Radiographic Detection of 100 Å Thickness Variations in 1-μm-Thick Coatings on Submillimeter-Diameter Laser Fusion Targets

Abstract

We have developed x-ray radiography to measure thickness variations of coatings on laser fusion targets. Our technique is based on measuring the variation in x-ray transmission through the targets. The simplest targets are hollow glass microshells* or microballoons** 100 to 500 μm in diameter (Fig. 1), that have several layers of metals or plastics, 1 to 100 ym thick (Fig. 2). Our goal is to examine these opaque coatings for thickness variations as small as 1% or 0.1%, depending on the type of defect. Using contact radiography we have obtained the desired sensitivity for concentric and elliptical defects of 1%. This percentage corresponds to thickness variations as small as 100 A in a 1-μm-thick coating. For warts and dimples, the desired sensitivity is a function of the area of the defect, and we are developing a system to detect 0.1% thickness variations that cover an area 10 μm by 10 μm.