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

摘要

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.