1448 Volume 55, Number 11, 2001 APPLIED SPECTROSCOPY0003-7028 / 01 / 5511-1448$2.00 / 0q 2001 Society for Applied Spectroscopy

submitted papers

Quanti� cation of Large Scale Micro-X-ray FluorescenceElemental Images

CHRISTOPHER G. WORLEY,* GEORGE J. HAVRILLA, and PAUL S. DUNNLos Alamos National Laboratory, MS G740, Los Alamos, New Mexico 87545

Niobium is commonly alloyed with uranium to prevent surface ox-idation, and determining how the niobium concentration is distrib-uted throughout a sample is useful in explaining observed materialproperties. The niobium concentration distribution was determinedacross the surface of depleted uranium samples using micro-X-ray� uorescence (MXRF). To date, MXRF has been employed primarilyas a qualitative tool for determining relative differences in elementalconcentrations across a sample surface. Here, a process was devel-oped to convert qualitative MXRF niobium distribution imagesfrom depleted uranium samples into images displaying concentra-tion values. Thus, MXRF was utilized to determine elemental con-centrations across a surface in a manner similar to that of the es-tablished method of electron microprobe X-ray analysis (EMPA).However, MXRF can provide such information from relativelylarge sample areas many cm 2 in size that are too large to examineby the higher spatial resolution technique of EMPA. Although thesample surfaces were polished to the same degree as the standards,little or no sample preparation should be necessary for sample sys-tems where a high energy analyte XRF line can be used for imaging.

Index Headings: Micro-X-ray � uorescence; Quantitative imaging;Elemental imaging; Niobium; Uranium.

INTRODUCTION

Electron microprobe X-ray analysis (EMPA) is a well-established method for determining how the concentra-tion of an element is distributed across a sample sur-face.1,2 Relative lateral elemental concentration differenc-es in a sample can be easily discerned using EMPA, andthe concentration gradients can even be accurately quan-ti� ed by using appropriate standards.3 In the past decade,micro-X-ray � uorescence (MXRF) has matured as a fac-ile means of determining relative differences in the con-centration of elements distributed across a sample sur-face.4–9 A key difference between EMPA and MXRF isthe sample area that can be analyzed. Whereas EMPAuses a micro-focused electron beam to probe mm 2 re-gions, MXRF utilizes a focused X-ray beam typically 10sto 100s of mm in diameter. Hence, MXRF is ideal forexamining considerably larger sample areas (mm 2 andgreater) but with the tradeoff of lower image lateral res-olution than EMPA. MXRF can therefore provide ele-mental distribution information across an entire sample

Received 31 January 2001; accepted 4 July 2001.* Author to whom correspondence should be sent.

surface, while EMPA is intended to map elemental dis-tributions over local micro-regions.

There are a number of advantages associated withMXRF vs. EMPA.10 A primary advantage is that themethod is nondestructive, whereas EMPA can damage asample if a high electron beam current is used. Minimalsample preparation is usually required with MXRF, butsamples analyzed by EMPA must be either conductors orcoated with a conducting layer.1 MXRF also provides agreater penetration depth from which elemental signalscan be gathered, resulting in a larger analysis volume and,therefore, better elemental sensitivity for medium andhigh atomic number elements. Finally, samples can beanalyzed in air by MXRF but must be placed under highvacuum with EMPA.

Despite these advantages, however, quanti� cation ofelemental image concentration distributions has onlybeen well-established with EMPA to date. A number ofMXRF reports11–22 demonstrate the quanti� cation of sin-gle points and/or quantitative line scans. In line-scanquanti� cation, the X-ray beam is moved in successivepoints in a line across a sample surface and elementalconcentrations are determined at each analysis spot.However, quantitative lateral elemental maps of the entiresample surface were not presented in these reports. A fewstudies demonstrate the quanti� cation of a specimen atspeci� ed points across the surface,19,20 and recentlyMXRF two-dimensional elemental concentration mapswere acquired of elements imbedded in a polymer ma-trix.23 In all of these quantitative line-scan and multiple-spot studies, samples were analyzed using a static mode(microprobe type mode), whereby the X-ray beam washeld stationary for a set amount of time to collect a netintensity value (converted to concentration) from theanalysis spot. The beam was then moved to a neighboringspot to determine the intensity and concentration for thatregion. By repeating this process either along a singleline or two dimensionally, elemental concentration gra-dients were determined.

In the present work, samples were imaged in a dynam-ic mode in which the X-ray beam was continuously ras-tered across a sample surface; an intensity value wasmeasured for each image pixel as the beam passed overthe corresponding sample region. A key advantage of this

APPLIED SPECTROSCOPY 1449

method is that images can be collected in a much shortertime than when using the static mode. The image lateralresolution is also better than when using the static mode.In the dynamic mode, there can be a continuous overlapof analysis areas forming the image, which leads to high-er spatial resolution. Due to the nature of the static-modedata-collection process, however, the image pixels do notusually overlap completely. To do so would require animpractical analysis time for large samples such as thosestudied here. This is due to the extra time necessary tomove the X-ray beam an incremental step prior to andfollowing the collection of the intensity at each point.Thus, images acquired in the static mode usually havelower spatial resolution than in the dynamic mode in or-der to collect the image in a practical amount of time.The time per pixel achieved with the dynamic methodcan be small, however, resulting in lower sensitivity thanthat obtained with the static approach.

Because this dynamic mode differs from the staticmethod of the previously mentioned studies, a somewhatdifferent approach was used here versus that taken withthe previous static mode studies to quantify MXRF imageelemental concentrations. Also, an image of the entiresample surface was quanti� ed instead of only selectedpoints. Brie� y, a set of standards was imaged, and anaverage image pixel intensity was determined from eachstandard to generate a linear calibration of intensity ver-sus concentration. The elemental intensity distribution(elemental image) was then acquired from an unknownsample, and the intensities of the image pixels were con-verted into concentration values using the standards cal-ibration. Thus, concentration information can be extract-ed from any desired region in the image (e.g., from asingle pixel, from a line across the image, or from a de-� ned area in the image). This dynamic imaging quanti-� cation method, therefore, provides a quick means rela-tive to the static mode for quantifying large sample sur-face areas (up to ;230 cm 2 using the instrument here)with relatively high spatial resolution. Any interesting ar-eas noted in post-analysis image processing can then bereanalyzed using a longer acquisition time per pixel toprovide better elemental sensitivity but over a smallerregion of the sample.

The objective of this work was to demonstrate the ca-pacity of MXRF for quantifying elemental concentrationdistributions across a large sample surface area. Depleteduranium containing 6% niobium was used for this study.A detailed discussion of the material properties respon-sible for niobium heterogeneity observed in these urani-um samples will not be presented here. Instead, the meth-od development for MXRF elemental image quanti� ca-tion will be discussed.

Niobium is alloyed with uranium to inhibit oxidation,but niobium heterogeneity can alter the desired uraniummaterial properties.24,25 Thus, determining the niobiumconcentration distribution in the samples studied was ofcritical importance. Also, the surface area of some ofthese samples was over 100 cm 2, so the higher spatialresolution method of EMPA was not bene� cial here.Only MXRF could provide this spatially-resolved ele-mental information over such large surface areas, andquanti� cation of the resulting elemental maps was real-ized using the previously discussed standards approach.

EXPERIMENTAL

Instrumentation. A Kevex Omicron MXRF systemwas employed for this work consisting of a 100-W poly-chromatic rhodium X-ray anode operated at 20 kV and1.9 mA. The source and detector were oriented at a 458incident angle with the sample. A liquid nitrogen cooledSi(Li) detector was used with an active area of 50 mm 2

and an energy resolution of 160 eV. To restrict the pri-mary beam diameter for the MXRF imaging studies, a500-mm-diameter aperture was placed over the source.The sample stage was moved laterally in the x and ydirections while the primary beam remained stationary toscan the X-ray beam. All the images consisted of 256 3256 pixels in the x and y directions. A helium atmospherewas used to prevent signi� cant attenuation of the niobiumL3-M 4,5(La) X-ray signal. Note: the IUPAC nomenclaturefor labeling XRF elemental lines will be used here.26

Samples. All the samples analyzed in this study con-sisted of depleted uranium metal. Although depleted ura-nium is radioactive, it is safe to handle outside of a glov-ebox since it does not release any airborne particulatesunder normal handling conditions. The sample surfaceswere machined and polished in a glovebox, and the wastewas treated in accordance with U.S. federal radioactivewaste handling regulations. Gloves were used to handlethe uranium samples, which is standard practice in han-dling any radioactive material.

The lateral niobium intensity distribution was deter-mined from a set of � ve uranium standards containing 0,2, 4, 6, and 8% niobium, respectively. These standardswere button-shaped and ;1.5 cm in diameter. The surfacewas machine � nished and then hand-sanded. The interiorof four cylindrical depleted uranium rods were then ex-amined. Each rod contained nominally 6% niobium andwas ;20 cm long. The diameters of the rods were 18,25, 35, and 50 mm, respectively. To analyze the interiorsurfaces, the rods were cut in half axially, and the � atinterior surfaces were machine � nished followed byhand-sanding using the same protocol applied to the stan-dards. One of the rod halves from each of the four sam-ples was then examined by MXRF. Because the rods weretoo long to � t in the spectrometer intact, they were cutin half along the radial axis, and the two resulting pieceswere imaged side by side. The images of the two pieceswere then merged together using image processing soft-ware in order to display the niobium distribution over theentire interior surface of each rod.

RESULTS AND DISCUSSION

Standards. The niobium distributions of the � ve de-pleted uranium standards were imaged by MXRF bymonitoring the Nb L3-M 4,5(La) peak intensity versus lat-eral position. Figure 1 shows a typical spectrum acquiredfrom a single spot on a depleted uranium sample. Thehigher energy Nb K-L2,3(Ka) peak was not used for im-aging because it overlaps with the U L3-N 5(Lb2), as seenin the spectrum. Unfortunately, due to a smaller infor-mation depth of the lower energy Nb L3-M 4,5 photons (;1mm) compared with Nb K-L2,3 photons (;10 mm), the NbL3-M 4,5 peak intensity varies more due to changes in sam-ple surface roughness. This will be discussed in greaterdetail below.

1450 Volume 55, Number 11, 2001

FIG. 1. Point spectrum acquired from a depleted uranium sample alloyed with nominally 6% niobium. The X-ray source was operated at 20 kV.The table lists the major niobium, uranium, and rhodium (X-ray anode material) peaks that could be detected as well as their corresponding energiesand the information depth for the Nb K-L2,3 and Nb L3-M4,5 lines.

A white light image of three of these standards isshown in Fig. 2a, and the MXRF niobium intensity imageacquired from all � ve standards is presented in Fig. 2b.(The pixel intensities in the niobium image and in all theother elemental images acquired by the instrument wereoriginally color coded, but most of the images are pre-sented in grayscale here for reproduction purposes.) Thetotal niobium X-ray counts per pixel is represented by adistinct shade of gray as depicted in the image grayscalebar; however, the image was collected in a digital formatwhich only contained this color information. Actual pixelintensity data was not saved in order to reduce the image� le size. An image maximum and minimum intensity val-ue was provided by the software, which was then usedto convert the colors (shown as grayscale shades here)back into intensity values, as will be discussed below. Anincrease in the niobium intensity is clearly evident withincreasing concentration. In addition, some niobium het-erogeneity is apparent in the standards, especially in the8% standard. Ideally, the standards should be homoge-neous, but regions of the standards in which the niobiumdistribution appeared to be homogeneous were used todetermine an average intensity value for each standard.As will be discussed below, this approach was successfulin providing a linear calibration and predicting the ex-pected niobium concentration in samples.

To prepare a linear calibration of the standards, theniobium distribution image of all � ve standards was con-verted to TIF format using the MXRF Omicron instru-ment software. Next, this TIF � le was imported into aseparate image processing program (Fortner ResearchTransform version 3.3.0.1). This imported � le consistedof a 256 3 256 pixel array in which the original pixel

colors from the Omicron instrument � le were convertedinto numeric values from 0 to 235. The correlation be-tween the original pixel colors and their correspondingelemental intensity values was maintained as a linearfunction. Black represented an intensity of 0 counts,white represented the image maximum intensity value,and intensities in between were denoted by the thermalcolor scale.

Using the original image maximum intensity valueprovided by the instrument software, the resulting pixelarray values from 0 to 235 were converted into actualniobium counts/pixel. This was achieved by dividing thearray by 235 and multiplying by the image maximumintensity provided by the MXRF instrument software.The uranium M5-N 6,7(Ma) peak intensity vs. lateral po-sition was also collected to determine the uranium dis-tribution from the standards, and the same color-to-inten-sity protocol used for niobium was followed. (The Rh L3-M4,5 and L2-M 4(Lb1) peaks were adequately resolvedfrom the U M5-N6,7 peak to allow use of this uraniumline for imaging.) The Nb/U intensity ratio was then cal-culated for each pixel, creating a new data array. A regionof pixels from each of the standards where the niobiumappeared to be evenly distributed was then extracted fromthis Nb/U intensity ratio array, and a mean Nb/U ratiovalue was determined from this extracted region of val-ues. Using this mean value for each standard, a linearcalibration (r2 5 0.999) was achieved.

Analysis of Unknown Rod Samples. The niobiumconcentration distribution was imaged from the interiorsurface of the four depleted uranium rods. Figure 3a is awhite light image of the 18-mm-diameter rod interior.(The rod was cut in half along the radial axis to � t into

APPLIED SPECTROSCOPY 1451

FIG. 2. White light image (a) of three depleted uranium standards containing 0% Nb, 2% Nb, and 4% Nb, respectively, and the MXRF niobiumL3-M4,5 intensity image (b) acquired from all � ve uranium standards. The niobium image acquisition time was ;6 h. The left side of the imagecorresponds to the edge facing the detector. The bottom of the image corresponds to the side facing the X-ray beam.

FIG. 3. White light image (a) and MXRF niobium L3-M4,5 intensity image (b) acquired from the 18-mm-diameter uranium rod interior. The rodwas cylindrical in shape and was cut in half along the long axis resulting in two � at interior surfaces. The interior of one of these two rod halveswas then examined. This half was further cut into two equal-length segments to � t into the MXRF spectrometer. The MXRF image acquisitiontime was ;25 h. The top of the image corresponds to the edge facing the detector. The right side of the image corresponds to the side facing theX-ray beam.

the spectrometer.) The niobium and uranium distributionsfrom the resulting two halves were then imaged. Figure3b shows the niobium L3-M 4,5 grayscale intensity mapfrom the 18-mm rod pieces, where black corresponds to

low intensity counts and white to high counts. Labels 1and 3 denote the ends of the pristine uncut rod, and label2 corresponds to the region where the rod was cut to � tit into the spectrometer. The other three rod samples were

1452 Volume 55, Number 11, 2001

FIG. 4. Histogram of the number of image pixels vs. % niobium fromthe MXRF niobium image acquired from the 18-mm-diameter rod in-terior.

FIG. 5. Composite MXRF niobium image of the 18-mm-diameter uranium rod interior (a), a plot of the % niobium along a line through anapparent niobium inclusion (b), and a plot of the % niobium along a line down the center of the rod interior (c).

also imaged, and the results were similar to those ob-served for the 18-mm-diameter rod. The images fromthese other rods are not presented here.

The presence of a relatively high concentration of ni-obium is clearly evident near the end of the rod labeled1, and a slightly higher niobium intensity is observeddown the center of the rod than is seen along the edges.This niobium heterogeneity is indicative of niobium seg-regation during processing as the rod cools from the melt.This segregation occurs both from the outside toward thecenter and from the bottom towards the top of the cyl-inder (end labeled 1 in Fig. 3). This information can beused to control the alloy composition by adjusting thecooling rate of the melt. All four rods exhibited similar

properties of a niobium rich core and niobium accumu-lation at the top of the rod.

While this qualitative information alone was useful, theniobium concentration in the inclusion and over the restof the rod interior was quanti� ed to better understand thespecimen niobium composition. The MXRF niobium in-tensity map was therefore converted into a niobium con-centration map using the protocol previously discussed,whereby the Nb/U intensity ratio from every pixel wastransformed into percent niobium using the standards cal-ibration. Figure 4 is a histogram of the number of imagepixels corresponding to each percent niobium value. Thehistogram is centered at approximately 6% niobium,which is the nominal concentration in the rods and dem-onstrates the accuracy of MXRF image quanti� cation forthe binary system studied. The spurious histogram barthat is over twice as large as neighboring bars is assumedto be a statistical anomaly, as the remainder of the his-togram follows approximately a Gaussian pro� le.

Once the niobium concentration distribution imagefrom the rod halves was collected, an image of the entirerod interior was merged together using image processingsoftware (Fig. 5a). (This image is presented in the orig-inal instrument color scale to best depict the niobiumconcentration distribution across the sample.) The percentniobium was then plotted along a line bisecting the nio-bium inclusion along the rod short axis (Fig. 5b) andalong the middle of the rod parallel to the long axis (Fig.5c). Since the niobium concentration corresponding toevery pixel was calculated, the concentration could bedetermined from any speci� ed region of the image orfrom a line drawn across any area of the specimen (not

APPLIED SPECTROSCOPY 1453

FIG. 6. Plot of the average niobium intensity vs. niobium concentrationacquired from the � ve uranium standards using four different surfacepreparations: rough machine-� nished, machine-� nished and hand-sand-ed, mirror-polished, and mirror-polished followed by hand-sanding.

just along the lines shown here). The niobium concentra-tion was found to be . 8% at the center of the inclusion.By extracting line pro� les in this manner, subtle devia-tions in concentration can be discerned across a sample.For example, several concentration spikes are present inthe Fig. 5c line scan, indicating the location of severalniobium inclusions which are not apparent from viewingonly the qualitative color map. Note that the line scanswere smoothed using a 15 and 5 point Savitzky–Golayalgorithm for Figs. 5b and 5c, respectively, to detect thesesubtle features.

Effects of Surface Finish on the Niobium Intensity.The niobium distribution was also quanti� ed in square-shaped depleted uranium plates, but due to signi� cant dif-ferences in the surface roughness of these samples com-pared with the standards, the average concentration cal-culated was only half that expected. Hence, this demon-strates that surface � nish can greatly affect the accuracyof MXRF quantitative elemental imaging. Some MXRFstudies have been published that discuss the effects of sur-face roughness on analyte peak intensities.18,27–30 As pre-viously discussed, this intensity (and concentration) de-pendence on surface roughness is a result of the relativelylow energy of the niobium L3-M 4,5 XRF line (2.2 keV).These low energy X-ray photons do not have a large sam-ple penetration or information depth (see table in Fig. 1).Therefore, two surfaces of differing roughness will atten-uate the photons to greatly different extents before reach-ing the detector, resulting in different intensities detectedfrom the two surfaces.

To study the dependence of the niobium intensity onsurface roughness, the surfaces of the standards were pre-pared using four different � nishes. First, the standardswere machine � nished, providing a visibly rough surface.The standards were then imaged by MXRF. Next, thesurfaces were hand-sanded to a smoother � nish and im-aged (� nish used for comparison to the rod sampleswhich were prepared the same way). The resulting sur-faces were still dull and non-mirror-like in appearance.Next, the surfaces were polished to a mirror appearanceand imaged, and � nally, the surfaces were hand-sandedagain to provide a surface with a roughness in betweenthat of the machine 1 hand-sand and that of the mirror� nish. Thus, four different roughnesses were preparedand analyzed for all of the standards.

Figure 6 is a comparison of the niobium L3-M 4,5 inten-sity obtained from the � ve standards using the four dif-ferent surface preparations. In addition to the increase inintensity due to higher-concentration standards, an in-crease was also correlated with a decrease in surfaceroughness. Niobium intensities from the smooth mirror-polished surfaces were ;2.5 times greater than those ob-served from the rough machine-� nished surfaces. Thus,the surfaces of unknown samples and standards must beprepared identically in order to most accurately quantifyMXRF elemental images when a low-energy analyte linesuch as the niobium L3-M 4,5 is used. Although using theNb/U intensity ratio should partially compensate for de-viations in surface preparation between samples and stan-dards, the U M5-N6,7 line (3.2 keV) was used for imaging,which is 1 keV higher in energy than the Nb L3-M 4,5 line.Thus, the surface � nish will affect the niobium peak in-

tensity more signi� cantly than the uranium peak inten-sity.

CONCLUSION

This work has demonstrated the ef� cacy of MXRF inquantifying niobium concentration distribution imagesacquired from depleted uranium samples. A majority ofMXRF imaging studies have involved qualitative imag-ing in which relative elemental concentration differencesare determined two-dimensionally across a sample sur-face, but the present report illustrates a novel approachto the quanti� cation of such images. While EMPA is amature technique for providing such elemental distribu-tion images, it is a higher spatial-resolution method, andlarge macroscopic sample regions can be imaged only byMXRF. Therefore, the current study is signi� cant sincequantitative elemental imaging can now be extended torelatively large sample sizes at least many cm 2 in area.

Although sample surface roughness was found to sig-ni� cantly affect the accuracy of MXRF image quanti� -cation, preparing all standards and unknowns with iden-tical surface roughnesses resulted in concentration valuesthat were expected for the samples. The utilization of therelatively low-energy Nb L3-M 4,5 analyte line was nec-essary for quanti� cation in the uranium/niobium system.This resulted in intensity values that were more suscep-tible to surface-roughness variations than if a higher en-ergy line were used. For systems in which a higher en-ergy line is amenable to analysis, less care in surfacepreparation would be necessary to achieve accurate re-sults. Thus, MXRF has the potential to provide accurateelemental image quanti� cation with little or no surfacepreparation for some sample types; no other techniquecan provide such information as easily.

ACKNOWLEDGMENT

The authors thank the Department of Energy for funding this work.This data was presented at the Denver X-ray Conference in August1999. Los Alamos is operated by the University of California for theDepartment of Energy. Identi� cation of speci� c vendors is not an en-dorsement of particular instrumentation.

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