740 research outputs found
Uncertainties of size measurements in electron microscopy characterization of nanomaterials in foods
Electron microscopy is a recognized standard tool for nanomaterial characterization, and recommended by the European Food Safety Authority for the size measurement of nanomaterials in food. Despite this, little data have been published assessing the reliability of the method, especially for size measurement of nanomaterials characterized by a broad size distribution and/or added to food matrices. This study is a thorough investigation of the measurement uncertainty when applying electron microscopy for size measurement of engineered nanomaterials in foods. Our results show that the number of measured particles was only a minor source of measurement uncertainty for nanomaterials in food, compared to the combined influence of sampling, sample preparation prior to imaging and the image analysis. The main conclusion is that to improve the measurement reliability, care should be taken to consider replications and matrix removal prior to sample preparation
The Clustering of Extragalactic Extremely Red Objects
We have measured the angular and spatial clustering of 671 K5
Extremely Red Objects (EROs) from a 0.98 square degree sub-region of the NOAO
Deep Wide-Field Survey (NDWFS). Our study covers nearly 5 times the area and
has twice the sample size of any previous ERO clustering study. The wide field
of view and BwRIK passbands of the NDWFS allow us to place improved constraints
on the clustering of z=1 EROs. We find the angular clustering of EROs is
slightly weaker than in previous measurements, and w(1')=0.25+/-0.05 for
K<18.40 EROs. We find no significant correlation of ERO spatial clustering with
redshift, apparent color or absolute magnitude, although given the
uncertainties, such correlations remain plausible. We find the spatial
clustering of K5 EROs is well approximated by a power-law, with
r_0=9.7+/-1.1 Mpc/h in comoving coordinates. This is comparable to the
clustering of 4L* early-type galaxies at z<1, and is consistent with the
brightest EROs being the progenitors of the most massive ellipticals. There is
evidence of the angular clustering of EROs decreasing with increasing apparent
magnitude, when NDWFS measurements of ERO clustering are combined with those
from the literature. Unless the redshift distribution of K>20 EROs is very
broad, the spatial clustering of EROs decreases from r_0=9.7+/-1.1 Mpc/h for
K20 EROs.Comment: Accepted for publication in the ApJ. 29 pages with 10 figures. The
NOAO Deep Wide-Field Survey Bootes data release is available online at
http://www.noao.edu/noao/noaodeep
From Stars to Super-planets: the Low-Mass IMF in the Young Cluster IC348
We investigate the low-mass population of the young cluster IC348 down to the
deuterium-burning limit, a fiducial boundary between brown dwarf and planetary
mass objects, using a new and innovative method for the spectral classification
of late-type objects. Using photometric indices, constructed from HST/NICMOS
narrow-band imaging, that measure the strength of the 1.9 micron water band, we
determine the spectral type and reddening for every M-type star in the field,
thereby separating cluster members from the interloper population. Due to the
efficiency of our spectral classification technique, our study is complete from
approx 0.7 Msun to 0.015 Msun. The mass function derived for the cluster in
this interval, dN/dlogM \propto M^{0.5}, is similar to that obtained for the
Pleiades, but appears significantly more abundant in brown dwarfs than the mass
function for companions to nearby sun-like stars. This provides compelling
observational evidence for different formation and evolutionary histories for
substellar objects formed in isolation vs. as companions. Because our
determination of the IMF is complete to very low masses, we can place
interesting constraints on the role of physical processes such as fragmentation
in the star and planet formation process and the fraction of dark matter in the
Galactic halo that resides in substellar objects.Comment: 37 pages, 16 figs, 6 tables (Table 4 is a separate LaTeX file)
Accepted for publication in Astrophysical Journal (Oct 1, 2000 issue
Simultaneous Optical Model Analyses of Elastic Scattering, Breakup, and Fusion Cross Section Data for the He + Bi System at Near-Coulomb-Barrier Energies
Based on an approach recently proposed by us, simultaneous
-analyses are performed for elastic scattering, direct reaction (DR)
and fusion cross sections data for the He+Bi system at
near-Coulomb-barrier energies to determine the parameters of the polarization
potential consisting of DR and fusion parts. We show that the data are well
reproduced by the resultant potential, which also satisfies the proper
dispersion relation. A discussion is given of the nature of the threshold
anomaly seen in the potential
172 ks Chandra Exposure of the LALA Bo\"{o}tes Field: X-ray Source Catalog
We present an analysis of a deep, 172 ks Chandra observation of the Large
Area Lyman Alpha Survey (LALA) Bo\"{o}tes field, obtained with the Advanced CCD
Imaging Spectrometer (ACIS-I) on the Chandra X-ray Observatory. This is one of
the deepest Chandra images of the extragalactic sky; only the 2 Ms CDF-N and 1
Ms CDF-S are substantially deeper. A total of 168 X-ray sources were detected.
The X-ray source counts were derived and compared with those from other Chandra
deep surveys; the hard X-ray source density of the LALA Bo\"{o}tes field is 33%
higher than that of CDF-S at the flux level of 2.0E-15 ergs/cm^2/s, confirming
the field-to-field variances of the hard band source counts reported by
previous studies. The deep exposure resolves > 72% of the 2-10 keV X-ray
background.
Our primary optical data are R-band imaging from NOAO Deep Wide-Field Survey
(NDWFS), with limiting magnitude of R = 25.7 (Vega, 3sigma, 4" diameter
aperture). We have found optical counterparts for 152 of the 168 Chandra
sources (90%). Among the R-band non-detected sources, not more than 11 of them
can possibly be at z > 5, based on the hardness ratios of their X-ray emission
and nondetections in bluer bands. The majority (~76%) of the X-ray sources are
found to have log(f_X/f_R) within 0.0+-1, which are believed to be AGNs.Most of
the X-ray faint/optically bright sources (log(f_X/f_R) < -1.0) are optically
extended, which are low-z normal galaxies or low luminosity AGNs. There is also
a population of sources which are X-ray overluminous for their optical
magnitudes (log(f_X/f_R) > 1.0), which are harder in X-ray and are probably
obscured AGNs. (abridged)Comment: AJ accepted, 37 pages, including 10 figures and 1 tabl
The IRAC Shallow Survey
The IRAC shallow survey covers 8.5 square degrees in the NOAO Deep Wide-Field
Survey in Bootes with 3 or more 30 second exposures per position. An overview
of the survey design, reduction, calibration, star-galaxy separation, and
initial results is provided. The survey includes approximately 370,000,
280,000, 38,000, and 34,000 sources brighter than the 5 sigma limits of 6.4,
8.8, 51, and 50 microJy at 3.6, 4.5, 5.8, and 8 microns respectively, including
some with unusual spectral energy distributions.Comment: To appear in ApJS, Spitzer special issue. For full resolution see
http://cfa-www.harvard.edu/irac/publication
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