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We acquire ferromagnetic resonance (FMR) spectra for an out-of-plane magnetized yttrium iron garnet sample with a vector network analyzer at microwave frequencies ranging from 4.2 to 5.2 GHz. The applied static magnetic field varies from approximately 253.6 to 260.1 kA/m. Based on an empirical model for resonant features in the S21 spectrum produced by the excitation of multiple modes and instrumental effects, we predict measured values of S21-parameters. For each of many microwave frequencies, for each of multiple modes, we determine the resonant field value of the applied magnetic field, an amplitude, and an FMR linewidth. Based on the frequency-dependent resonant field values produced by excitation of the main mode, we determine the effective magnetization and the gyromagnetic ratio of the sample. [ABSTRACT FROM AUTHOR]

In experiments in a range of felds including fast neutron spectroscopy and astroparticle physics, one can discriminate events of interest from background events based on the shapes of electronic pulses produced by energy deposits in a detector. Here, I focus on a well-known pulse shape discrimination method based on the ratio of the temporal integral of the pulse over an early interval Xp and the temporal integral over the entire pulse Xt. For both event classes, for both a Gaussian noise model and a Poisson noise model, I present analytic expressions for the conditional distribution of Xp given knowledge of the observed value of Xt and a scaled energy deposit corresponding to the product of the full energy deposit and a relative yield factor. I assume that the energy-dependent theoretical prompt fraction for both classes are known exactly. With a Bayesian approach that accounts for imperfect knowledge of the scaled energy deposit, I determine the posterior mean background acceptance probability given the target signal acceptance probability as a function of the observed value of Xt. My method enables one to determine receiver-operating-characteristic curves by numerical integration rather than by Monte Carlo simulation for these two noise models. [ABSTRACT FROM AUTHOR]

Despite their uniform crystallinity, the shape and faceting of semiconducting nanowires (NWs) can give rise to variations in structure and associated electronic properties. Here, we develop a hybrid scanning probe-based methodology to investigate local variations in electronic structure across individual n-doped GaN NWs integrated into a transistor device. We perform scanning microwave microscopy (SMM), which we combine with scanning gate microscopy to determine the free-carrier SMM signal contribution and image local charge carrier density variations. In particular, we find significant variations in free carriers across NWs, with a higher carrier density at the wire facets. By increasing the local carrier density through tip-gating, we find that the tip injects current into the NW with strongly localized current when positioned over the wire vertices. These results suggest that the strong variations in electronic properties observed within NWs have significant implications for device design and may lead to new paths to optimization. [ABSTRACT FROM AUTHOR]

We used a broadband, atomic-force-microscope-based, scanning microwave microscope (SMM) to probe the axial dependence of the charge depletion in a p-n junction within a gallium nitride nanowire (NW). SMM enables the visualization of the p-n junction location without the need to make patterned electrical contacts to the NW. Spatially resolved measurements of S'11, which is the derivative of the RF reflection coefficient S11 with respect to voltage, varied strongly when probing axially along the NW and across the p-n junction. The axial variation in S'11 effectively mapped the asymmetric depletion arising from the doping concentrations on either side of the junction. Furthermore, variation of the probe tip voltage altered the apparent extent of features associated with the p-n junction in S'11 images. [ABSTRACT FROM AUTHOR]

At the NIST Neutron Imaging Facility, we collect neutron projection data for both the dry and wet states of a Proton-Exchange-Membrane (PEM) fuel cell. Transmitted thermal neutrons captured in a scintillator doped with lithium-6 produce scintillation light that is detected by an amorphous silicon detector. Based on joint analysis of the dry and wet state projection data, we reconstruct a residual neutron attenuation image with a Penalized Likelihood method with an edge-preserving Huber penalty function that has two parameters that control how well jumps in the reconstruction are preserved and how well noisy fluctuations are smoothed out. The choice of these parameters greatly influences the resulting reconstruction. We present a data-driven method that objectively selects these parameters, and study its performance for both simulated and experimental data. Before reconstruction, we transform the projection data so that the variance-to-mean ratio is approximately one. For both simulated and measured projection data, the Penalized Likelihood method reconstruction is visually sharper than a reconstruction yielded by a standard Filtered Back Projection method. In an idealized simulation experiment, we demonstrate that the cross validation procedure selects regularization parameters that yield a reconstruction that is nearly optimal according to a root-mean-square prediction error criterion. [ABSTRACT FROM AUTHOR]

The theory of quantum electrodynamics (QED) predicts that beta decay of the neutron into a proton, electron and antineutrino should be accompanied by a continuous spectrum of soft photons. While this inner bremsstrahlung branch has been previously measured in nuclear beta and electron capture decay, it has never been observed in free neutron decay. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework and heavy baryon chiral perturbation theory (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined a branching ratio of (3.13 ± 0.34) × 10-3 (68 per cent level of confidence) in the energy region between 15 and 340 keV, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; the characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. This result may provide opportunities for more detailed investigations of the weak interaction processes involved in neutron beta decay. [ABSTRACT FROM AUTHOR]

We estimate the root-mean-square (RMS) value of timing jitter noise in simulated signals similar to measured highspeed sampled signals. The simulated signals are contaminated by additive noise, timing jitter noise, and time shift errors. Before estimating the RMS value of the jitter noise, we align the signals (unless there are no time shift errors) based on estimates of the relative shifts from cross-correlation analysis. We compute the mean and sample variance of the aligned signal based on repeated measurements at each time sample. We estimate the derivative of the noise-free signal based, in part, on a regression spline fit to the average of the aligned signals. Our initial estimate of the RMS value of the jitter noise depends on estimated derivatives and sample variances at time samples where the magnitude of the estimated derivative exceeds a selected threshold. This initial estimate is generally biased. Using a parametric bootstrap approach, we adaptively adjust this initial estimate of the RMS value of the jitter noise based on an estimate of this bias. We apply our method to real data collected at NIST. We study how results depend on the derivative threshold. [ABSTRACT FROM AUTHOR]

Estimates the quality factor Q and resonant frequency f[sub 0] of a microwave cavity based on observations of a resonance curve on an equally spaced frequency grid. Variance of the additive noise in the observed resonance curve; Use of weighted least squares in estimating associated model parameters.

Presents a study that examined several problems related to the characterization of the timebase in high-speed sampling oscilloscopes. Estimation of additive and jitter noises; Analysis of the uncertainty of timebase estimates; Effects of amplitude, phase drifts and jitter error on the estimation of timebase distortion.

Investigates the relative performance of various methods for aligning noisy one-dimensional signals. Centroid method; Cross correlation methods; Comparison study.

Analyzes daily maximum temperature data collected at the San Francisco International Airport from 1949 to 1994. Findings that the warmest day of the week tends to occur on the first or last day of that week; Author's belief that the phenomenon results from positive temporal correlations in the daily maximum temperature data.

Reconstructions of positron emission tomography images are obtained with the iterative expectation maximization (EM) algorithm. The EM algorithm is halted according to a cross-validation procedure. For the cases studied, this method yields a reconstruction with high variability about its expected value. The variability of the reconstruction about its expected value is reduced by computing its bootstrap expectation. Based on the reconstruction computed from the observed projection data, synthetic projection data sets are simulated. Reconstructions of the synthetic projection data sets are averaged to yield the bootstrap expectation. This bootstrap procedure is a nonlinear filtering method. The procedure is automatic; no smoothing kernel or bandwidth parameter need be specified. For simulated data, the bootstrap method yielded somewhat sharper reconstructions than did an optimized linear approach. The method is applied to real data from a fluorodeoxiglucose study of the human brain. Near the boundaries, the resampling procedure yielded a sharper reconstruction. [ABSTRACT FROM AUTHOR]

An asymmetry statistic of physical interest is an estimate of the ratio of the difference and the sum of the Poisson rate parameters for two scattering processes. Typically, an additive background signal contributes to measurements of each signal. Background is measured in a third experiment. Data are corrected by subtracting measured background. When the measured background is larger than one of the other measurements, background-corrected counts are negative. For this case, it would seem that no useful information can be extracted from the experiment. Here, asymmetry and an associated uncertainty interval are estimated for such cases using a bootstrap approach. [ABSTRACT FROM AUTHOR]

In many scattering experiments, the quantity of most direct physical interest is a measure of the difference between two closely related scattering signals, each generated by a Poisson scattering process. This difference is often expressed in terms of an asymmetry statistic, that is, the difference normalized to the sum of the two signals, corrected for an additive background contribution. Typically, a propagation of errors approach is used to compute confidence intervals for asymmetry. However, these confidence intervals are not reliable in general. In this work, generally accurate confidence intervals for asymmetry are obtained using a parametric bootstrap approach. Based on the observed data, data are simulated using a Monte Carlo resampling scheme. The resampled data sets satisfy a constraint that ensures that background-corrected count rates are not negative. [ABSTRACT FROM AUTHOR]

We have made photovoltaic cells by infiltrating the conjugated polymer regioregular poly(3-hexylthiophene) into films of mesoporous titania, which are self-assembled using a structure directing block copolymer. The mesoporous titania films were chosen because they have pores with a diameter slightly less than 10 nm, which is the exciton diffusion length in many conjugated polymers, and because they provide continuous pathways for electrons to travel to an electrode after electron transfer has occurred. The photovoltaic cells have an external quantum efficiency of 10% and a 1.5% power conversion efficiency under monochromatic 514 nm light. Experiments that vary the amount of polymer in the titania films suggest that the performance of the cells is limited by poor hole transport in the polymer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Comments on the role of quantitative analysis of images in a variety of fields including acoustics, astronomy, combustion science, electromagentics, geophysics, material science, medicine and physics. Advancement in imaging techniques; Discussion on an emerging experimental method for two-dimensional imaging of the compositional analysis of elements in materials by the energy spectrum of emitted X-rays; Implementation of method for image characterization in dendronic image characterization environment.