and Meth. Phys. Res.

The silicon micro-strip detector plane for the LOFT/Wide Field Monitor

The main objective of the Wide Field Monitor (WFM) on the LOFT mission is to provide unambiguous detection of the high-energy sources in a large field of view, in order to support science operations of the LOFT primary instrument, the LAD. The monitor will also provide by itself a large number of results on the timing and spectral behaviour of hundreds of galactic compact objects, Active Galactic Nuclei and Gamma-Ray Bursts. The WFM is based on the coded aperture concept where a position sensitive detector records the shadow of a mask projected by the celestial sources. The proposed WFM detector plane, based on Double Sided micro-Strip Silicon Detectors (DSSD), will allow proper 2-dimensional recording of the projected shadows. Indeed the positioning of the photon interaction in the detector with equivalent fine resolution in both directions insures the best imaging capability compatible with the allocated budgets for this telescope on LOFT. We will describe here the overall configuration of this 2D-WFM and the design and characteristics of the DSSD detector plane including its imaging and spectral performances. We will also present a number of simulated results discussing the advantages that this configuration offers to LOFT. A DSSD-based WFM will in particular reduce significantly the source confusion experienced by the WFM in crowded regions of the sky like the Galactic Center and will in general increase the observatory science capability of the mission.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-89, 201

Background Light in Potential Sites for the ANTARES Undersea Neutrino Telescope

The ANTARES collaboration has performed a series of {\em in situ} measurements to study the background light for a planned undersea neutrino telescope. Such background can be caused by $^{40}$K decays or by biological activity. We report on measurements at two sites in the Mediterranean Sea at depths of 2400~m and 2700~m, respectively. Three photomultiplier tubes were used to measure single counting rates and coincidence rates for pairs of tubes at various distances. The background rate is seen to consist of three components: a constant rate due to $^{40}$K decays, a continuum rate that varies on a time scale of several hours simultaneously over distances up to at least 40~m, and random bursts a few seconds long that are only correlated in time over distances of the order of a meter. A trigger requiring coincidences between nearby photomultiplier tubes should reduce the trigger rate for a neutrino telescope to a manageable level with only a small loss in efficiency.Comment: 18 pages, 8 figures, accepted for publication in Astroparticle Physic

Compton Large Area Silicon Timing Tracker for Cosmic Vision M3

International audienceProposed in response to the ESA call for the third Medium size mission (M3), CAPSiTT is a small mission designed for a 3-year survey of the non-thermal high energy sky from an equatorial LEO orbit. With a large effective area and a very wide field of view, its single instrument, a silicon tracker, provides good imaging, spectroscopic and polarimetric capabilities with a sensitivity 10-100 times better than COMPTEL. Nucleosynthesis and particle acceleration mechanisms in various sites are the main scientific topics addressed by CAPSiTT

Construction and test of a fine-grained liquid argon preshower prototype

A separate liquid argon preshower detector consisting of two layers featuring a fine granularity of 2.5~10$^{\mathrm{-3}}$ was studied by the RD3 collaboration. A prototype covering approximately 0.8 in pseudo-rapidity and 9 degrees in azimuth was built and tested at CERN in July 94. CMOS and GaAs VLSI preamplifiers were designed and tested for this occasion. The combined response of this detector and an accordion electromagnetic calorimeter prototype to muons, electrons and photons is presented. For minimum ionizing tracks a signal-to-noise ratio of 4.5 per preshower layer was measured. Above 150~GeV the space resolution for electrons is better than 250~$\mu$m in both directions. The precision on the electromagnetic shower direction, determined together with the calorimeter, is better than 4 mrad above 50~GeV. It is concluded that the preshower detector would adequately fulfil its role for future operation at CERN Large Hadron Collider

Performance of an endcap prototype of the Atlas accordion electromagnetic calorimeter

The design and construction of a lead-liquid argon endcap calorimeter prototype using an accordion geometry and conceived as a sector of the inner wheel of the endcap calorimeter of the future ATLAS experiment at the LHC is described. The performance obtained using electron beam data is presented. The main results are an energy resolution with a sampling term below $11\%/\sqrt{E(\rm GeV)}$ and a small local constant term, a good linearity of the response with the incident energy and a global constant term of 0.8\% over an extended area in the rapidity range of $2.2\eta 2.9$. These properties make the design suitable for the ATLAS electromagnetic endcap calorimeter

Performance of the front-end electronics of the ANTARES neutrino telescope

ANTARES is a high-energy neutrino telescope installed in the Mediterranean Sea at a depth of 2475 m. It consists of a three-dimensional array of optical modules, each containing a large photomultiplier tube. A total of 2700 front-end ASICs named Analogue Ring Samplers (ARS) process the phototube signals, measure their arrival time, amplitude and shape as well as perform monitoring and calibration tasks. The ARS chip processes the analogue signals from the optical modules and converts information into digital data. All the information is transmitted to shore through further multiplexing electronics and an optical link. This paper describes the performance of the ARS chip; results from the functionality and characterization tests in the laboratory are summarized and the long-term performance in the apparatus is illustrated.Comment: 20 pages, 22 figures, published in Nuclear Instruments and Methods