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preface to fifteenth edition

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SPECTROSCOPY 7.15<br />

TABLE 7.7<br />

Analyzing Crystals for X-Ray Spectroscopy<br />

Crystal<br />

Reflecting<br />

Plane<br />

2d Spacing,<br />

Å<br />

Reflectivity<br />

Quartz 50521.624 Low<br />

Aluminum 111 2.338 High<br />

Topaz 303 2.712 Medium<br />

Quartz 2023 2.750 Low<br />

Lithium fluoride 220 2.848 High<br />

Silicon 111 3.135 High<br />

Quartz 1123.636 Medium<br />

Lithium fluoride 200 4.028 High<br />

Sodium chloride 200 5.639 High<br />

Calcium fluoride 111 6.32High<br />

Quartz 1011 6.686 High<br />

Quartz 1010 8.50 Medium<br />

Pentaerythri<strong>to</strong>l (PET)<br />

0028.742High<br />

Ethylenediamine tartrate (EDT) 020 8.808 Medium<br />

Ammonium dihydrogen phosphate (ADP) 110 10.648 Low<br />

Gypsum 020 15.185 Medium<br />

Mica<br />

00219.92Low<br />

Potassium hydrogen phthalate (KAP) 1011 26.4 Medium<br />

Lead palmitate 45.6<br />

Strontium behenate 61.3<br />

Lead stearate 100.4 Medium<br />

The long-wavelength analyzers are prepared by dipping an optical flat in<strong>to</strong> the film of the metal<br />

fatty acid about 50 times <strong>to</strong> produce a layer 180 molecules in thickness.<br />

Lithium fluoride is the optimum crystal for all wavelengths less than 3 Å. Pentaerythri<strong>to</strong>l (PET)<br />

and potassium hydrogen phthalate (KAP) are usually the crystals of choice for wavelengths from 3<br />

<strong>to</strong> 20 Å. Two crystals suppress even-ordered reflections: silicon (111) and calcium fluoride (111).<br />

Mass Absorption Coefficients. Radiation traversing a layer of substance is diminished in intensity<br />

by a constant fraction per centimeter thickness x of material. The emergent radiant power P, in<br />

terms of incident radiant power P 0 , is given by<br />

P P exp (x)<br />

0<br />

which defines the <strong>to</strong>tal linear absorption coefficient . Since the reduction of intensity is determined<br />

by the quantity of matter traversed by the primary beam, the absorber thickness is best expressed<br />

on a mass basis, in g/cm 2 . The mass absorption coefficient /, expressed in units cm 2 /g, where <br />

is the density of the material, is approximately independent of the physical state of the material and,<br />

<strong>to</strong> a good approximation, is additive with respect <strong>to</strong> the elements composing a substance.<br />

Table 7.8 contains values of / for the common target elements employed in X-ray work. A<br />

more extensive set of mass absorption coefficients for K, L, and M emission lines within the wavelength<br />

range from 0.7 <strong>to</strong> 12Å is contained in Heinrich’s paper in T. D. McKinley, K. F. J. Heinrich,<br />

and D. B. Wittry (eds.), The Electron Microprobe, Wiley, New York, 1966, pp. 351–377. This<br />

article should be consulted <strong>to</strong> ascertain the probable accuracy of the values and for a compilation of<br />

coefficients and exponents employed in the computations.

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