81 research outputs found
PVF2 Transducers for NDE
We report on recent calculations and experiments on the broadband properties and impulse characteristics of PVF2 transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitation of surface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic substrates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF2 films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency measurements on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60°, and control of acceptance angle profiles, in face plates using multilayer PVF2 films
PVF2 Transducers for NDT
We have investigated the spatial dependence of the longitudinal piezoelectric stress constant e of PVF2. Experiments were performed on a series of no.minally identical brass-backed PVF2 longitudinal wave transducers in water using commercial PVF2 film. Computer programs were designed to predict the performance of the transducers as a function of ezz (Z) , the thickness mode piezoelectric stress constant as a function of position Z through the thickness of the film. Our experiments indicate that the coupling coefficient is uniform across the film thickness. These computer programs were also used to model the insertion loss and bandwidth performance of the transducers. High voltage pulses were applied to PVF2 transducers to determine the region of linearity and to find the maximum nondestructive voltage that can be used. Transducers of this type were also made using PVF films fabricated in the Stanford Center for Materials Research using commercial resin, and found to pertorm as well as transducers using commercial film. Transducers of the above kind have also been used as bulk wave sources in wedge transducer assemblies for the production of surface acoustic waves on ceramic plates, and initial observations of reflections from surface cracks have been made
A comparison of the optical properties of radio-loud and radio-quiet quasars
We have made radio observations of 87 optically selected quasars at 5 GHz
with the VLA in order to measure the radio power for these objects and hence
determine how the fraction of radio-loud quasars varies with redshift and
optical luminosity. The sample has been selected from the recently completed
Edinburgh Quasar Survey and covers a redshift range of 0.3 < z < 1.5 and an
optical absolute magnitude range of -26.5 < M_{B} < -23.5 (h, q_{0} = 1/2). We
have also matched up other existing surveys with the FIRST and NVSS radio
catalogues and combined these data so that the optical luminosity-redshift
plane is now far better sampled than previously. We have fitted a model to the
probability of a quasar being radio-loud as a function of absolute magnitude
and redshift and from this model infer the radio-loud and radio-quiet optical
luminosity functions. The radio-loud optical luminosity function is featureless
and flatter than the radio-quiet one. It evolves at a marginally slower rate if
quasars evolve by density evolution, but the difference in the rate of
evolutions of the two different classes is much less than was previously
thought. We show, using Monte-Carlo simulations, that the observed difference
in the shape of the optical luminosity functions can be partly accounted for by
Doppler boosting of the optical continuum of the radio-loud quasars and explain
how this can be tested in the future.Comment: 33 pages, 9 postscript figures, uses the AAS aaspp4 LaTeX style file,
to appear in the 1 February 1999 issue of The Astrophysical Journa
Nonlinear multimode fiber optics: recent advances
We start by providing an overview of the emerging field of nonlinear optics in multimode optical fibers [1]. These fibers provide a simple testbed for observing complex wave propagation dynamics, in analogy with other fields of physics ranging from two-dimensional hydrodynamic turbulence and Bose-Einstein condensation. In addition, nonlinear multimode optical fibers enable new methods for achieving the ultrafast, light-activated control of temporal, spatial and spectral degrees of freedom of intense, pulsed light beams, for a range of different technological applications
Expected performance of the ASTRI-SST-2M telescope prototype
ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) is an
Italian flagship project pursued by INAF (Istituto Nazionale di Astrofisica)
strictly linked to the development of the Cherenkov Telescope Array, CTA.
Primary goal of the ASTRI program is the design and production of an end-to-end
prototype of a Small Size Telescope for the CTA sub-array devoted to the
highest gamma-ray energy region. The prototype, named ASTRI SST-2M, will be
tested on field in Italy during 2014. This telescope will be the first
Cherenkov telescope adopting the double reflection layout in a
Schwarzschild-Couder configuration with a tessellated primary mirror and a
monolithic secondary mirror. The collected light will be focused on a compact
and light-weight camera based on silicon photo-multipliers covering a 9.6 deg
full field of view. Detailed Monte Carlo simulations have been performed to
estimate the performance of the planned telescope. The results regarding its
energy threshold, sensitivity and angular resolution are shown and discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
The Origin of the Cosmic Soft X-Ray Background: Optical Identification of an Extremely Deep ROSAT Survey
We present the results of the deepest optically identified X-ray survey yet
made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of
1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg
cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are
identified. Some (5) other sources have tentative identifications and in a
further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux
levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep
X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however,
the survey is dominated by a population of galaxies with narrow optical
emission lines (NELGs). In addition, a number of groups and clusters of
galaxies are found at intermediate fluxes. Most of these are poor systems of
low X-ray luminosity and are generally found at redshifts of > 0.3. Their
numbers are consistent with a zero evolutionary scenario, in contrast to the
situation for high luminosity clusters at the same redshift. We discuss the
significance of these results to the determination of the cosmic soft X-ray
background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved
more than 50% of the background. We also briefly consider the probable
importance of NELG objects to the residual background and look at some of the
properties of these unusual objects.Comment: 28 pages, LaTeX, 17 figures from 32 postscript files. Uses mn.sty and
epsf.sty. Accepted by MNRAS. Revised version now contains data table. For
more information see http://www.astro.soton.ac.uk/~amn/UKdee
Probing the Coevolution of Supermassive Black Holes and Galaxies Using Gravitationally Lensed Quasar Hosts
In the present-day universe, supermassive black hole masses (MBH) appear to
be strongly correlated with their galaxy's bulge luminosity, among other
properties. In this study, we explore the analogous relationship between MBH,
derived using the virial method, and the stellar R-band bulge luminosity (Lr)
or stellar bulge mass (M*) at epochs of 1 < z < 4.5 using a sample of 31
gravitationally lensed AGNs and 20 non-lensed AGNs. At redshifts z > 1.7
(10--12 Gyrs ago), we find that the observed MBH--Lr relation is nearly the
same (to within ~0.3 mag) as it is today. When the observed Lr are corrected
for luminosity evolution, this means that the black holes grew in mass faster
than their hosts, with the MBH/M* mass ratio being a factor of > 4(+2)(-1)
times larger at z > 1.7 than it is today. By the redshift range 1<z<1.7 (8-10
Gyrs ago), the MBH/M* ratio is at most two times higher than today, but it may
be consistent with no evolution. Combining the results, we conclude that the
ratio MBH/M* rises with look-back time, although it may saturate at ~6 times
the local value. Scenarios in which moderately luminous quasar hosts at z>1.7
were fully formed bulges that passively faded to the present epoch are ruled
out.Comment: ApJ accepted, includes Referee comments and statistics to better
quantify the statistical significance of results. 23 pages, 11 figures, 4
table
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