inclined slighty to B (Fig. 4.4), it might be possible for

B' to satisfy the Bragg condition for the above value of

h2+k2+12. Fortnuately, it is not necessary to calculate

the indices of B'. One first calculates the Bragg angles

for the planes that can diffract in the FOLZ (those planes

listed in Table 4.1) and then calculates the actual angle

these planes make with the given beam direction, in this

case, the (114). If the difference between theta a, the

angle the low index beam makes with the plane, and theta

b, the calculated Bragg angle for that plane, is less than

alpha, the angle of convergence, then that diffracting

plane will produce a FOLZ line. This situation is

described in Figure 4.5. If theta a and theta b are both

plotted against h2+k2+l2, the two intersecting curves in

Figure 4.5 result. Note that the two curves intersect at

the value of h2+k2+12 calculated from the equation. Any

convergence angle up to 4.8 mrad can be superimposed onto

this figure and the resulting range of h2+k2+12 values

determined. For an alpha of 2.5 x 10-3 rads, this range

of values is 100 to 83, a slightly smaller range than

determined by the first method.

 Once the range of h2+k2+12 values has been determined

and the actual indices assigned as in Table 4.1, it is a

simple matter to index any pattern for the FCC crystal.

One first indexes the zero order Laue zone (ZOLZ) in the

normal way (Edington, 1976). This zero order indexed

pattern is shown in Figure 4.6 for a [114] CBED pattern