146 BELL SYSTEM TECHNICAL JOURNAL 



3.1 The Double Crystal Spectrometer 



In the double crystal spectrometer (Fig. 8), X-rays reflected from a crystal 

 plate are again reflected from a second crystal plate into the ionization cham- 

 ber. Their intensity is indicated by a meter showing the amplified ioniza- 

 tion current, as in the case of the single crystal spectrometer. The divergent 

 white rays from the collimating slits are reflected from crystal plate A as 

 shown in Fig. 8: those of longer wave-length at higher angles and those of 

 shorter wave-length at lower angles in accordance with Bragg's law. If 



nX = 2d SIN e 



|9lK \ CRYSTAL 



Q \ PLATE 



ACTION OF CRYSTAL A '^2 



ACTION OF A AND B TOGETHER 



Fig. 8.— Double crystal spectrometer, parallel position. Si and S2 are collimating slits 



crystal plates A and B are placed so that similar sets of atomic planes in the 

 two plates are parallel, the same rays that were reflected from plate A will 

 also reflect from plate B as shown in Fig. 9 since each ray will meet this plate 

 at the particular angle, d, which will satisfy Bragg's law for that wave-length 

 for the atomic planes in question. 



If plate B is rotated a few seconds away from this position, however, and 

 if the crystal is perfect, the conditions for reflection are destroyed for all rays 

 so that no X-rays enter the ionization chamber. However, a plate with a 

 surface laver of misoriented crvstal material will still reflect when thus 



