Quantitative Analysis of Ultralow-Density Materials Using Laboratory-Based Quasi-Monochromatic Radiography

The measurement of the density of materials, especially ultralow-density foams, is difficult in that the measurement must be precise and localizable. The density of the material is often governed by its cellular (i.e., porous) structure, and many techniques exist to create that structure. Often, the cellular structure can vary from one location within the material to another, and when at low densities (i.e., densities lower than similar to 500 mg/cm(3)), it can vary due to shrinkage during syneresis, collapse under the weight of gravity, or gas/water vapor uptake. Quantifying this variation is important for a variety of applications, especially when used in plasma physics targets. Knowing the density and its variation across the sample is critical for experimental results to be accurately predicted by physics calculations and for modeling the results of the physics targets. The use of quasi-monochromatic radiography provides a means to image the two-dimensional (2-D) distribution of density variation within silica aerogel materials and to quantitatively measure that variation from sample to sample and lot to lot. For this study, two batches of silica aerogels with targeted densities of similar to 20 mg/cm(3) were created, one batch at Lawrence Livermore National Laboratory, and the other batch at Los Alamos National Laboratory. Outlined here is a quasi-monochromatic radiography system using various X-ray sources coupled to a doubly curved crystal optic and X-ray charge-coupled device camera to image and characterize these materials. It was found that measuring the density both gravimetrically and using quasi-monochromatic radiography were statistically identical, although the two batches were found to be slightly higher than their targeted density due to shrinkage. The radiography system also provided 2-D information as to the aerogel quality, i.e., presence of voids, chipped material, or inclusions.