Three‐dimensional density measurements of ultra low density materials by X‐ray scatter using confocal micro X‐ray fluorescence spectroscopy

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Abstract
Targets used in high energy density physics experiments, such as those fielded at the National Ignition Facility, are typically made of multi‐component systems that include metals, metal coatings, and very low density materials. These very low density materials with densities as low as ~10 mg/cm 3 must have uniform density throughout. Characterizing their density in 3D is a very difficult problem. One technique used is confocal micro X‐ray fluorescence. This technique, which uses a polycapillary optic to focus the X‐rays from the X‐ray source and another on the detector, measures the density of these materials based upon their X‐ray scatter. In order to gain a complete picture of their X‐ray scatter, the sample is rastered in 3D to generate a complete 3D density map. As proof of technique, the examination of very low density poly(styrene‐divinylbenzene) foams, poly(methylpentene) foams, as well as silica aerogels were completed. Results for the polymer foam materials show a linear correlation ( R 2 = 0.99) between X‐ray scatter intensity and bulk density. However, for higher atomic number materials (e.g. aerogels) the amount of X‐ray scatter is very dependent upon the depth of data collection as a result of the absorption of the X‐rays by the upper portions of the sample. This self‐absorption reduces the ability of this technique to quantify the density of the material in full 3D. Self‐absorption modeling will be required to compensate. Scans through the aerogel surface indicate an increased density at the surface due processing. Finally, a 3D image of a machined aerogel tube is presented. Copyright © 2012 John Wiley & Sons, Ltd.

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