Measurement of x-ray intensity

In the examples just given, the observed intensity was estimated simply by visual comparison of one line with another. Although this simple procedure is satisfactory in a surprisingly large number of cases, there are problems in which a more precise measurement of diffracted intensity is necessary. Two methods are available for making such measurements, one dependent on the photographic effect of x-rays and the other on the ability of x-rays to activate an electronic counter. These methods have already been mentioned briefly in Sec. 1-8 and will be described more fully in Chaps. 6 and 7, respectively. 

4-3 Consider a hypothetical element whose structure can be based on either of the following  

Determine simplified structure-factor equations for each cell and the positions (20 values) of the first four lines that would be observed (F 0) on a powder pattern made with 

Cu Ka radiation. Plot the 26 values of these lines in the manner of Fig. lD-2, and label each line with its indices relative to Cell A and Cell B. Draw the two cells in the proper relation to one another, and show that the indices of any one observed line, other than obvious ones of the form 00/, refer to the same plane of atoms. 

4-5 A certain tetragonal crystal has four atoms of the same kind per unit cell, located 0 i i i 0 i. i 0 0 i (Do not change axes.) 

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The principal functions performed by electronic circuitry in the measurement of x-ray intensity are listed below. 

The following general observations apply to both kinds of problems. The scale of operations is fixed by the necessity of comparing adjacent sections of tissue only microns in width, which makes it advisable to use the photographic plate as detector, in order that the necessary measurements of x-ray intensity can be made on greatly enlarged areas. Inas-... 

THE UNIT CELL BY THE METHOD OF TRIAL AND ERROR 204 Measurement of X-ray intensities 206... 

X being the true coefficient of absorption, and a a constant) has been adopted in practical measurements of X-ray intensity. For great intensities ap > pL strong extinction and internal mutual action) we obtain the relationships given by the dispersion theory, while for small intensities ap pi i.e. slight mutual action) we obtain the simple relationships given by the kinematic theory. 

Specimen preparation has been dealt with previously (see Chap. 7), and it will be assumed from now on that the specimen can be considered homogeneous as far as the penetration of the x-ray beam is concerned. Similarly, the selection of excitation conditions and the basis of measurement of x-ray intensity have also been dealt with in previous sections. The following paragraphs are thus concerned mainly with the conversion of x-ray intensities to elemental concentrations, although further reference will be made to special problems in the measurement of x-ray intensities where this is relevant. 

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