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Notes: We report the observation by photoluminescence (PL) spectroscopy of cross-barrier recombination between spatially separated two-dimensional electron and hole gases confined respectively in the quantum well (QW) and collector accumulation layer of a GaAs/AlGaAs double-barrier resonant tunneling structure. At the onset of the n=3(E3) resonance in the current–voltage characteristic, the energy of the cross-barrier transition Ecr is found to coincide with that of the PL peak arising from recombination of electrons from the E3 confined level in the QW with n=1 confined hole states (E3lh recombination). Similarly, at the onset of the E4 resonance, Ecr≈E4lh. We show that this behavior arises as a consequence of the symmetrical potential distribution within the structure at the onsets of the resonances. © 1996 American Institute of Physics.

Notes: Theoretical results recently obtained for polarizability of intersecting spherical particles have been compared with those for polarizability of intersecting cardioidal particles resembling the spherical shape. Calculations based on a modified Parseval Rule have been performed for a number of metals in a dielectric matrix in a quasistatic approximation. A large difference in the polarizabilities has been found in spite of a striking similarity in their shapes. An unexpected small value for both the real and the imaginary part of the polarizability of cardioidal shaped metal particles occurs at wavelengths where simple metal spheres absorb strongly. A Maxwell-Garnett model has been used to compare the optical properties of composite materials containing the cardioidal and intersecting spherical particles of interest. It has been found that a composite containing cardioid shaped particles in a dielectric matrix can have an unusual response. Its absorption is very close to zero, while the refractive index is close to that of the fully dense host. © 1996 American Institute of Physics.

Notes: We have employed Langmuir monolayers of polydimethylsiloxane-polystyrene diblock copolymers as a model system for examining layers of tethered chains under good solvent conditions. The range of surface density accessible with this system coincides with the ranges reported in the literature for chains tethered onto solid substrates from dilute solution in good solvents. We have varied both the surface density (σ) and the molecular weight (M) of the submerged polystyrene block independently, covering over a decade in each variable. Both the form of the segmental concentration profile and the variation of the layer height with σ and M are in good agreement with numerical self-consistent-field (SCF) calculations. On the other hand, we do not observe precise agreement with the scaling predictions for strongly stretched chains, in contrast to some previous reports. Through measurements of the surface pressure (Π), we present the first direct comparison between anchoring energies and differential chain energies in tethered layers. We find these to be of equal magnitude at the desorption transition. However, the chain energies reach values roughly an order of magnitude larger than can be accounted for by the osmotic interaction of the polystyrene segments in the good solvent. In addition, the dependence of Π on σ is more consistent with a model of soft spheres with hard cores than that of a semidilute polymer mesh. The hard core areas seem to be loosely related to Rg. We attribute these observations to a steric effect which limits lateral interpenetration of the submerged blocks. The sharp rise in Π with σ is of great practical importance as it limits the maximum surface coverage in this system, and may also do so for other systems. © 1995 American Institute of Physics.

Notes: Objectives To establish the relation between fetal abdominal circumference and birthweight in a large population of fetuses; to identify whether the error in estimating birthweight by abdominal circumference varied with the magnitude of abdominal circumference; and to establish whether adding femur length to abdominal circumference caused a clinically important reduction of error in predicting birthweight.Design A retrospective study.Setting The ultrasound department of a teaching maternity hospital offering a tertiary referral service.Sample From 3512 nondiabetic women with a normally formed singleton fetus, an abdominal circumference measurement of the infant was made within seven days of delivery; of these, 1213 had a femur length measurement performed at the same time.Results There was a linear relation between abdominal circumference and birthweight. There was a strong inverse correlation between the proportional error in predicting birthweight from the abdominal circumference and the magnitude of the abdominal circumference. Both the Campbell and Wilkin equation (abdominal circumference alone) and the Hadlock equation (abdominal circumference and femur length) were associated with systematic errors, especially with larger birthweight infants. The median absolute errors for the two equations were not significantly different overall (6.98% and 6.86% respectively), although the Hadlock equation was significantly more accurate in predicting birthweight in infants weighing greater than 4500 g. However, no threshold value of abdominal circumference or of estimated fetal weight using the Hadlock equation had a positive predictive value in estimating infants of 〉 4500 g of greater than 35%.Conclusions Prediction of birthweight should be by abdominal circumference alone. 〈link href="#t1"〉Table 1 presents robust estimates of the error of predicting birthweight using fetal abdominal circumference.〈tabular xml:id="t1"〉1〈title type="main"〉 The relation between fetal abdominal circumference (AC) and birthweight (BW). 〈table colsep="0" rowsep="0" frame="topbot" pgwide="0" orient="port"〉〈tgroup cols="5" align="left"〉〈colspec colnum="1" colname="col1" align="left"/〉〈colspec colnum="2" colname="col2" align="center"/〉〈colspec colnum="3" colname="col3" align="center"/〉〈colspec colnum="4" colname="col4" align="center"/〉〈colspec colnum="5" colname="col5" align="center"/〉〈thead valign="bottom"〉〈row rowsep="1"〉AC (mm)nMedian BW(g)10th-90th centile BW (g)Range BW(g)〈tbody valign="top"〉200-20913900750-1030740-1040210-219201040830-1370780-1400220–229201060750-1410650-1460230-239281255980-1470900-1860240-2493614351200-17901080-1950250-2593715801290-19251180-2260260-2695618351490-21901340-2400270-2798920001640-23201390-2620280-28913422651920-26601530-2910290-29921925302130-29001820-3100300-30935026852340-30802010-3420310-31938728502470-32902110-3650320-32948430602700-34702350-3770330-33943932602880-37002570-3980340-34942333803040-38602670-4240350-35931436153240-40402890-4460360-36924537503330-11903020-4610370-37911738403480-43603180-4790380-3896641403660-46403470-4820390-3993542903665-46753640-5000

Notes: We describe a novel cw millimeter-wave electron spin resonance (ESR) spectrometer designed to operate in the frequency range of 80–200 GHz and in the temperature range of 2.5–300 K, which may be easily scaled to higher frequencies. The spectrometer uses a bimodal reflection cavity coupled to a circular corrugated guide and uses Gaussian quasi-optics for most of the front-end signal processing. This technique has very low insertion loss and allows a number of sophisticated measurement techniques to be employed including induction operation, which significantly reduces the effect of microphonics and stray reflections. A number of examples are given illustrating the sensitivity of the instrument and the advantages of using ESR at high fields. © 1998 American Institute of Physics.

Notes: Some important characteristics are described of one- and two-dimensional gas proportional detectors with delay line position readout which are currently in use at beam line X12B of the National Synchrotron Light Source. The importance of careful choice of geometric design parameters is emphasized with respect to position linearity and electrical stability; position resolution of 100 μm FWHM and counting rates up to several times 105 s−1 are obtained in calibration studies. Results from some recent beam-line experiments further illustrate the excellent position resolution and linearity, and demonstrate the usefulness of these detector systems for small-angle scattering, particularly in dynamic studies. © 1995 American Institute of Physics.

Notes: A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys. © 1998 American Institute of Physics.

Notes: The use of primary electron counting techniques as an alternative to the more usual parallel plate avalanche chamber that has been employed in soft x-ray scattering experiments is being investigated at the National Synchrotron Light Source. The theoretical aspects of primary electron counting and motivation behind building a primary electron counting detector are described, as well as characteristics and future improvements of the device constructed at the NSLS. The detector consists of a low electric field drift region and a low pressure multistep avalanche region which can be operated with two or three stages of electron multiplication. The device has worked well in extensive tests as a simple parallel plate avalanche chamber, providing energy resolutions of 58% and 43% at 277 and 500 eV, respectively. Operated as a primary electron counter, preliminary results show an energy resolution of 38% at 500 eV. © 1995 American Institute of Physics.

Notes: A brief description of the most common encoding methods, and their relative merits, used with position sensitive proportional detectors at synchrotron facilities will be made. While some of these systems are reaching maturity, continued development is resulting in further improvements, especially in terms of linearity and resolution. The use of detectors with special geometries for specific experiments will be described. The microstrip gas chamber will be addressed, with particular emphasis on a comparison of its capabilities with those of the multiwire proportional chamber. The development of proportional detectors intended for dynamic studies at second- and third-generation sources, with counting rate capabilities well in excess of these generally available at present, will be described. The use of gas based detectors, albeit with modest energy resolution, for photon detection in the energy regime of a few hundred electron volts will be mentioned. © 1995 American Institute of Physics.

Notes: We report measurements of the polarization and time-averaged intensity of four-wave mixing signals from GaAs quantum wells as functions of the pump beam polarizations, laser photon energy, and optical injection density, the latter covering a range of higher levels than has previously been reported. Excitation of heavy-hole excitons produced a sin2 variation of the signal intensity as the pump beam polarizations changed from parallel to perpendicular, while injection of light-hole excitons led to an approximately cos2 dependence. The tuning dependence of the heavy-hole-exciton signal revealed a low-energy signal which is attributed to biexcitons. The dominance of this signal for perpendicular polarization causes the observed sin2 variation of the heavy-hole-exciton signal. © 1995 American Institute of Physics.