File:X rays and crystal structure (1915) (14591373038).jpg

Identifier: xrayscrystalstru00braguoft (find matches)
Title: X rays and crystal structure
Year: 1915 (1910s)
Authors: Bragg, William Henry, Sir, 1862-1942 Bragg, William Lawrence, Sir, 1890-1971
Subjects: Crystallography X-rays
Publisher: London : G. Bell
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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mall changes of position, recording thecurrent for each position. Let 6 (Fig. 5) be thesource, AB be the crystal: and let the slit of theionisation chamber move over the arc CD. All thereflected rays appear to come from S\ the image ofS in the line AB. For simplicity we suppose thereflecting layer to be very thin, so that S may beassumed to be a point. In reality, it is drawn outinto a line SS, which is a continuation of ^►S. Therays reflected at A are of a wave length (considering THE X-RAY SPECTROMETER 27 the reflection of the first order only) given byA = 2rtsin 6, where 6 is the angle SAN. The wavesreflected along BC have a wave length equal to2^sin SEN, and so on. Thus the radiation isanalysed into a spectrum along CD ; and if thevarious distances and angles are measured, the wavelength of any particular ray can be determined interms of 2d, the constant of the crystal grating. In this experiment the slit at A (Fig. 4) may bemade narrow in order that it may be taken as the point
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Fig. 5. source 5. The slit at B is not wanted, and may beleft wide ; the slit of the ionisation chamber must benarrow. Unless the cathode rays impinge on arather large area of the anticathode, so that a pencilof some divergence passes through A, the range of thespectrum on CD (Fig. 5) is only small, but it is oftenquite sufficient. For example, in Fig. 6 the crystalis sylvine (KCl), and the table carrying the crystal 28 THE X-RAY SPECTROMETER is set at such an angle that the primary rays passingthrough the axis make an angle of 5 40 with thecrystal face. The ordinates of thefigure show the currents obtained atvarious settings of the ionisation cham-ber. The curve shows the existenceof a strong radiation at 11^ 20, whichis in fact the principal rhodium line. If the source of the rays on theanticathode is fine enough, both theA and the B slits may be left wide.If the crystal is large and uniform,quite a considerable length of spectrummay be mapped in this way. A photo-graphic plate may

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