Increased Time Resolution Research Articles

Integrated spectral evolution of Galactic open clusters

We present a library of 47 open-cluster integrated spectra, mostly obtained at CASLEO (San Juan, Argentina) in the range 3600 < 1 < 7400 A, which are made available at CDS. The data are combined with previous spectra to obtain 10 high signal-to-noise ratio basic templates in the young and intermediate-age domains, which are also provided in the library. These Galactic disc templates represent the increased time resolution spectral evolution of a stellar population unit around the Solar metallicity level. The improved signal-to-noise ratio of the present templates with respect to previous template lists, together with their increased time resolution, allowed us to improve the fundamental parameters of some open clusters. The present spectral library will be useful for several astrophysical applications, particularly for population syntheses of star-forming giant galaxies.

Influence of Contrast Media Dose in Elliptical Ordered MR Angiography Image Quality of the Carotid Arteries: Preliminary Results

The contrast timing is crucial in fast imaging magnetic resonance (MR), particularly in the angiographic techniques. In MR sequences the central lines of k space own the image contrast information (1,2). Hence, the synchronization of the acquisition of central views of k-space during the maximum concentration of contrast media (CM) into the examined vessel is mandatory to obtain images of higher quality (2–7). The lack of adequate timing with premature acquisition yields to poor examination with no visualization of the vessels. On the opposite, the continuous infusion or a delayed acquisition cause the enhancement of veins and soft tissues that may obscure the examined structures (5, 8–11). In fact, although fast imaging sequences increase the image quality and lessen the contrast media injected volume, a shorter acquisition time allows a smaller error in contrast timing (6, 10). Additional factors occurs with high flow vessels such as renal and carotid arteries in which the window of selective enhancement is very narrow (4, 10–13). The delay of the scan is not accurately predicted by patient’s physiological parameters (3, 7, 12). Several techniques have been developed to ensure a proper timing (14). The first one is based on a test bolus administered prior to angiographic acquisition during continuous monitoring scans at the level of the region of interest. The scan delay is estimated from the measurement of the transit time of the test bolus from the site of injection to the examined vessel (3, 7, 12, 15). The second technique is based on a series of consecutive short angiographic sequences expecting that the vessels of interest fill at least during the central period of one of them (16, 17). The third one consists on partial k-space updating with increased time resolution through more frequent sampling of the k-space’s centre (5, 14, 18). All these techniques suffer from several drawbacks because of the inverse relationship between the spatial resolution and the acquisition time. Moreover they are less accurate in high flow vessels. Magnetic resonance angiography (MRA) with centric ordered acquisition is a relatively new technique which has been already investigated by several Authors (6, 10, 13, 19– 22). Basically in this technique the central views of k-space are acquired at the very beginning of the acquisition. The triggering is automatic or manual as the vessel enhancement is detected through an automatic volume sampling with a threshold or through a monitoring “fluoroscopic” slice. The asymmetric structure of this sequence and the advantages on contrast timing lead to a different use of the contrast media. The aim of this study is to assess the influence of the contrast media dose in MRA with elliptical ordering of the carotid arteries and to identify the parameters affecting this technique.

Surface materials: the frontier of solid state chemistry

Atomic scale studies of surfaces over the past 30 years greatly increased our knowledge of the chemical, mechanical, electrical, magnetic, and optical properties of surfaces. As a result, new technologies developed using these properties of surfaces, creating wealth, new scientific knowledge, and the need for new surface instrumentation with ever increasing time resolution, spatial and energy resolution. Over 60 techniques have been developed, most recently, those that can scrutinize, on the molecular level, the buried interfaces, solid–high pressure gas, solid–liquid, and solid–solid. The surface may be treated as a separate phase with different structure, composition and electronic properties. The new concepts of molecular surface science that were discovered include clean surface reconstruction, adsorbate induced restructuring, the high mobility of surface atoms at high pressures, and the unique importance of surface defects in controlling surface phenomena ranging from adsorption to heterogeneous catalysis. Future directions include studies of electrode surface under external potential conditions, polymer surfaces, studies of biomaterials, and studies of nanoclusters produced by chemical means or by electron beam lithography.

Analysis of the first year of Earth rotation parameters with a sub-daily resolution gained at the CODE processing center of the IGS

The solutions of the CODE Analysis Center submitted to the IGS, the International Global Position System (GPS) Service for Geodynamics, are based on three days of observation of about 80–100 stations of the IGS network. The Earth rotation parameters (ERPs) are assumed to vary linearly over the three days with respect to an a priori model. Continuity at the day boundaries as well as the continuity of the first derivatives are enforced by constraints. Since early April 1995 CODE has calculated a new ERP series with an increased time resolution of 2 hours. Again continuity is enforced at the 2-hours-interval boundaries. The analysis method is described, particularly how to deal with retrograde diurnal terms in the ERP series which may not be estimated with satellite geodetic methods. The results obtained from the first year of data covered by the time series (time interval from 4 April 1995 to 30 June 1996) are also discussed. The series is relatively homogeneous in the sense of the used orbit model and the a priori model for the ERPs. The largest source of excitation at daily and sub-daily periods is likely to be the effect of the ocean tides. There is good agreement between the present results and Topex/Poseidon ocean tide models, as well as with models based on Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) data. Non-oceanic periodic variations are also observed in the series. Their origin is most probably a consequence of the GPS solution strategy; other possible sources are the atmospheric tides.

Accurate Antarctic dating technique sought by Quaternary community

Late Quaternary paleoenvironmental records from the circum‐Antarctic and Southern Ocean play an increasing role in deciphering the interplay among the atmosphere, ocean, and cryosphere climate systems. Correlating such records at increasing time resolution is vital for evaluating the global phasing of scenarios that involve rapid climate changes in the tropics and at one or both poles. Because interhemispheric correlations are subject to greater scrutiny [Crowley, 1999] and a higher level of detail than in the past, we must be certain that our chronologies are both accurate and precise.Although radiocarbon dating remains the single best chronologic tool for marine and terrestrial sequences, the technique has serious problems when applied to Antarctic samples. To assess the state of the problem (s), to consider potential solutions, and to review new continuous time series, a workshop was convened earlier this year.

Dithered-edge sampling of terahertz pulses

We introduce a technique for ultrawideband measurement of terahertz pulses called dithered-edge sampling (DES). The technique makes use of a photoconductive receiver, but the detection bandwidth is much wider than that of the receiver alone. The key to increasing the bandwidth is the addition of an ultrafast optically triggered attenuator that chops the terahertz pulse before its detection. The bandwidth is limited only by the duration of the optical pulse used to trigger the attenuator. We use a combination of derivative dither and an integrating receiver to recover the terahertz field directly from the measured signal. When used alone, the slow receiver blurs the measured terahertz pulse width to 1.3 ps. However, the increased time resolution of the DES system (triggered attenuator plus receiver) allows one to measure source-limited terahertz pulse widths (400 fs in this case).

A synthetic bandwidth measurement system for acoustic remote sensing of geoacoustic properties of bottom sediments

A series of narrowband acoustic transmissions may be combined into an equivalent broadband signal - with a corresponding increase in resolution in the time domain. This procedure is called synthetic bandwidth processing. If ray methods are used in combination with inverse theory to determine the otacoustic properties of bottom sediments, the increased time resolution aids in the identification of arrivals which are closely spaced in time. This, in turn, permits improved estimates of the spatial distribution of properties and more accurate otacoustic models. Signal processing techniques and guidelines for synthesizing multioctave broadband pulse-like signals are presented. Also described is the instrumentation used to implement the technique in real ocean environments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Light baton system: A system for conducting computer music performance

Abstract We describe an interface designed and developed to allow an automatic synthesis system to be controlled by a human conductor. The aim is to endow that electronic music with the expressiveness and feeling which are characteristic of live performances. The system recognises those movements and gestures made by a conductor during a live performance which conform with international standards. The system consists of a special conductor's baton and a personal computer with an image acquisition board connected to a camera. The movements of a small lamp placed on the tip of the baton are detected by the camera, digitised as light point position changes and stored. A best‐fitting interpolating method is used to perform the corresponding path, increase time resolution and predict the beat points. The data acquired by the system, i.e., beat point prediction, order number of the beats, amplitude of gesture, end of movement and absence of light points, are applied as input parameters of an algorithmic compositional environment. The system can be used to synchronise human and machine performances, to learn conducting movements, and to control algorithmic compositions in real time.

Development of a geometry-compensated neutron time-of-flight detector for ICF applications with approximately 200 ps time response

Current-mode neutron time-of-flight detectors are used on Nova for neutron yield, ion temperature, and neutron emission time measurements. Currently used detectors are limited by the time response of the microchannel plate photomultiplier tubes used with the scintillators, scintillator decay time, scintillator thickness, and oscilloscope response time. A change in the geometry of the scintillator allows one to take advantage of the increased time resolution made possible by more advanced transient recorders and microchannel plate photomultiplier tubes. A prototype detector has been designed to incorporate these changes, and could potentially yield time resolution of less than 150 ps. Experimental results are presented demonstrating an ion temperature measurement of a direct-drive DT implosion on Nova.

Picosecond dynamics of degenerate orthoexcitons inCu2O

Previous time-resolved luminescence studies of orthoexcitons in ${\mathrm{Cu}}_{2}$O have shown a ``quantum saturation'' behavior. Basically, it was found that for 10-ns laser pulses, which are comparable to the exciton lifetime, the exciton-gas temperature rises along the phase boundary for Bose-Einstein condensation as the density is increased. The microscopic processes that lead to such an unusual effect remained unresolved. To isolate the various kinetic processes, we have now conducted a study using 100-ps pulses, which are short compared with the orthoexciton lifetime. Combined with an increased time resolution, this short-pulse excitation has allowed us to observe some of the intrinsic rates of thermalization and decay of the orthoexcitons. At low-to-intermediate excitation levels, the decay of the gas temperature is found to agree with a model of acoustic-phonon emission, and the deduced exciton-phonon coupling is in basic agreement with that obtained from diffusion experiments. Another principal result is that the decay time of the gas temperature is significantly increased with increasing degeneracy, i.e., at high excitation level. After the short excitation pulse the orthoexcitons follow the Bose-Einstein phase boundary in density and temperature in a way similar to their behavior during the ``steady-state'' conditions of the long-pulse excitation. We conclude that the temperature rise is not simply due to insufficient thermalization caused by a shortening of the exciton lifetime; at high density there must be a heating mechanism such as Auger recombination.

Automating the recording and improving the presentation of the anesthesia record.

Although anesthesia records have been kept for over a hundred years, there is still discussion of their value and content. Two uses of the record are widely accepted: (1) review after the anesthetic event (as in medicolegal disputes), and (2) support of patient care during the delivery of an anesthetic. Although the anesthetic record is mandatory in much of the world, there is not a single standard for its format. Automating the generation and presentation of the record will enhance its value and help develop a consensus as to content. Merely automating the steps used to produce the manually generated record does not realize the full benefit of automation. For maximum benefit, the primary goal of automation should be to support the uses of the record. Specific techniques that are discussed include increasing time resolution, optimizing the type and location of input and display equipment, and tailoring the human interface. Particular attention is paid to the issue of how much detail is acceptable in the record, how to use visual cues to present detail properly, how to exclude extraneous detail, and how to avoid misleading presentations (erroneous interpretation of the data). Specific elements discussed include line width, the use of color, presentation of gradients, statistical summaries, contexts for reporting data, graphical techniques for increasing data content, and pictorial presentations. Current records are more often confusing because presented information is inconsistently displayed or irrelevant than because too much information is offered, and automation can ameliorate this problem.

California Current Eddy Formation: Ship, Air, and Satellite Results

Until recently, quantitative measurements of the circulation of the California Current were limited to hydrographic determinations [See figure in the PDF file] of temperature and salinity. This information is now being augmented by satellite data. Clouds permitting, satellite scanner systems can locate major ocean frontal boundaries if they are associated with even quite weak horizontal sea-surface temperature gradients. The satellite data are most usefully interpreted in a region such as that encompassing the California Current, where the surface and main thermocline temperature distributions bear some relation to each other. In such a region, it is possible to make interpretations of circulation based on satellite-derived sea-surface temperature patterns. The correctness of these interpretations depends heavily on the availability of historical and present-day subsurface data, collected by conventional methods from ships and aircraft. Satellite infrared scanners, in addition to providing information on circulation with vastly increased spatial resolution, have the potential (with cooperative weather) for providing increased time resolution. These improvements in resolution have permitted us to see that much of the spatial variation in the California Current takes place along welldefined fronts and to observe the evolution of one particular meander.

A general purpose delay unit and its application to kinetic flash photolysis spectroscopy.

A delay unit is described which is capable of producing time delays of from one microsecond to several milliseconds duration. The delay times may be selected at one microsecond intervals throughout the range. It has been designed to fit this specification but can easily be extended to produce longer delays. By suitable modification shorter delay times and increased time resolution may be obtained. The unit has a totally reproducible performance, is adaptable to other applications and is insensitive to external fields.