Page 16 - Modern physical chemistry

1.6 Bragg Reflection 5

positions and emit radiation. The resulting line spectrum is characteristic of the atoms
present in the target because the pertinent energy levels are.
For nearly monochromatic X rays, a person adjusts the tube voltage to obtain as much
contrast as possible between the desired line and the background bremsstrahlung. Also,
one interposes some material that absorbs the unwanted lines more effectively than the
desired line. This filter may be used as a window in the X-ray tube.

1.5 The Action of Crystals on the Radiation
Max von Laue and his students Friedrich and Knipping were the first to find that X
rays are suitable for studying the internal structure of crystals. In 1912, they reported
having put photographic plates (1) between the crystal and the X-ray source, (2) on either
side of the crystal, and (3) behind the crystal. They obtained positive results with the
third arrangement, finding a definite diffractive pattern of spots. Laus's method will not
be analyzed here; instead, we will consider the simpler Bragg method.
w. H. Bragg and others early found that the radiation scattered by crystals possessed
the same ionizing power and was absorbed like the incident radiation. When polychro-
matic X rays were employed, the scattered radiation consisted of X rays varying in inten-
sity and composition with direction.
W. L. Bragg (son of W. H. Bragg) found that each set of diffracted rays converged as
though the incident beam was being reflected from parallel planes within the crystal. See
figure 1.3. Turning the crystal altered the reflection. In fact, the emerging beam moved
as light does from a similarly rotating optical mirror. The diffracting substance was acting
as a pack of parallel mirrors.
But what are these X-ray mirrors in a crystal? W. L. Bragg suggested that they are
planes of high atomic density, since he found that reflection occurred from planes par-
allel to cleavage surfaces. Those planes parallel to good cleavage surfaces were very good
X-ray reflectors. Evidence that the mirrors were within the crystal was obtained by Hupka
when he showed that roughening the surfaces had no effect on the reflection of X rays
although it did cause ordinary light to be scattered diffusely.

1.6 Bragg ReDection
The younger Bragg went on to treat the reflection of monochromatic X rays from a
set of planes. The following discussion is based on his work.
Let X rays of given wavelength A. strike parallel partially reflecting planes spaced dis-
tance d apart as figure 1.4 shows. In the approximation that specular reflection occurs,
the angle between a reflected ray and the pertinent plane equals the angle between the
corresponding incident ray and the plane, 8.

Source Detector

FIGURE 1.3 Diverging incident
beam producing a converging
Crystal reflected beam.