Development of small particle speciation for nuclear forensics by soft X-ray scanning transmission spectromicroscopy.

Synchrotron radiation spectromicroscopy provides a combination of submicron spatial resolution and chemical sensitivity that is well-suited to analysis of heterogeneous nuclear materials. The chemical and physical characteristics determined by scanning transmission X-ray microscopy (STXM) are complementary to information obtained from standard radiochemical analysis methods. In addition, microscopic quantities of radioactive material can be characterized rapidly by STXM with minimal sample handling and intrusion, especially in the case of particulate materials. The STXM can accommodate a diverse range of samples including wet materials, complex mixtures, and small quantities of material contained in a larger matrix. In these cases, the inventory of species present in a sample is likely to carry information on its process history; STXM has the demonstrated capability to identify contaminants and sample matrices. Operating in the soft X-ray regime provides particular sensitivity to the chemical state of specimens containing low-Z materials, via the K-edges of light elements. Here, recent developments in forensics-themed spectromicroscopy, sample preparation, and data acquisition methods at the Molecular Environmental Science Beamline 11.0.2 of the Advanced Light Source are described. Results from several initial studies are presented, demonstrating the capability to identify the distribution of the species present in heterogeneous uranium-bearing materials. Future opportunities for STXM forensic studies and potential methodology development are discussed.

Plutonium segregation in glassy aerodynamic fallout from a nuclear weapon test.

This study combines electron microscopy equipped with energy dispersive spectroscopy to probe major element composition and autoradiography to map plutonium in order to examine the spatial relationships between plutonium and fallout composition in aerodynamic glassy fallout from a nuclear weapon test. A sample set of 48 individual fallout specimens were interrogated to reveal that the significant chemical heterogeneity of this sample set could be described compositionally with a relatively small number of compositional endmembers. Furthermore, high concentrations of plutonium were never associated with several endmember compositions and concentrated with the so-called mafic glass endmember. This result suggests that it is the physical characteristics of the compositional endmembers and not the chemical characteristics of the individual component elements that govern the un-burnt plutonium distribution with respect to major element composition in fallout.

Spatially-resolved analyses of aerodynamic fallout from a uranium-fueled nuclear test.

Five silicate fallout glass spherules produced in a uranium-fueled, near-surface nuclear test were characterized by secondary ion mass spectrometry, electron probe microanalysis, autoradiography, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Several samples display compositional heterogeneity suggestive of incomplete mixing between major elements and natural U ((238)U/(235)U = 0.00725) and enriched U. Samples exhibit extreme spatial heterogeneity in U isotopic composition with 0.02 < (235)U/(238)U < 11.84 among all five spherules and 0.02 < (235)U/(238)U < 7.41 within a single spherule. In two spherules, the (235)U/(238)U ratio is correlated with changes in major element composition, suggesting the agglomeration of chemically and isotopically distinct molten precursors. Two samples are nearly homogenous with respect to major element and uranium isotopic composition, suggesting extensive mixing possibly due to experiencing higher temperatures or residing longer in the fireball. Linear correlations between (234)U/(238)U, (235)U/(238)U, and (236)U/(238)U ratios are consistent with a two-component mixing model, which is used to illustrate the extent of mixing between natural and enriched U end members.

A combined theoretical and experimental investigation of uranium dioxide under high static pressure.

We have investigated the behavior of uranium dioxide (UO2) under high static pressure using a combination of experimental and theoretical techniques. We have made Raman spectroscopic measurements up to 87 GPa, electrical transport measurements up to 50 GPa from 10 K to room temperature, and optical transmission measurements up to 28 GPa. We have also carried out theoretical calculations within the GGA + U framework. We find that Raman frequencies match to a large extent, theoretical predictions for the cotunnite (Pnma) structure above 30 GPa, but at higher pressures some behavior is not captured theoretically. The Raman measurements also imply that the low-pressure fluorite phase coexists with the cotunnite phase up to high pressures, consistent with earlier reports. Electrical transport measurements show that the resistivity decreases by more than six orders of magnitude with increasing pressure up to 50 GPa but that the material never adopts archetypal metallic behavior. Optical transmission spectra show that while UO2 becomes increasingly opaque with increasing pressure, a likely direct optical band gap of more than 1 eV exists up to at least 28 GPa. Together with the electrical transport measurements, we conclude that the high pressure electrical conductivity of UO2 is mediated by variable-range hopping.

When the dust settles: stable xenon isotope constraints on the formation of nuclear fallout.

Nuclear weapons represent one of the most immediate threats of mass destruction. In the event that a procured or developed nuclear weapon is detonated in a populated metropolitan area, timely and accurate nuclear forensic analysis and fallout modeling would be needed to support attribution efforts and hazard assessments. Here we demonstrate that fissiogenic xenon isotopes retained in radioactive fallout generated by a nuclear explosion provide unique constraints on (1) the timescale of fallout formation, (2) chemical fractionation that occurs when fission products and nuclear fuel are incorporated into fallout, and (3) the speciation of fission products in the fireball. Our data suggest that, in near surface nuclear tests, the presence of a significant quantity of metal in a device assembly, combined with a short time allowed for mixing with the ambient atmosphere (seconds), may prevent complete oxidation of fission products prior to their incorporation into fallout. Xenon isotopes thus provide a window into the chemical composition of the fireball in the seconds that follow a nuclear explosion, thereby improving our understanding of the physical and thermo-chemical conditions under which fallout forms.

Improving precision in resonance ionization mass spectrometry: influence of laser bandwidth in uranium isotope ratio measurements.

The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of (235)U/(238)U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of uranium oxide targets with essentially no sample preparation.

Evaluation of menus planned in Mississippi child-care centers participating in the Child and Adult Care Food Program.

OBJECTIVES: To collect information from licensed child-care centers in Mississippi on their foodservice operations relative to participation in the Child and Adult Care Food Program sponsored by the US Department of Agriculture (USDA); to collect information on who planned the menus; and to evaluate the energy and nutrient content of the planned menus relative to the suggested goal of one third of the Recommended Dietary Allowances (RDAs) and the recommendations made in the Dietary Guidelines for Americans. DESIGN: Survey questionnaire. SETTING: Licensed child-care centers in Mississippi. SAMPLES: Ninety-two licensed child-care centers provided cycle menus for analysis. One hundred eighteen centers returned the questionnaire. MAIN OUTCOME MEASURES: Content of the menus relative to the meal-pattern guidelines established for the Child and Adult Care Food Program; the energy and nutrient content of the menus relative to the RDAs and the Dietary Guidelines for Americans. STATISTICAL ANALYSES PERFORMED: One-way analysis of variance and frequencies. RESULTS: Seventy-five percent of the centers reported participating in the Child and Adult Care Food Program. All but one of the centers planned menus that met the meal-pattern requirements established by the program, licensure, and Head Start Performance Standards. Results showed that following the established meal-pattern guidelines for the child nutrition programs may not guarantee consistent nutritional quality of planned menus in child-care centers. The mean amounts of energy and many nutrients were significantly lower (P < .05) for centers that reported participating in the Child and Adult Care Food Program. Mean fat levels for all centers exceeded the recommendation of no more than 30% of total energy from fat: 40.8% of total energy from fat was reported by centers that participated in the program and 38% by those that said they did not. APPLICATIONS: Additional guidance is needed for menu planning in child-care centers to ensure compliance with the nutritional goal of meeting one third of the RDAs and Dietary Guidelines for Americans.

Calcium supplementation on normotensive and hypertensive pregnant women.

Normotensive and hypertensive pregnant women participated in a study to determine the effects of calcium supplementation on blood pressure. Subjects were randomly assigned to control or supplemented groups (1000 mg Ca/d). Blood pressure and serum total and ionic calcium were measured during the 20-wk supplementation period. Calcium supplementation had a significant lowering effect on diastolic blood pressure over the course of the study in the hypertensive group only. The hypertensive control subjects' mean serum ionic calcium value decreased significantly (P less than 0.05) over the course of the experiment. A significant (P less than 0.05) inverse relationship was observed between dietary calcium intake and blood pressure (r = -0.386 for systolic pressure and -0.359 for diastolic pressure).