Use of diamond sensors for a high-flux, high-rate X-ray pass-through diagnostic

X-ray free-electron lasers (XFELs) deliver pulses of coherent X-rays on the femtosecond time scale, with potentially high repetition rates. While XFELs provide high peak intensities, both the intensity and the centroid of the beam fluctuate strongly on a pulse-to-pulse basis, motivating high-rate beam diagnostics that operate over a large dynamic range. The fast drift velocity, low X-ray absorption and high radiation tolerance properties of chemical vapour deposition diamonds make these crystals a promising candidate material for developing a fast (multi-GHz) pass-through diagnostic for the next generation of XFELs. A new approach to the design of a diamond sensor signal path is presented, along with associated characterization studies performed in the XPP endstation of the LINAC Coherent Light Source (LCLS) at SLAC. Qualitative charge collection profiles (collected charge versus time) are presented and compared with those from a commercially available detector. Quantitative results on the charge collection efficiency and signal collection times are presented over a range of approximately four orders of magnitude in the generated electron-hole plasma density.

Perfect absorption in GaAs metasurfaces near the bandgap edge

Perfect optical absorption occurs in a metasurface that supports two degenerate and critically-coupled modes of opposite symmetry. The challenge in designing a perfectly absorbing metasurface for a desired wavelength and material stems from the fact that satisfying these conditions requires multi-dimensional optimization often with parameters affecting optical resonances in non-trivial ways. This problem comes to the fore in semiconductor metasurfaces operating near the bandgap wavelength, where intrinsic material absorption varies significantly. Here we devise and demonstrate a systematic process by which one can achieve perfect absorption in GaAs metasurfaces for a desired wavelength at different levels of intrinsic material absorption, eliminating the need for trial and error in the design process. Using this method, we show that perfect absorption can be achieved not only at wavelengths where GaAs exhibits high absorption, but also at wavelengths near the bandgap edge. In this region, absorption is enhanced by over one order of magnitude compared a layer of unstructured GaAs of the same thickness.

Dual-gate operation and carrier transport in SiGe p-n junction nanowires

We investigate carrier transport in silicon-germanium nanowires with an axial p-n junction doping profile by fabricating these wires into transistors that feature separate top gates over each doping segment. By independently biasing each gate, carrier concentrations in the n- and p-side of the wire can be modulated. For these devices, which were fabricated with nickel source-drain electrical contacts, holes are the dominant charge carrier, with more favorable hole injection occurring on the p-side contact. Channel current exhibits greater sensitivity to the n-side gate, and in the reverse biased source-drain configuration, current is limited by the nickel/n-side Schottky contact.

Application and experimental validation of an integral method for simulation of gradient-induced eddy currents on conducting surfaces during magnetic resonance imaging

The time-varying magnetic fields created by the gradient coils in magnetic resonance imaging can produce negative effects on image quality and the system itself. Additionally, they can be a limiting factor to the introduction of non-MR devices such as cardiac pacemakers, orthopedic implants, and surgical robotics. The ability to model the induced currents produced by the switching gradient fields is key to developing methods for reducing these unwanted interactions. In this work, a framework for the calculation of induced currents on conducting surface geometries is summarized. This procedure is then compared to two separate experiments: (1) the analysis of the decay of currents induced upon a conducting cylinder by an insert gradient set within a head only 7 T MR scanner; and (2) analysis of the heat deposited into a small conductor by a uniform switching magnetic field at multiple frequencies and two distinct conductor thicknesses. The method was shown to allow the accurate modeling of the induced time-varying field decay in the first case, and was able to provide accurate estimation of the rise in temperature in the second experiment to within 30% when the skin depth was greater than or equal to the thickness of the conductor.

Fabrication of a nanostructure thermal property measurement platform

Measurements of the electrical and thermal transport properties of one-dimensional nanostructures (e.g. nanotubes and nanowires) are typically obtained without detailed knowledge of the specimen's atomic-scale structure or defects. To address this deficiency, we have developed a microfabricated, chip-based characterization platform that enables both transmission electron microscopy (TEM) of the atomic structure and defects as well as measurement of the thermal transport properties of individual nanostructures. The platform features a suspended heater line that physically contacts the center of a suspended nanostructure/nanowire that was placed using in situ scanning electron microscope nanomanipulators. Suspension of the nanostructure across a through-hole enables TEM characterization of the atomic and defect structure (dislocations, stacking faults, etc) of the test sample. This paper explains, in detail, the processing steps involved in creating this thermal property measurement platform. As a model study, we report the use of this platform to measure the thermal conductivity and defect structure of a GaN nanowire.

A hot-wire probe for thermal measurements of nanowires and nanotubes inside a transmission electron microscope

A hot wire probe has been developed for use inside a transmission electron microscope to measure the thermal resistance of individual nanowires, nanotubes, and their contacts. No microfabrication is involved. The probe is made from a platinum Wollaston wire and is pretensioned to minimize the effects of thermal expansion, intrinsic thermal vibrations, and Lorentz forces. An in situ nanomanipulator is used to select a particular nanowire or nanotube for measurement, and contacts are made with liquid metal droplets or by electron-beam induced deposition. Detailed thermal analysis shows that for best sensitivity, the thermal resistance of the hot-wire probe should be four times that of the sample, but a mismatch of more than two orders of magnitude may be acceptable. Data analysis using the ratio of two ac signals reduces the experimental uncertainty. The range of detectable sample thermal resistances spans from approximately 10(3) to 10(9) KW. The probe can also be adapted for measurements of the electrical conductance and Seebeck coefficient of the same sample. The probe was used to study a multiwalled carbon nanotube with liquid Ga contacts. The measured thermal resistance of 3.3 x 10(7) KW had a noise level of approximately +/-3% and was repeatable to within +/-10% upon breaking and re-making the contact.

Near-diffraction-limited diode laser arrays by wavelength beam combining

We demonstrate 35 W output peak power with M2 approximately 1.35 in both dimensions from a 100 element, 100 microm pitch slab-coupled optical waveguide laser diode array using wavelength beam combining.

Comparison of bone graft matrices for human mesenchymal stem cell-directed osteogenesis

Scaffolds to support cell-based tissue engineering are critical determinants of clinical efforts to regenerate and repair the body. Bone tissue engineering requires materials that are biocompatible, well vascularized, mechanically suited for bone function, integrated with the host skeleton, and support osteoinduction of the implanted cells that form new bone. The aim of this study was to compare the osteogenic potential of bone marrow-derived, culture expanded human mesenchymal stem cells (hMSCs) adherent to different scaffolds composed of various calcium phosphates. Cells were loaded onto 2 x 2 mm cubes of coral calcium carbonate-derived apatite, bovine bone-derived apatite, synthetic hydroxyapatite (HA)/tricalcium phosphate (TCP) (60:40%), or synthetic HA/TCP (20:80%) and placed into the dorsum of SCID mice for 5 weeks. Subsequent histomorphometric analysis of bone formation within the cubes revealed the absence of bone formation within the coral-derived apatite and the bovine bone-derived apatite. Bone formation within synthetic HA/TCP scaffolds was measured to be 8.8% (+/-2.7%) and 13.8% (+/-3.6%) of the total tissue present for the 60:40% and 20:80% materials, respectively. Minimal resorption was observed at this early time point. Scanning electron microscopy evaluation of loaded scaffolds indicates that cell loading was not a variable affecting the different bone formation outcomes in these four scaffolds. In this ectopic model, different apatite-containing scaffolds of similar morphology and porosity demonstrated marked differences in their ability to support osteoinduction by implanted hMSCs. The necessary induction of hMSCs along the osteoblastic lineage may be dependent, in part, on the local microenvironment established by the scaffold chemistry and interactions with the host.

Women, marital status, and symptoms of depression in a midlife national sample

Previous studies of the correlates of depression among women have not generally been based on adequate midlife samples or precision in the specification of marital status categories. The present analysis is designed to address these deficiencies and is based on data from the Health and Retirement Survey-Wave 1 (i.e., respondents 51 to 61 years old). Results indicate that married women are less likely to report symptoms of depression than their unmarried counterparts. The mental health benefits of marriage are greater for men than for women. Moreover, other variables, such as marital satisfaction, self-rated health, and employment status are more powerful predictors of emotional well-being in midlife than marital status per se. The quality of marriage affects depressive symptoms more strongly for women than men.

Incipient analysis of mesenchymal stem-cell-derived osteogenesis

Tissue regeneration strategies invoke cell-based therapies for effective tissue formation. Current assessment of mesenchymal stem cell (MSC) directed bone regeneration during in vivo assays is dependent on histologic determination of bone formation. It was the aim of this study to determine the relationship between bone sialoprotein (BSP) expression and osteocalcin expression with subsequent osteogenesis occurring in MSC-based implants. RT-PCR assessment of human actin, collagen type I, BSP, and osteocalcin indicated that undifferentiated cells did not express BSP or osteocalcin. Three weeks following implantation, human BSP could be identified in RNAs isolated from the retrieved implants. For every implant from which human BSP cDNA was amplified, parallel implants harvested at 6 weeks demonstrated bone formation at the histologic level. This study confirms that, in the context of the severe combined immunodeficiency disease (SCID) mouse model, culture-expanded, cryopreserved human MSCs have osteogenic potential and demonstrates that implanted cell gene expression can reveal the early onset of bone formation.

Settlement has many faces: physicians, attorneys and medical malpractice

We conduct an analysis of the jurisdictional dispute over the management of medical malpractice lawsuits, focusing on the process through which liability is defined. We utilize a North Carolina sample of physicians who have been sued, their defense counsel, and counsel for the plaintiff in the case. A comparison of the perspectives of these three parties reveals that over half of the physicians who settle perceive themselves as not liable. Defense counsel are more adept at predicting both negotiated resolutions and whether or not money will be paid than either plaintiffs' counsel or physicians. Almost two-thirds of physicians who thought they were not liable expressed a desire for vindication. Almost half the time when the physicians denied liability money was nonetheless paid to resolve the claim. Physician responses to the outcome of their cases focus on the need for reform, especially in terms of a call for peer or expert review. We identify and discuss culture conflict between law and medicine. For lawyers "settlement" is not a negative thing, but for physicians it implies fault. We challenge existing literature which analyzes the settlement of medical malpractice claims solely in terms of rational economic models, and we argue that social psychological variables are equally important.