Radio age research, manufacturing, communications, broadcasting, television (1941)

left: crystals of graphite as they appear under the diffraction camera, the symmetry of the beads is a result of the hexagonal structure of graphite. center: combination electron microscope and electron diffraction pattern of a single flake of graphite. the center is a true electron micrograph at about 5,000 magnification. right: diffraction pattern of bauxite crystals. out and the detector receives nothing. However, if conditions are just right so that all the reflected signals are vibrating together when they leave the crystal then the de- tector receives a very strong signal. In the diffraction camera this means that no reflection is obtained from the crystal unless it is situated at just the right angle to the beam. Such a reflection leaves a spot on the photographic plate. So far our explanation accounts for only one spot on the pattern yet the pattern shown here is a series of rings. The specimen was not a single crystal, however, but literally millions of very small ones arranged completely at random. If we exam- ined one of the rings closely we should find that it is really made up of a large number of spots; each one due to a different crystal. Each of those crystals happened to be at just the right angle relative to the beam to reflect part of it by a defi- nite amount. However, they do not necessarily do so in the same direc- tion. In fact, since in every other regard the arrangement of the crys- tals is random, the result is the circle we observe. Accounting for Rings Accounting for the different rings in the pattern requires a little fur- ther understanding of the Hature of a crystal. We have said that the crystal is made up of row upon row, and plane upon plane, of atoms reg- ularly spaced in all directions. That means there are manv different ways of picking the rows and planes which describe the crystal. It is like looking over a field of regularly planted corn. The corn was planted in regular rows both along and across the field so that the farmer could use a machine cultivator in both directions and thus cultivate all around each hill without doing hand work. Crystals Have Many Planes As we look over the field, how- ever, we see that the corn is also arranged in rows other than those the farmer originally laid out but which came about as a result of the original regular arrangement. The same effect occurs in a crystal. There are many ways of finding regularly spaced planes in the crys- tal each of which can produce a reflection if the crystal is situated properly. In the corn field we would notice that the distance between extra rows is different than the dis- tance between the intended ones. In the crystal this is also true and means that the angle of reflection from each one of these extra sets of possible planes is different. The crystallographer with a good knowledge of geometry can work out what the diameter of each ring in the pattern, due to some special crystal, ought to be. Thus, the actual diffraction pattern he obtains quickly substantiates or reputes his guesses as to the nature of the specimen. Diffraction as a method of analy- sis had been well worked out, using X-rays long before Thomson's ex- periments. When he obtained his results it was obvious that electrons could be used as an alternate means of accomplishing the same result. Quite a lot of work was done using early cameras. Unfortunately, the X-ray technique was better known and better developed so that little was done about designing good elec- tron diffraction cameras in spite of a number of inherent advantages which they have for precise work in a general laboratory. Electrons can be focused. X-rays cannot; an elec- tron diffraction pattern can be made in a matter of seconds while it takes hours in the X-ray camera. Microscope Becomes a Camera With these and a number of other points in mind the RCA Labora- tories developed an adapter for en- abling an electron microscope to be used as an electron diffraction cam- era whenever further analysis of the microscope specimen seemed to be desirable. This was the first ap- plication of precision electron optics to the problem of electron diffrac- tion and the results were so encour- aging that even further develop- ment was initiated. The result has been the construction of an experi- mental model of a new type of elec- tron diffraction camera. The new instrument has several new features which the designers feel will make it an important scien- tific tool in the not too distant fu- ture. For instance, the diffraction patterns which it gives are ten to [RADIO AGE 271