Application of nuclear reaction analysis to trace oxygen analysis in metal fluoride materials
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F. N. Tebbe, L. E. Firment, and L. H. Brixner E. I. du Pont de Nemours & Company, Wilmington, Delaware 19898 (Received 22 August 1988; accepted 23 February 1989) Trace oxygen contamination of metal fluoride glasses degrades their performance in applications such as optical fibers or x-ray phosphors. Traditional methods for oxygen determination based on reaction with carbon to form carbon monoxide are slow and insensitive. We have used nuclear reaction analysis, in particular the l8 O(p, a) reaction, to determine oxygen content in a variety of metal fluorides. Careful choice of the energy of the incident proton eliminates interference from fluorine. Standardless quantification of our measurements yields good agreement with the known oxygen content of mixtures of oxides and fluorides and with several (non-fluoride) NBS standards over four orders of magnitude. Measurement of 30 ppm oxygen has been demonstrated.
I. INTRODUCTION Improved methods of analysis are needed to support development of fluoride materials for new applications such as optical fibers and digital radiography. In this paper, we present the application of nuclear reaction analysis (NRA) as a direct, quantitative method with great dynamic range for measuring the oxygen content of metal fluoride materials. The optical transmission of fluoride glasses is significantly higher than that of corresponding oxide glasses. The greater electronegativity and lower polarizability of fluorine decrease absorption in the UV and visible, while the weaker bond force constants and larger mass of fluorine lower the phonon frequencies and their overtones in the infrared. In addition, fluorides have a lower probability of incorporating impurity H than oxides, which decreases the problem of high frequency HX vibrational absorptions. For these reasons there have been considerable interest and success in producing optical data transmission fibers from metal fluoride glasses.1 The desirable optical properties of fluoride glasses are degraded by the presence of oxygen in the glass.2 In addition to increasing the absorption of light, oxide phases may precipitate and cause scattering losses.3 Development of low-loss fluoride glass optical fibers, and their subsequent manufacture, will depend on reliable methods of analyzing impurity oxygen. Another metal fluoride application sensitive to the presence of low levels of oxygen is that of barium fluorohalides in digital radiography. It has been shown4 that the controlled introduction of anion vacancies in BaFCl: Eu or BaFBr:Eu screens5 is important for trapping of electrons. "'Permanent address: Department of Physics, Nankai University, Tianjin, People's Republic of China. 916
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Oxygen in these phosphors introduces electron traps in an uncontrolled manner. It was of interest to us to attempt to prepare BaFCl and BaFBr with low levels of oxygen, and to measure the oxygen present. A direct analytical approach to the analysis of oxygen in metal fluorides is optical absorption spectroscopy, but the low leve
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