SURFACE LAYERS OF CRYSTALS 



141 



current produced in the ionization chamber which is at twice the Bragg angle 

 to the incident beam. A fihn in a paper envelope is then placed before the 

 ionization chamber in a holder which permits a small portion of it to be ex- 

 posed at a time. A brief (5 or 10-second) exposure is then made with the 

 crystal plate in this "zero position",^ (see Figs. 4 and 5), recording the strong 

 characteristic radiation reflected from the main crystal plate. At the same 



(film)- 



(10 SECONDS)- 



(FILM)- 

 (10 MINUTES )- 



I 



(FILM) 



(FILM)- 



SHORT )( 



ZERO POSITION 



POSITION AFTER 

 NEGATIVE ROTATION 



Fig. 4. — Spectrometer photography of misoriented crystalline material 



time the weak white radiation is being reflected from the disturbed material, 

 but this radiation is relatively so weak and the disturbed material of such 



* To check the correctness of the "zero" setting, a "rocking" exposure is taken, during 

 which the plate is rocked through the Bragg angle. The upper half of the beam should be 

 shielded for the "zero" ex]iosure and the lower half for the "rocking" exposure so that the 

 film need not be moved between the two exposures. In the "rocking" exposure the beam 

 is reflected during only a fraction of the exjiosure time and because the ex|3osure is so brief 

 only the reflection of the strongest part of the incident beam (the part that is going to pro- 

 duce the reflections in later exposures) is recorded. In the "zero" exposure the crystal 

 plate may have been set so as to reflect the divergent, weaker rays of the beam which may 

 differ in direction from the strong part of the beam by as much as 15 minutes. The terms 

 "stronger" and "weaker" do not refer here to characteristic and white radiation, but to 

 parts of the beam that have more or fewer X-rays due to the geometry of the collimating 

 system with relation to the target. 



