4,076 research outputs found
Temperature perturbation model of the opto-galvanic effect in CO2-laser discharges
A detailed discharge model of the opto-galvanic effect in molecular laser gas mixtures is developed based on the temperature perturbation or discharge cooling mechanism of Smith and Brooks (1979). Excellent agreement between the model and experimental results in CO2 laser gas mixtures is obtained. The model should be applicable to other molecular systems where the OGE is being used for laser stabilisation and as a spectroscopic tool
A Search for 6.7 GHz Methanol Masers in M33
We report the negative results from a search for 6.7 GHz methanol masers in
the nearby spiral galaxy M33. We observed 14 GMCs in the central 4 kpc of the
Galaxy, and found 3 sigma upper limits to the flux density of ~9 mJy in
spectral channels having a velocity width of 0.069 km/s. By velocity shifting
and combining the spectra from the positions observed, we obtain an effective
3sigma upper limit on the average emission of ~1mJy in a 0.25 km/s channel.
These limits lie significantly below what we would expect based on our
estimates of the methanol maser luminosity function in the Milky Way. The most
likely explanation for the absence of detectable methanol masers appears to be
the metallicity of M33, which is modestly less than that of the Milky Way
Full characterization and analysis of a terahertz heterodyne receiver based on a NbN hot electron bolometer
We present a complete experimental characterization of a quasioptical twin-slot antenna coupled small area (1.0×0.15 µm^2) NbN hot electron bolometer (HEB) mixer compatible with currently available solid state tunable local oscillator (LO) sources. The required LO power absorbed in the HEB is analyzed in detail and equals only 25 nW. Due to the small HEB volume and wide antenna bandwidth, an unwanted direct detection effect is observed which decreases the apparent sensitivity. Correcting for this effect results in a receiver noise temperature of 700 K at 1.46 THz. The intermediate frequency (IF) gain bandwidth is 2.3 GHz and the IF noise bandwidth is 4 GHz. The single channel receiver stability is limited to 0.2–0.3 s in a 50 MHz bandwidth
Knowledge Rich Natural Language Queries over Structured Biological Databases
Increasingly, keyword, natural language and NoSQL queries are being used for
information retrieval from traditional as well as non-traditional databases
such as web, document, image, GIS, legal, and health databases. While their
popularity are undeniable for obvious reasons, their engineering is far from
simple. In most part, semantics and intent preserving mapping of a well
understood natural language query expressed over a structured database schema
to a structured query language is still a difficult task, and research to tame
the complexity is intense. In this paper, we propose a multi-level
knowledge-based middleware to facilitate such mappings that separate the
conceptual level from the physical level. We augment these multi-level
abstractions with a concept reasoner and a query strategy engine to dynamically
link arbitrary natural language querying to well defined structured queries. We
demonstrate the feasibility of our approach by presenting a Datalog based
prototype system, called BioSmart, that can compute responses to arbitrary
natural language queries over arbitrary databases once a syntactic
classification of the natural language query is made
The Arecibo Methanol Maser Galactic Plane Survey - II: Statistical and Multi-wavelength Counterpart Analysis
We present an analysis of the properties of the 6.7 GHz methanol maser sample
detected in the Arecibo Methanol Maser Galactic Plane Survey. The distribution
of the masers in the Galaxy, and statistics of their multi-wavelength
counterparts is consistent with the hypothesis of 6.7 GHz maser emission being
associated with massive young stellar objects. Using the detection statistics
of our survey, we estimate the minimum number of methanol masers in the Galaxy
to be 1275. The l-v diagram of the sample shows the tangent point of the
Carina-Sagittarius spiral arm to be around 49.6 degrees, and suggests
occurrence of massive star formation along the extension of the Crux-Scutum
arm. A Gaussian component analysis of the maser spectra shows the mean
line-width to be 0.38 km/s which is more than a factor of two larger than what
has been reported in the literature. We also find no evidence that faint
methanol masers have different properties than those of their bright
counterparts.Comment: Accepted by ApJ; Revised footnote number 3 on page 8 based on private
communicatio
K-shell photoionization of ground-state Li-like boron ions [B]: Experiment and Theory
Absolute cross sections for the K-shell photoionization of ground-state
Li-like boron [B(1s2s S)] ions were measured by employing the
ion-photon merged-beams technique at the Advanced Light Source synchrotron
radiation facility. The energy ranges 197.5--200.5 eV, 201.9--202.1 eV of the
[1s(2s\,2p)P]P and [1s(2s\,2p)P] P
resonances, respectively, were investigated using resolving powers of up to
17\,600. The energy range of the experiments was extended to about 238.2 eV
yielding energies of the most prominent
[1s(2\,n)]P resonances with an absolute accuracy
of the order of 130 ppm. The natural linewidths of the [1s(2s\,2p)P]
P and [1s(2s\,2p)P] P resonances were measured
to be meV and meV, respectively, which compare
favourably with theoretical results of 4.40 meV and 30.53 meV determined using
an intermediate coupling R-matrix method.Comment: 6 figures and 2 table
Dynamic buckling and fragmentation in brittle rods
We present experiments on the dynamic buckling and fragmentation of slender
rods axially impacted by a projectile. By combining the results of Saint-Venant
and elastic beam theory, we derive a preferred wavelength lambda for the
buckling instability, and experimentally verify the resulting scaling law for a
range of materials including teflon, dry pasta, glass, and steel. For brittle
materials, buckling leads to the fragmentation of the rod. Measured fragment
length distributions show two clear peaks near lambda/2 and lambda/4. The
non-monotonic nature of the distributions reflect the influence of the
deterministic buckling process on the more random fragmentation processes.Comment: 4 pages, 5 figures, submitted to Physical Review Letter
Assessment of the Radiological Impact of the Inactive Uranium-Mill Tailings at Mexican Hat, Utah
High surface soil concentrations of 226Ra and high above-ground measurements of gamma-ray intensity in the vicinity of the inactive uranium-mill tailings at Mexican Hat show both wind and water erosion of the tailings. The former mill area, occupied by a trade school at the time of this survey, shows a comparatively high level of contamination, probably from unprocessed ore on the surface of the ore storage area near the location of the former mill buildings. However, the estimated health effect of exposure to gamma rays during a 2000-hr work year in the area represents an increase of 0.1% in the risk of death from cancer. Exposure of less than 600 persons within 1.6 km of the tailings to radon daughters results in an estimated 0.2%/year increase in risk of lung cancer.
It appears that nothing has been done to implement the recommendation of an earlier (1968) survey that tailings at this site be stabilized or that access be restricted with periodic monitoring to assure that population exposures are acceptably low. Data provided in this report and in an engineering assessment report by Ford, Bacon and Davis Utah Inc. will provide guidance for remedial action if that should be undertaken
The AzTEC mm-Wavelength Camera
AzTEC is a mm-wavelength bolometric camera utilizing 144 silicon nitride
micromesh detectors. Herein we describe the AzTEC instrument architecture and
its use as an astronomical instrument. We report on several performance metrics
measured during a three month observing campaign at the James Clerk Maxwell
Telescope, and conclude with our plans for AzTEC as a facility instrument on
the Large Millimeter Telescope.Comment: 13 pages, 15 figures, accepted for publication in Monthly Notice
Self-diffusion in granular gases
The coefficient of self-diffusion for a homogeneously cooling granular gas
changes significantly if the impact-velocity dependence of the restitution
coefficient is taken into account. For the case of a constant
the particles spread logarithmically slow with time, whereas the
velocity dependent coefficient yields a power law time-dependence. The impact
of the difference in these time dependences on the properties of a freely
cooling granular gas is discussed.Comment: 6 pages, no figure
- …