Distribution of chlorine in quartz determined by neutron beam focusing prompt gamma activation analysis and micro-x-ray
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William J. Heward GE Global Research Center, Niskayuna, New York 12309
Rick L. Paul National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Frederic J. Klug and Yan Gao GE Global Research Center, Niskayuna, New York 12309 (Received 15 May 2003; accepted 21 July 2003)
This paper describes two spatially resolved analytical techniques for chlorine distribution analysis in high-purity quartz glass. The first, prompt gamma activation analysis (PGAA), to which most of this paper is devoted, is emphasized because a new neutron focusing technique has made this study feasible. Despite the low concentration of chlorine, the neutron absorption cross section for Cl is about 200 times greater than for Si, making Cl in Si an ideal system for PGAA. The second technique described is micro-x-ray fluorescence using a laboratory-based spectrometer. The results from each technique and their ability to quantify low levels of chlorine (400–1800 g Cl/g SiO2) are described.
I. INTRODUCTION
High-purity quartz is an important material for optical communication applications. The quartz is formed by flame hydrolysis of silicon tetrachloride producing silica soot deposited as cylinder that is sintered to high optical clarity. During the formation process, (OH)− ions are introduced into the quartz, and metal impurities may be present as well. The (OH)− presence is detrimental to wave transmission in the 1.3–1.5-m wavelength range. Typically, the (OH)− is removed by replacing the (OH)− with Cl− during sintering of the quartz cylinder in a He– Cl2 atmosphere. This procedure also serves to convert metal impurities into metal chlorides, which are known to have high vapor pressures. Because the metal chlorides subsequently sublime, the metal impurities are removed. The viscosity of the glass, an important material property during product fabrication, is a function of the chlorine content. Characterization of the bulk and radial chlorine concentration is therefore important. Despite the long history of using chlorine to reduce (OH)− and metal impurities in quartz, few analysis details are available.1,2 Neutron activation analysis (NAA) has been performed initially on a large number of samples because the bulk concentration of chlorine was the only information desired early in the study. However, to observe fully 2486
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J. Mater. Res., Vol. 18, No. 10, Oct 2003 Downloaded: 17 Mar 2015
the composition–processing–property relationships of the quartz glass, it has become necessary to observe the radial distribution of chlorine in the samples. Initial micro-x-ray fluorescence (micro-XRF) measurements have confirmed the radial-dependent distribution of chlorine in the samples, thus necessitating the need for spatially resolved chlorine concentration and distribution measurements. Typical wavelength-dispersive x-ray fluorescence spectrometers are able to quantify chlorine, but generally do not have adequate spatial resolution. In such a case, the quartz cylinder would need to be sectioned layer by l
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