4,052 research outputs found
A Giant Sample of Giant Pulses from the Crab Pulsar
We observed the Crab pulsar with the 43-m telescope in Green Bank, WV over a
timespan of 15 months. In total we obtained 100 hours of data at 1.2 GHz and
seven hours at 330 MHz, resulting in a sample of about 95000 giant pulses
(GPs). This is the largest sample, to date, of GPs from the Crab pulsar taken
with the same telescope and backend and analyzed as one data set. We calculated
power-law fits to amplitude distributions for main pulse (MP) and interpulse
(IP) GPs, resulting in indices in the range of 2.1-3.1 for MP GPs at 1.2 GHz
and in the range of 2.5-3.0 and 2.4-3.1 for MP and IP GPs at 330 MHz. We also
correlated the GPs at 1.2 GHz with GPs from the Robert C. Byrd Green Bank
Telescope (GBT), which were obtained simultaneously at a higher frequency (8.9
GHz) over a span of 26 hours. In total, 7933 GPs from the 43-m telescope at 1.2
GHz and 39900 GPs from the GBT were recorded during these contemporaneous
observations. At 1.2 GHz, 236 (3%) MP GPs and 23 (5%) IP GPs were detected at
8.9 GHz, both with zero chance probability. Another 15 (4%) low-frequency IP
GPs were detected within one spin period of high-frequency IP GPs, with a
chance probability of 9%. This indicates that the emission processes at high
and low radio frequencies are related, despite significant pulse profile shape
differences. The 43-m GPs were also correlated with Fermi gamma-ray photons to
see if increased pair production in the magnetosphere is the mechanism
responsible for GP emission. A total of 92022 GPs and 393 gamma-ray photons
were used in this correlation analysis. No significant correlations were found
between GPs and gamma-ray photons. This indicates that increased pair
production in the magnetosphere is likely not the dominant cause of GPs.
Possible methods of GP production may be increased coherence of synchrotron
emission or changes in beaming direction.Comment: 33 pages, 10 figures, 6 tables, accepted for publication in Ap
Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage
The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close (not, vert, similar1 μm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)64−, a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale
Thermodynamics of nanodomain formation and breakdown in Scanning Probe Microscopy: Landau-Ginzburg-Devonshire approach
Thermodynamics of tip-induced nanodomain formation in scanning probe
microscopy of ferroelectric films and crystals is studied using the
Landau-Ginzburg-Devonshire phenomenological approach. The local redistribution
of polarization induced by the biased probe apex is analyzed including the
effects of polarization gradients, field dependence of dielectric properties,
intrinsic domain wall width, and film thickness. The polarization distribution
inside subcritical nucleus of the domain preceding the nucleation event is very
smooth and localized below the probe, and the electrostatic field distribution
is dominated by the tip. In contrast, polarization distribution inside the
stable domain is rectangular-like, and the associated electrostatic fields
clearly illustrate the presence of tip-induced and depolarization field
components. The calculated coercive biases of domain formation are in a good
agreement with available experimental results for typical ferroelectric
materials. The microscopic origin of the observed domain tip elongation in the
region where the probe electric field is much smaller than the intrinsic
coercive field is the positive depolarization field in front of the moving
counter domain wall. For infinitely thin domain walls local domain breakdown
through the sample depth appears. The results obtained here are complementary
to the Landauer-Molotskii energetic approach.Comment: 35 pages, 8 figures, suplementary attached, to be submitted to Phys.
Rev.
Potentiometric Multisensory Systems with Novel Ion-Exchange Polymer-Based Sensors for Analysis of Drugs
This paper examines potentiometric multisensory systems that consist of novel cross-sensitive PD-sensors (Potential Donnansensors). The analytical signal of PD-sensors is the Donnan potential at the ion-exchange polymer/electrolyte test solution interface. The use of novel sensors for the quantitative analysis of multicomponent aqueous solutions of amino acids, vitamins and medical substances is based on protolytic and ion-exchange reactions at the interfaces of ion-exchangers and test solutions. The potentiometric sensor arrays consist of PD-sensors and ion-selective electrodes. Such systems were developed for the multicomponent quantitative analysis of lysine monohydrochloride, thiamine chloride and novocaine hydrochloride solutions that contained salts of alkaline and alkaline-earth metals, as well as for mixed solutions of nicotinic acid and pyridoxine hydrochloride. Multivariate methods of analysis were used for sensor calibration and the analysis of the total response of sensor arrays. The errors of measurement of the electrolytes in aqueous solutions did not exceed 10%. The developed multisensory systems were used to determine the composition of a therapeutic "Mineral salt with low content of sodium chloride" and to determine concentrations of novocaine in sewage samples from a dental clinic
The screening effects influence on the nano-domain tailoring in ferroelectrics-semiconductors
We calculate the realistic sizes of nano-domains recorded by the electric
field of atomic force microscope tip in BaTiO3 and LiNbO3
ferroelectric-semiconductors in contrast to the over-estimated ones obtained in
the previous works. We modified the existing models of domain formation
allowing for the Debye screening, recharging of sluggish surface screening
layers caused by emission current between the tip apex and the domain butt
surface and the redistribution of domain depolarization field induced by the
charged tip apex. We have shown that the depolarization field energy of the
domain butt, Debye screening effects and field emission at high voltages lead
to the essential decrease of the equilibrium domain sizes. We obtained, that
the domain length and radius do not decrease continuously with voltage
decrease: the domain appears with non-zero length and radius at definite
critical voltage. Such "threshold" domain formation is similar to the first
order phase transition and correlates with recent theoretical and experimental
investigations.Comment: 20 pages, 4 figures, new references are added, content is changed,
sent to Physica Status Solid
Secondary somatic mutations restoring RAD51C and RAD51D associated with acquired resistance to the PARP inhibitor rucaparib in high-grade ovarian carcinoma
High-grade epithelial ovarian carcinomas (OC) containing mutated BRCA1 or BRCA2 (BRCA1/2) homologous recombination (HR) genes are sensitive to platinum-based chemotherapy and poly(ADP-ribose) polymerase inhibitors (PARPi), while restoration of HR function due to secondary mutations in BRCA1/2 has been recognized as an important resistance mechanism. We sequenced core HR pathway genes in 12 pairs of pre-treatment and post-progression tumor biopsy samples collected from patients in ARIEL2 Part 1, a phase 2 study of the PARPi rucaparib as treatment for platinum-sensitive, relapsed OC. In six of 12 pre-treatment biopsies, a truncation mutation in BRCA1, RAD51C or RAD51D was identified. In five of six paired post-progression biopsies, one or more secondary mutations restored the open reading frame. Four distinct secondary mutations and spatial heterogeneity were observed for RAD51C. In vitro complementation assays and a patient-derived xenograft (PDX), as well as predictive molecular modeling, confirmed that resistance to rucaparib was associated with secondary mutations
- …