Kirk K. Nielson

Matrix corrections for energy dispersive X-ray fluorescence analysis of environmental samples with coherent/incoherent scattered X-rays.

A numerical method is given for computing matrix effects in relatively thick (63 mg/cm/sup 2/), pelletized homogeneous or particulate samples of biological and environmental origin. The method relies on estimating an approximate light element (Z less than or equal to 13) content of the sample from the coherent and incoherent scatter peaks after they have been corrected for absorption and for scattering from the heavy elements (Z greater than 13). The total sample absorption due to both light and heavy elements is computed by an iterative procedure which yields self-absorption corrections with a relative precision of several percent. Enhancement and particle size effects are also computed in the iterative procedure on the same basis as the absorption corrections. Since the corrections are based on a thin sample x-ray calibration, repeated calibrations for specific sample types are avoided. The method yields matrix corrected quantities of 24 elements from 1024 channels of raw data in approximately 20 to 50 s (5 to 15 iterations) using a 24K, PDP-15 computer.

The Raetrad model of radon generation and transport from soils into slab-on-grade houses

Remediation planning and 222Rn-related construction zoning require knowledge of how close and strong 226Ra sources can be in different foundation soils under different groundwater conditions without excessively elevating indoor 222Rn levels. A two-dimensional numerical-analytical model was developed to simulate (a) 222Rn emanation, decay, and movement by diffusion and advection in soils around houses and in their understructures; and (b) 222Rn accumulation in a single-zone house. The model represents foundation soils and a house in elliptical-cylindrical geometry. 222Rn may diffuse through its floor slab or may enter via idealized cracks and openings. The model was validated with analytical calculations of two-dimensional air pressure fields and with one-dimensional calculations of 222Rn generation with diffusion and diffusion combined with advection. Agreement generally was within < 1% when finite-difference approximations were minimized. Benchmark comparisons with indoor 222Rn measurements in two test-cell structures under passive and depressurized conditions averaged within 11% of measured values, well within measurement uncertainty. The corresponding average bias was only 3%. Larger variations were observed when applying the model to 50 houses. In this application, a negative bias of nearly 50% was observed due to data gaps and to poorly-characterized floor slabs and crack distributions.

X-ray fluorescence measurements of Mg, P, S, CI, K, Ca, Mn, Fe, Cu, and Zn in fruits, vegetables, and grain products

Abstract Concentrations of Mg, P, S, Cl, K, Ca, Mn, Fe, Cu, and Zn were measured in quadruplicate for 190 samples of 72 different fruits, vegetables, and grain products by a multielement X-ray fluorescence (XRF) method. Validation measurements on standard reference materials provided two to five certified values for each element and averaged within 5.9% of reference values from the National Institute of Standards and Technology, with an average bias of 2.6%. Accuracies also were verified by separate atomic absorption spectrophotometry measurements on the food samples. Long-term variations monitored with statistical control charts showed that 76 batch measurements of P and Zn in standards all were within 3σ confidence limits despite apparent temporal variation. Variations representing short-term analytical precision and sample homogeneity averaged 4% relative standard deviation (RSD) and 4.7% RSD, respectively, as computed from statistical analyses of duplicate XRF measurements on duplicate sample aliquots.

Radon diffusion coefficients for aged residential concretes

This note reports radon gas pore diffusion coefficient measurements for residential concretes from Florida, ranging in age from 12 y to 45 y. The coefficients ranged from 1.5 x 10(-7) m2 s-1 to 5.5 x 10(-7) m2 s-1. On the average, these values are about a factor of 1.6 higher than average values previously reported for new residential concretes in Florida.

Radon penetration of concrete slab cracks, joints, pipe penetrations, and sealants

Radon movement through 12 test slabs with different cracks, pipe penetrations, cold joints, masonry blocks, sealants, and tensile stresses characterized the importance of these anomalous structural domains. Diffusive and advective radon transport were measured with steady-state air pressure differences controlled throughout the deltaP = 0 to 60 Pa range. Diffusion coefficients (deltaP = 0) initially averaged 6.5 x 10(-8) m2 s(-1) among nine slabs with only 8% standard deviation, but increased due to drying by 0.16% per day over a 2-y period to an average of 2.0 x 10(-7) m2 s(-1). An asphalt coating reduced diffusion sixfold but an acrylic surface sealant had no effect. Diffusion was 42 times higher in solid masonry blocks than in concrete and was not affected by small cracks. Advective transport (deltaP < or = 60 Pa) was negligible for the slabs (10(-16) m2 permeability), pipe penetrations, and caulked gaps, but was significant for cracks, disturbed pipe penetrations, cold joints, masonry blocks, and concrete under tensile stress. Crack areas calculated to be as small as 10(-7) m2 significantly increased radon advection. Algebraic expressions predict air velocity and effective crack width from enhanced radon transport and air pressures. Masonry blocks, open cracks, and slab cold joints enhance radon penetration but stressed slabs, undisturbed pipe penetrations, and sealed cracks may not.

A sensitive effluent method for measuring radon gas emanation from low-emanating materials

The fraction of radon (222Rn) gas emanated from soils and building materials is critical in estimating radiation source strengths for human dosimetry. A sensitive method has been developed to measure emanation fractions (E) from sample radon effluents. The method equilibrates samples with their emanated radon in sealed cans for 24–30 days. Samples are then assayed for radium (226Ra) by high-efficiency gamma scintillation spectrometry that eliminates thorium (232Th) interferences. Radon in the can head space is then measured, and E is calculated as the ratio of emanated radon activity to sample radium activity. Values of E measured by the effluent method were compared with corresponding measurements by the differential gamma assay method. Precisions evaluated on 259 Florida soil samples indicated 1000-s detection limits for emanating radium of Ra · E = 0.02 pCi g−1 by the effluent method and Ra · E = 0.5 pCi g−1 by the differential gamma method. The effluent method has superior precision by a factor of eight for background soils containing Ra · E = 0.25 pCi g−1. The effluent method averaged about 8% lower than the differential gamma method on 21 comparison samples. Twenty were within a ±20% data accuracy objective.

Screening for elevated lead and tin in fruits and vegetables by Nondestructive X-ray fluorescence

Abstract Respective lead and tin concentrations in the dry matter of 525 food samples analyzed by nondestructive X-ray fluorescence exceeded detection limits in only 24 and 36 samples, principally fruits and vegetables. However, both elements were detected simultaneously in 22 of the samples, indicating a highly significant correlation in their elevated occurrences. The distributions of measured concentrations suggested two populations. A predominantly canned-fruit group averaged 2.5 ± 1.0 μg/g lead and 360 ± 270 μg/g tin (dry), and a vegetable-dominated group averaged 8.0 ± 4.0 μg/g lead and 10 ± 4.6 μg/g tin. Inside surfaces of nine food cans averaged 2 lead and 700 μg/cm 2 tin, sufficient to explain the observed occurrences of tin (0.2 to 6% leaching) but not of lead. The correlated lead and tin occurrences are consistent with biological uptake from soils during plant growth and may comprise the upper parts of broader natural distributions that are centered in the sub microgram per gram range. Tin from food cans may account for most dietary tin ingestion, but is approximately 40 times lower than the estimated threshold for adverse health effects. Intermittent lead occurrences in certain vegetables could exceed acceptable daily intake levels, depending on consumption patterns. Nondestructive X-ray fluorescence is useful for detecting high lead and tin levels while analyzing for minerals of nutritional importance.

Raetrad model extensions for radon entry into multi-level buildings with basements or crawl spaces.

The RAETRAD model was generalized to characterize radon generation and movement from soils and building materials into multi-level buildings with basements or crawl spaces. With the generalization, the model retains its original simplicity and ease of use. The model calculates radon entry rates that are consistent with measurements published for basement test structures at Colorado State University, confirming approximately equal contributions from diffusion and pressure-driven air flow at indoor-outdoor air pressure differences of deltaP(i-o) = -3.5 Pa. About one-fourth of the diffusive radon entry comes from concrete slabs and three-fourths comes from the surrounding soils. Calculated radon entry rates with and without a barrier over floor-wall shrinkage cracks generally agree with Colorado State University measurements when a sustained pressure of deltaP(i-o) = -2 Pa is used to represent calm wind (<1 m s(-1)) conditions. Calculated radon distributions in a 2-level house also are consistent with published measurements and equations.

Occurrence of arsenic in seafoods from fast foods analyzed by X-ray fluorescence

Abstract Arsenic was detected in 22 of 525 samples of fruits, vegetables, grain products, fast foods, dairy products, and seafoods analyzed by a multielement X-ray fluorescence method. The arsenic occurrences corresponded almost completely and exclusively with seafoods, and the correspondence was confirmed by additional analyses of separated parts of fast-food fish sandwiches. The geometric mean arsenic concentration for all seafoods was 2.1 μg/g (dry weight), with a geometric standard deviation of 3.2. Relative analytical precisions averaged 5.9% standard deviation, and arsenic inhomogeneity among sample aliquots averaged 4.5%. Accuracies validated by six National Institute of Standards and Technology reference materials averaged within 0.2 μg/g, or 5.1 % of certified values, with a net bias of less than 0.1 μg/g. Total arsenic contents in single servings of the seafoods (geometric mean = 109 μg) may account completely for the average U.S. arsenic intake if one serving is consumed every 2 to 10 days. Average total arsenic contents are similar among single servings of fish sandwiches, fish fillets, shrimp, and clam chowder; however, variation among specific samples of these seafoods amounts to a factor of 2 to 3. Arsenic concentrations in fish and shrimp agree with values predicted from bioaccumulation from seawater, suggesting consistency with a broader range of seafoods.

Deuterium concentration and cold fusion; Rate distributions in palladium

Cold fusion reactions and excess heat production have been reported in the electrolysis of heavy water with a palladium metal cathode. Solution of the standard diffusion equation for deuterium without fusion indicates that the deuterium concentration distribution rapidly becomes constant in the palladium lattice. Solution of the nonlinear diffusion equation for deuterium undergoing fusion also gives constant deuterium concentrations, suggesting that any fusion occurs uniformly throughout the palladium lattice. The hypothesis that fusion reactions occur predominantly at the palladium surface is shown to be inconsistent with experimental data.