Solidification of Ni-Re Peritectic Alloys.

Differential thermal analysis (DTA) and microstructural and microprobe measurements of DTA and as-cast Ni-Re alloys with compositions between 0.20 and 0.44 mass fraction Re provide information to resolve differences in previously published Ni-Re phase diagrams. This investigation determines that the peritectic invariant between liquid, Re-rich hexagonal close packed and Ni-rich face center cubic phases, L + HCP → FCC, occurs at 1561.1 °C ± 3.4 °C (1σ) with compositions of liquid, FCC and HCP phases of 0.283 ± 0.036, 0.436 ± 0.026, and 0.828 ± 0.037 mass fraction Re, respectively. Analysis of the microsegregation in FCC alloys yields a partition coefficient for solidification, k = 1.54 ± 0.09 (mass frac./mass frac.). A small deviation from Scheil behavior due to dendrite tip kinetics is documented in as-cast samples. No evidence of an intermetallic phase is observed.

Postdetonation nuclear debris for attribution.

On the morning of July 16, 1945, the first atomic bomb was exploded in New Mexico on the White Sands Proving Ground. The device was a plutonium implosion device similar to the device that destroyed Nagasaki, Japan, on August 9 of that same year. Recently, with the enactment of US public law 111-140, the "Nuclear Forensics and Attribution Act," scientists in the government and academia have been able, in earnest, to consider what type of forensic-style information may be obtained after a nuclear detonation. To conduct a robust attribution process for an exploded device placed by a nonstate actor, forensic analysis must yield information about not only the nuclear material in the device but about other materials that went into its construction. We have performed an investigation of glassed ground debris from the first nuclear test showing correlations among multiple analytical techniques. Surprisingly, there is strong evidence, obtainable only through microanalysis, that secondary materials used in the device can be identified and positively associated with the nuclear material.

Maximum pixel spectrum: a new tool for detecting and recovering rare, unanticipated features from spectrum image data cubes.

A new software tool, the maximum pixel spectrum, detects rare events within a spectrum image data cube, such as that generated with electron-excited energy-dispersive X-ray spectrometry in a scanning electron microscope. The maximum pixel spectrum is a member of a class of 'derived spectra' that are constructed from the spectrum image data cube. Similar to a conventional spectrum, a derived spectrum is a linear array of intensity vs. channel index that corresponds to photon energy. A derived spectrum has the principal characteristics of a real spectrum so that X-ray peaks can be recognized. A common example of a derived spectrum is the summation spectrum, which is a linear array in which the summation of all pixels within each energy plane gives the intensity value for that channel. The summation spectrum is sensitive to the dominant features of the data cube. The maximum pixel spectrum is constructed by selecting the maximum pixel value within each X-ray energy plane, ignoring the remaining pixels. Peaks corresponding to highly localized trace constituents or foreign contaminants, even those that are confined to one pixel of the image, can be seen at a glance when the maximum pixel spectrum is compared with the summation spectrum.

Measures for spectral quality in low-voltage X-ray microanalysis.

Characteristic x-ray production with energetic electrons depends strongly on the overvoltage, the ratio of the incident beam energy to the critical excitation energy for the atomic species of interest. Low-voltage x-ray microanalysis (beam energy < or = 5 keV) is especially susceptible to artifacts due to sample charging because the overvoltage is low and even slight charging can strongly affect peak intensities. The Duane-Hunt bremsstrahlung limit is a good diagnostic to detect sample charging. Dynamic charging effects, however, can influence spectra despite an apparently satisfactory Duane-Hunt limit. Dynamic charging effects must be examined by time series experiments, or through use of dynamic energy windows continuously measuring count rates placed across the spectrum. When charging is a problem, conductive surface coatings can eliminate the effects. When pristine surfaces must be examined without coating, the use of a conductive grid can control charging so that useful x-ray spectra can be obtained.

Visibility of objects in computer simulations of noisy micrographs.

Thresholds of visibility for objects in images with random pixel noise are predicted in terms of the signal-to-noise ratio. From trials with volunteers marking test images, we determined visibility thresholds of objects obscured by random pixel noise. The test images had objects with a variety of simple shapes and relatively little internal structure. Aside from the noise, the background of the test images was smooth and featureless. We extend the threshold signal-to-noise ratio measurements of Rose and others to a variety of object sizes and shapes. For objects with areas less than a disc subtending 2 degrees at the eye, visibility depends on the averaged difference in intensity from background, the noise level and the number of pixels in the object. Visibility does not seem to depend on object shape.

"Standardless" quantitative electron probe microanalysis with energy-dispersive X-ray spectrometry: is it worth the risk?

"Standardless" procedures for quantitative electron probe X-ray microanalysis attempt to eliminate the need for standardization through calculation of standard (pure element) intensities. Either "first principles" calculations, which account for all aspects of X-ray generation, propagation, and detection, or "fitted standards" calculations, which use mathematical fits to measured intensities from a limited set of pure standards, can form the basis for standardless analysis. The first principles standardless analysis procedure embedded in the National Institutes of Health/National Institute of Standards and Technology comprehensive X-ray calculation engine and database, Desktop Spectrum Analyzer, has been tested against spectra measured on NIST standard reference materials, research materials, and binary compounds. The resulting distribution of errors is broad, ranging from -90% to +150% relative. First principles standardless analysis can thus lead to unacceptably large errors.

Trace elemental analysis at nanometer spatial resolution by parallel-detection electron energy loss spectroscopy.

Parallel-detection electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) and a field emission source provides an unprecedented sensitivity for elemental microanalysis. By deflecting the energy loss spectrum across a parallel detector and computing the difference spectrum from sequentially collected energy-shifted spectra, the effects due to detector pattern noise are nearly eliminated so that signals less than 0.1% of the background can be readily detected. Measurements on a series of glass standard reference materials show that EELS provides both high spatial resolution and trace sensitivity at the 10 atomic ppm level for a wide range of elements including the alkaline earths, 3-d transition metals, and the lanthanides. For analytical volumes with dimensions of the order of 10 nm, this translates into near-single atom detectability.

On the use of ionization cross sections in analytical electron microscopy.

There are two approaches to the utilization of the ionization cross section, Q, for use in the determination of kappa AB factors for quantitative microanalysis in the analytical electron microscope. The first approach is to interpolate a value of Q from experimentally determined kappa AB factors at a fixed accelerating voltage (kV). The second approach uses a theoretical parameterization of Q generated by fitting the fundamental Bethe expression to selected experimental values of Q over a wide range of kV. This paper discusses the relative merits of the two approaches.

Artifacts in energy dispersive x-ray spectrometry in the scanning electron microscope (II).

The quality of x-ray spectra obtained with an energy dispersive x-ray spectrometer on an electron beam instrument can be severely compromised by the presence of electromagnetic interference. Sources of electromagnetic interference include ground currents and signals generated by time-varying currents in instrument components such as scan coils. Spectrometer resolution can be degraded by the accumulation of ice and vaccum oil on critical components of the device. Operation at high electron energy can cause artifacts in spectra due to direct entry of electrons and spurious x-rays into the detector. Processing high energy photons (above 40 keV) can lead to detector saturation effects which degrade resolution and affect dead time correction. Transmission of high energy x-rays through the detector accompanied by Compton scattering can lead to a distortion of the low energy portion of the spectrum.

Raman microprobe studies of two mineralizing tissues: enamel of the rat incisor and the embryonic chick tibia.

The laser-Raman microprobe developed at the National Bureau of Standards has been applied to the study of the mineralization process in rat incisor enamel and embryonic chick tibia. Cryostat sections were prepared from fresh frozen tissues and allowed to air dry. In these mineralizing tissues two forms of phosphorus compounds have been observed: (1) an inorganic phase identified as apatitic phosphate and (2) an organic phosphate. The distribution of these components from the mineralizing front to regions of higher mineralization has been determined with a spatial resolution of approximately 15 micrometer. The studies suggest the existence of a carbonate, with a Raman band corresponding to that of the mineral huntite, Mg3Ca(CO3)4, and found in regions of low phosphate mineral content.