THE DIFFRACTION OF ELECTRONS BY A CRYSTAL 



95 



If we turn the crystal to any arbitrarily chosen azimuth, set the 

 bombarding potential at any arbitrarily chosen value, and measure 

 the intensity of scattering as a function of colatitude, what we find 

 ordinarily is the type of relation represented by the curve on the 

 left in Fig. 3. 



4 8V, 54 V. 



64 V. 68 V. 



A 



AZIMUTH 



CAC Bc^c Bc^c Bc 

 AZIMUTH CURVE F0R[e = 50», V = 54 VOLTS] 



Fig. 3 — Curves showing development of diffraction beam in the A-azimuth 



. . . and variation of intensity with the azimuth at colat. 50° 



for which beam is strongest in the A-azimuth 



This curv'e is actually one found for scattering in the A-azimuth 

 when the bombarding potential is 36 volts. It is typical, however, 

 of the curves that are obtained when no diffraction beam is showing. 

 The intensity of scattering in a given direction is indicated by the 

 length of the vector from the point of bombardment to the curve. 

 The intensity is zero in the plane of the crystal surface, and increases 

 regularly as the colatitude angle is decreased. This type of scattering 

 forms a background upon which the diffraction beams are superposed. 



The occurrence of a diffraction beam is illustrated in the series of 

 curves to the right in Fig. 3. When the bombarding potential is 

 increased from 36 to 40 volts, the curve is characterized by a slight 

 hump at colatitude 60 degrees. With further increase in bombarding 

 potential this hump moves upward, and at the same time develops 



