Polarization dependence of x-ray absorption spectra in Na_xCoO_2

In order to shed light on the electronic structure of Na_xCoO_2, and motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a CoO_6 cluster including all interactions between 3d orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states obtained by promotion of a Co 2p electron to a 3d electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large Co-O covalency and cubic crystal field effects, that result in low spin Co 3d configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure

Bobcats Do Not Exhibit Rub Response Despite Presence at Hair Collection Stations

We evaluated the detection rate for hair snare sampling for bobcats (Lynx rufus) using colocated hair snares and infrared-triggered cameras at 20 locations on private property in eastern Texas. Hair snares and cameras were placed together at survey stations that included both visual and olfactory attractants. In 1,680 trap-nights we photographically documented 15 visits by bobcats but collected only one bobcat hair sample. Our observations suggested limited rubbing behavior by bobcats at hair snares despite presence at hair collection stations. The explanation for this behavior remains unclear but is consistent with observations of low and variable hair trap success for this species. Although presence of other carnivores, especially gray fox, may inhibit rub response of felids, we did not document gray foxes at our hair collection stations. Low rub frequency may limit the utility of these techniques for bobcats and we suggest that alternate techniques such as camera surveys or fecal DNA collection may be more suitable

Estimating Bobcat Abundance in East Texas Using Infrared-triggered Cameras

Reliable population estimation techniques for cryptic forest predators generally are lacking. Development of an efficient and reliable technique to estimate predator abundance directly would be a valuable tool for wildlife managers concerned with predator management. We evaluated the potential for camera survey techniques to provide abundance estimates for bobcats (Lynx rufus) in southeastern forest habitats. We also determined our ability to capture other forest carnivores photographically using these techniques. We used TrailMaster 1500 active infrared-triggered cameras to estimate abundance on a 1318-ha private land holding in eastern Texas. Camera stations were located along roads and wildlife travel corridors using a 65-ha block grid overlaying the property. We established 20 camera stations yielding a mean coverage of approximately 1 camera/65.9 ha. All camera stations were baited with bobcat urine and a visual attractant and monitored for 12 weeks. We recorded 15 bobcat photographic events of seven separate individuals that were identified using spot pattern and other distinctive markings. These data were used to derive a population abundance estimate of seven bobcats during the 12-week study period using the computer program CAPTURE. This corresponded to a density of 0.29–0.58 bobcats/km2, which compared favorably with other studies conducted in similar habitats. In addition to providing a monitoring technique for bobcats, photographic survey techniques could be adapted for monitoring other cryptic carnivores

Hammerhead, an ultrahigh resolution ePix camera for wavelength-dispersive spectrometers

Wavelength-dispersive spectrometers (WDS) are often used in synchrotron and FEL applications where high energy resolution (in the order of eV) is important. Increasing WDS energy resolution requires increasing spatial resolution of the detectors in the dispersion direction. The common approaches with strip detectors or small pixel detectors are not ideal. We present a novel approach, with a sensor using rectangular pixels with a high aspect ratio (between strips and pixels, further called "strixels"), and strixel redistribution to match the square pixel arrays of typical ASICs while avoiding the considerable effort of redesigning ASICs. This results in a sensor area of 17.4 mm x 77 mm, with a fine pitch of 25 $\mu$m in the horizontal direction resulting in 3072 columns and 176 rows. The sensors use ePix100 readout ASICs, leveraging their low noise (43 e$^-$, or 180 eV rms). We present results obtained with a Hammerhead ePix100 camera, showing that the small pitch (25 $\mu$m) in the dispersion direction maximizes performance for both high and low photon occupancies, resulting in optimal WDS energy resolution. The low noise level at high photon occupancy allows precise photon counting, while at low occupancy, both the energy and the subpixel position can be reconstructed for every photon, allowing an ultrahigh resolution (in the order of 1 $\mu$m) in the dispersion direction and rejection of scattered beam and harmonics. Using strixel sensors with redistribution and flip-chip bonding to standard ePix readout ASICs results in ultrahigh position resolution ($\sim$1 $\mu$m) and low noise in WDS applications, leveraging the advantages of hybrid pixel detectors (high production yield, good availability, relatively inexpensive) while minimizing development complexity through sharing the ASIC, hardware, software and DAQ development with existing versions of ePix cameras.Comment: 8 pages, 6 figure

Zones of Gelatinous Fibers in Populus Balsamifera L.

Although balsam poplar (Populus balsamifera L.) is a logical supplement to the aspen (Fopulus tremuloides Michx.) resource for the manufacture of oriented strandboard (OSB), it has not been utilized extensively because of reported machining problems. The machining difficulties usually have been attributed to fibers with thick gelatinous layers in their cell walls. This study employed scanning electron microscopy (SEM) to observe the cell-wall structure of balsam poplar grown in Minnesota. In a previous study, balsam poplar samples were identified as difficult to waferize on the basis of wood fibers plugging the waferizing knives. The balsam poplar samples that were difficult to waferize often contained areas that appeared as "white rings" to the naked eye. Observation with the scanning electron microscope revealed that the "white rings" in the wood were zones that contained very high concentrations of gelatinous fibers. In these fibers, the cell wall typically consisted of a gelatinous layer that occupied 30 to 90% of the cell wall. In most cases, the initiation and termination of zones with high concentrations of gelatinous fibers took place within one annual growth increment or slightly more. Additional observation of trees within the sample showed that the "white rings" and the accompanying high concentrations of gelatinous layers were usually restricted to one side of the tree. This observation has led us to believe that the "white ring" areas and the corresponding zones of gelatinous fibers were the result of tension wood formation in the balsam poplar trees and not a part of normal wood formation

The temporal pattern and lifestyle associations of respiratory virus infection in a cohort study spanning the first two years of life

Background: Respiratory virus infection is common in early childhood, and children may be symptomatic or symptom-free. Little is known regarding the association between symptomatic/asymptomatic infection and particular clinical factors such as breastfeeding as well as the consequences of such infection. Method: We followed an unselected cohort of term neonates to two years of age (220 infants at recruitment, 159 who remained in the study to 24 months), taking oral swabs at birth and oropharyngeal swabs at intervals subsequently (at 1.5, 6, 9, 12, 18 and 24 months and in a subset at 3 and 4.5 months) while recording extensive metadata including the presence of respiratory symptoms and breastfeeding status. After 2 years medical notes from the general practitioner were inspected to ascertain whether doctor-diagnosed wheeze had occurred by this timepoint. Multiplex PCR was used to detect a range of respiratory viruses: influenza (A&B), parainfluenza (1–4), bocavirus, human metapneumovirus, rhinovirus, coronavirus (OC43, 229E, NL63, HKU1), adenovirus, respiratory syncytial virus (RSV), and polyomavirus (KI, WU). Logistic regression and generalised estimating equations were used to identify associations between clinical factors and virus detection. Results: Overall respiratory viral incidence increased with age. Rhinovirus was the virus most frequently detected. The detection of a respiratory virus was positively associated with respiratory symptoms, male sex, season, childcare and living with another child. We did not observe breastfeeding (whether assessed as the number of completed months of breastfeeding or current feed status) to be associated with the detection of a respiratory virus. There was no association between early viral infection and doctor-diagnosed wheeze by age 2 years. Conclusion: Asymptomatic and symptomatic viral infection is common in the first 2 years of life with rhinovirus infection being the most common. Whilst there was no association between early respiratory viral infection and doctor-diagnosed wheeze, we have not ruled out an association of early viral infections with later asthma, and long-term follow-up of the cohort continues

Secondary aerosol formation from atmospheric reactions of aliphatic amines

Although aliphatic amines have been detected in both urban and rural atmospheric aerosols, little is known about the chemistry leading to particle formation or the potential aerosol yields from reactions of gas-phase amines. We present here the first systematic study of aerosol formation from the atmospheric reactions of amines. Based on laboratory chamber experiments and theoretical calculations, we evaluate aerosol formation from reaction of OH, ozone, and nitric acid with trimethylamine, methylamine, triethylamine, diethylamine, ethylamine, and ethanolamine. Entropies of formation for alkylammonium nitrate salts are estimated by molecular dynamics calculations enabling us to estimate equilibrium constants for the reactions of amines with nitric acid. Though subject to significant uncertainty, the calculated dissociation equilibrium constant for diethylammonium nitrate is found to be sufficiently small to allow for its atmospheric formation, even in the presence of ammonia which competes for available nitric acid. Experimental chamber studies indicate that the dissociation equilibrium constant for triethylammonium nitrate is of the same order of magnitude as that for ammonium nitrate. All amines studied form aerosol when photooxidized in the presence of NOx with the majority of the aerosol mass present at the peak of aerosol growth consisting of aminium (R3NH+) nitrate salts, which repartition back to the gas phase as the parent amine is consumed. Only the two tertiary amines studied, trimethylamine and triethylamine, are found to form significant non-salt organic aerosol when oxidized by OH or ozone; calculated organic mass yields for the experiments conducted are similar for ozonolysis (15% and 5% respectively) and photooxidation (23% and 8% respectively). The non-salt organic aerosol formed appears to be more stable than the nitrate salts and does not quickly repartition back to the gas phase

Kink-induced transport and segregation in oscillated granular layers

We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counter-rotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let

Pathways of bond topology transitions at the interface of silicon nanocrystals and amorphous silica matrix

The interface chemistry of silicon nanocrystals (NCs) embedded in amorphous oxide matrix is studied through molecular dynamics simulations with the chemical environment described by the reactive force field model. Our results indicate that the Si NC-oxide interface is more involved than the previously proposed schemes which were based on solely simple bridge or double bonds. We identify different types of three-coordinated oxygen complexes, previously not noted. The abundance and the charge distribution of each oxygen complex is determined as a function of the NC size as well as the transitions among them. The oxidation at the surface of NC induces tensile strain to Si-Si bonds which become significant only around the interface, while the inner core remains unstrained. Unlike many earlier reports on the interface structure, we do not observe any double bonds. Furthermore, our simulations and analysis reveal that the interface bond topology evolves among different oxygen bridges through these three-coordinated oxygen complexes.Comment: 5 pages 6 figures 1 tabl