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Using an RCA EMD-2 electron diffraction unit or comparable equipment, it is felt that interplanar spacings or d values can be determined to within 0.1 per cent or better for spacings below 1.25 Å. A Joyce, Loebl Mark III double-beam automatic recording microdensitometer is used to measure the positions of the diffraction rings to the nearest 0.01 mm or 0.001 deg 2θ,. It is possible to measure the positions of diffraction rings or spots with still greater precision since line breadths at half maximum have been measured to be as low as 0.15 mm or 0.017 deg 2θ,. However, a number of instrument errors and uncertainties introduced by the specimen itself make greater precision of pattern measurement unwarranted. These errors are discussed, and methods are proposed for their reduction or elimination by such means as the use of internal standards, high-resolution photographic plates, and a simple correction for the approximation, 2 sin θ, = tan 2θ, generally made in the Bragg equation for small angles.
Dorsey, John R.
U.S. Department of Defense, Washington, D. C.