140,356 research outputs found
Robust visualization and discrimination of nanoparticles by interferometric imaging
Single-molecule and single-nanoparticle biosensors are a growing frontier in diagnostics. Digital biosensors are those which enumerate all specifically immobilized biomolecules or biological nanoparticles, and thereby achieve limits of detection usually beyond the reach of ensemble measurements. Here we review modern optical techniques for single nanoparticle detection and describe the single-particle interferometric reflectance imaging sensor (SP-IRIS). We present challenges associated with reliably detecting faint nanoparticles with SP-IRIS, and describe image acquisition processes and software modifications to address them. Specifically, we describe a image acquisition processing method for the discrimination and accurate counting of nanoparticles that greatly reduces both the number of false positives and false negatives. These engineering improvements are critical steps in the translation of SP-IRIS towards applications in medical diagnostics.R01 AI096159 - NIAID NIH HHSFirst author draf
Shadows and cavities in protoplanetary disks: HD163296, HD141569A, and HD150193A in polarized light
The morphological evolution of dusty disks around young (few Myr-old) stars
is pivotal to better understand planet formation. Since both dust grains and
the global disk geometry evolve on short timescale, high-resolution imaging of
a sample of objects may provide important hints towards such an evolution. We
enlarge the sample of protoplanetary disks imaged in polarized light with
high-resolution by observing the Herbig Ae/Be stars HD163296, HD141569A, and
HD150193A. We integrate our data with previous datasets to paint a larger
picture of their morphology. We report a weak detection of the disk around
HD163296 in both H and Ks band. The disk is resolved as a broken ring structure
with a significan surface brightness drop inward of 0.6 arcsec. No sign of
extended polarized emission is detected from the disk around HD141569A and
HD150193A. We propose that the absence of scattered light in the inner 0.6
arcsec around HD163296 and the non-detection of the disk around HD150193A may
be due to similar geometric factors. Since these disks are known to be flat or
only moderately flared, self-shadowing by the disk inner wall is the favored
explanation. We show that the polarized brightness of a number of disks is
indeed related to their flaring angle. Other scenarios (such as dust grain
growth or interaction with icy molecules) are also discussed. On the other
hand, the non-detection of HD141569A is consistent with previous datasets
revealing the presence of a huge cavity in the dusty disk.Comment: 10 pages, 5 figures; accepted by Astronomy & Astrophysic
Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4
Polarization measurements in X-rays can provide unique opportunity to study
the behavior of matter and radiation under extreme magnetic fields and extreme
gravitational fields. Unfortunately, over past two decades, when X-ray
astronomy witnessed multiple order of magnitude improvement in temporal,
spatial and spectral sensitivities, there is no (or very little) progress in
the field of polarization measurements of astrophysical X-rays. Recently, a
proposal has been submitted to ISRO for a dedicated small satellite based
experiment to carry out X-ray polarization measurement, which aims to provide
the first X-ray polarization measurements since 1976. This experiment will be
based on the well known principle of polarization measurement by Thomson
scattering and employs the baseline design of a central low Z scatterer
surrounded by X-ray detectors to measure the angular intensity distribution of
the scattered X-rays. The sensitivity of such experiment is determined by the
collecting area, scattering and detection efficiency, X-ray detector
background, and the modulation factor. Therefore, it is necessary to carefully
select the scattering geometry which can provide the highest modulation factor
and thus highest sensitivity within the specified experimental constraints. The
effective way to determine optimum scattering geometry is by studying various
possible scattering geometries by means of Monte Carlo simulations. Here we
present results of our detailed comparative study based on Geant4 simulations
of five different scattering geometries which can be considered within the
weight and size constraints of the proposed small satellite based X-ray
polarization measurement experiment.Comment: 14 pages, 6 figures, accepted for publication in "Nuclear Inst. and
Methods in Physics Research, A
Hard x-ray polarimeter for gamma-ray bursts and solar flares
We report on the development of a dedicated polarimeter design that is capable of studying the linear polarization of hard X-rays (50-300 keV) from gamma-ray bursts and solar flares. This compact design, based on the use of a large area position-sensitive PMT (PSPMT), is referred to as GRAPE (Gamma-RAy Polarimeter Experiment). The PSPMT is used to determine the Compton interaction location within an array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. One feature of GRAPE that is especially attractive for studies of gamma-ray bursts is the significant off-axis response (at angles \u3e 60 degrees). The modular nature of this design lends itself toward its accomodation on a balloon or spacecraft platform. For an array of GRAPE modules, sensitivity levels below a few percent can be achieved for both gamma-ray bursts and solar flares. Here we report on the latest results from the testing of a laboratory science model
Hard X-ray polarimetry with Caliste, a high performance CdTe based imaging spectrometer
Since the initial exploration of soft gamma-ray sky in the 60's, high-energy
celestial sources have been mainly characterized through imaging, spectroscopy
and timing analysis. Despite tremendous progress in the field, the radiation
mechanisms at work in sources such as neutrons stars and black holes are still
unclear. The polarization state of the radiation is an observational parameter
which brings key additional information about the physical process. This is why
most of the projects for the next generation of space missions covering the
tens of keV to the MeV region require a polarization measurement capability. A
key element enabling this capability is a detector system allowing the
identification and characterization of Compton interactions as they are the
main process at play. The hard X-ray imaging spectrometer module, developed in
CEA with the generic name of Caliste module, is such a detector. In this paper,
we present experimental results for two types of Caliste-256 modules, one based
on a CdTe crystal, the other one on a CdZnTe crystal, which have been exposed
to linearly polarized beams at the European Synchrotron Radiation Facility.
These results, obtained at 200-300 keV, demonstrate their capability to give an
accurate determination of the polarization parameters (polarization angle and
fraction) of the incoming beam. Applying a selection to our data set,
equivalent to select 90 degrees Compton scattered interactions in the detector
plane, we find a modulation factor Q of 0.78. The polarization angle and
fraction are derived with accuracies of approximately 1 degree and 5%. The
modulation factor remains larger than 0.4 when essentially no selection is made
at all on the data. These results prove that the Caliste-256 modules have
performances allowing them to be excellent candidates as detectors with
polarimetric capabilities, in particular for future space missions.Comment: 17 pages, 14 figures, 2 tables in Experimental Astronomy, 201
Recent laboratory tests of a hard x-ray solar flare polarimeter
We report on the development of a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (50 - 300 keV) from solar flares. Such measurements would be useful for studying the directivity (or beaming) of the electrons that are accelerated in solar flares. We initially used a simple prototype polarimeter to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. We have recently fabricated a science model based on a modular design concept that places a self-contained polarimeter module on the front-end of a 5-inch position- sensitive PMT (PSPMT). The PSPMT is used to determine the Compton interaction location within an annular array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. The independence of the two PMT readout schemes provides appropriate timing information for event triggering. We are currently testing this new polarimeter design in the laboratory to evaluate the performance characteristics of this design. Here we present the initial results from these laboratory tests. The modular nature of this design lends itself toward its accommodation on a balloon or spacecraft platform. A small array of such modules can provide a minimum detectable polarization (MDP) of less than 1% in the integrated 50 - 300 keV energy range for X-class solar flares
Development of a hard X-ray polarimeter for astrophysics
We have been developing a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (100-300 keV) from astrophysical sources. A laboratory prototype polarimeter has been used to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. Our design concept places a self-containedpolarimeter module on the front-end of a a 5-inch position sensitive PMT (PSPMT). We are currently working on the fabrication of a science model based on this PSPMT concept. Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our design has the advantage that it is sensitive over a rather large field-of-view (\u3e1 steradian), a feature that makes it especially attractive for γ-ray burst studies
Mueller matrix polarimetry of plasmon resonant silver nano-rods: biomedical prospects
Fundamental understanding of the light-matter interaction in the context of
nano-particles is immensely bene- fited by the study of geometry dependent
tunable Localized Surface Plasmon Resonance (LSPR) and has been demonstrated to
have potential applications in various areas of science. The polarization
characteristics of LSPR in addition to spectroscopic tuning can be suitably
exploited in such systems as contrast enhancement mech- anisms and control
parameters. Such polarization characteristics like diattenuation and retardance
have been studied here using a novel combination of Muller-matrix polarimetry
with the T-matrix matrix approach for silver nano-rods to show unprecedented
control and sensitivity to local refractive index variations. The study carried
out over various aspect ratios for a constant equal volume sphere radius shows
the presence of longitu- dinal (dipolar and quadrupolar) and transverse
(dipolar) resonances; arising due to differential contribution of
polarizabilities in two directions. The overlap regions of these resonances and
the resonances themselves exhibit enhanced retardance and diattenuation
respectively. The spectral and amplitude tunability of these polarimetric
parameters through the aspect ratios to span from the minimum to maximum ([0,
1] in the case of diattenuation and [0, {\pi}] in the case of retardance)
presents a novel result that could be used to tailor systems for study of
biological media. On the other hand, the high sensitivity of diattenuation dip
(caused by equal contribution of polarizabilities) could be possibly used for
medium characterization and bio-sensing or bio-imaging studies.Comment: 8 pages, 6 figures, Proceedings of the Saratov Fall Meeting, 201
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