130 



MICROSCOPIC TECHNIQUES 



wavelengths are equal, i.e., /i = h, the equation may be simplified 

 as follows : 



ii = 42e-(Mi/p-M2/p)a: = ^26-** 



Values for (i) and (/) are determined experimentally, and (m/p) can 

 be obtained from tables of mass absorption coefficients for different 

 elements and wavelengths, or it may be calculated. 



10- 



10' 



10- 



10' 



10' 



1 

 z 



K Edge 31 

 L,,, Edge 

 Mv Edge 



10 ' 20 30 40 50 60 70 80 90 ^ 



]3,06 1.28 0.69 0.42 0.29 0.20 0.15 0.11 A 



9M 5.56 3.15 1.90 1.39 1.01 0.76 A 



5.33 3.72 A 



10-' 



10" 



E 

 ^10"' 



o 10 



< 



10" 

 10" 



6C 7N 80 llNa 12 Mg 15P 16 S 17 CI 19 K 20Ca 26re 



Fig. 16. Value of k, that is, (mi/p) 

 — (m2/p), for different absorption edges 

 and different elements. For 15 P the 

 wavelength for the L edge is Ljj 



10 20 30 40 50 60 70 



ATOMIC NUMBER, Z 



80 90 



Fig. 17. The smallest determinable 

 amount of an element. 

 From Engsirom (1946) 



-L 



III- 



Fro7n Engstrom (1946) 



From the preceding equation it follows that the smallest deter- 

 minable quantity of the element per unit of surface is a function of k 

 or (mi/p) — (m2/p) . Accordingly, an absorption edge should be chosen 

 at which the difference between the mass absorption coefficients is 

 as large as possible. The variation of k with elements of different 

 atomic numbers (Z) is shown in Figure 16, which illustrates that the 

 K-absorption edge must be used for elements of low atomic number, 

 the Lni edge for elements in the middle of the periodic table (the 

 Lni is of greater magnitude than Lj or Ln), and the My edge for the 



