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

ELECTRONS MAKE PATTERNS Neu) Instrument Extends Scope of Crystal Analysis — Device May Be Changed Instflntly to Act as a Diffraction Camera or Electron Microscope. By Dr. James Hillier RCA Laboratories, Princeton, N. J. ALMOST twenty years ago Dr. ^ C. J. Davisson and Dr. L. H. Germer, working in the Bell Tele- phone Laboratories, were bouncing relatively slow electrons on the sur- face of a piece of nickel in a vacuum tube. The results were not particu- larly spectacular. In the course of the experiments, however, they sub- jected the nickel to a heat treat- ment, probably to clean the surface, which happened to be just right to cause the normally small crystals in the nickel to rearrange themselves and form very large ones. Then the bouncing electrons started behaving in a very unusual fashion. Being good scientists, Davisson and Germer found the new phenomenon much more inter- esting than their intended experi- ment and followed it up. They then built a tube in which the piece of nickel was a single large crystal and found that electrons bouncing off the surface tended to leave in a special direction. They deduced correctly that they were observing the effects of elec- tron waves which a Frenchman by the name of de Broglie had pre- dicted should exist. Davisson and Germer's observations proved the correctness of de Broglie's theory and gave terrific stimulus to the newly developing quantum mechan- ics and to the development of elec- tronic tools of analysis ultimately resulting in the development of the electron microscope and the electron diffraction camera. A short time later Dr. G. P. Thomson, working in England, greatly simplified the Davisson- Germer experiment by using much faster electrons and by shooting them through thin metallic films in which the crystals were very small. He found that if he shot a narrow beam of electrons through the foil and put a photographic plate some distance on the other side that, in addition to the exposed spot due to the original beam, he obtained the series of concentric rings which we now call an electron diffraction pattern. Thomson's experiments o r i g - inated in a very clear-cut way the science of electron diffraction. Since it was already well known from similar work with X-rays that very few crystalline materials give the same diffraction pattern, it was obvious to Thomson and his con- temporaries that the electron dif- fraction camera was an excellent method for analysis. There are a number of everyday phenomena, which most of us have noticed, which make it quite easy for even the non-scientist to under- stand the formation of a diffraction pattern. Let us look at the factors involved. First there is a beam of electrons (or light or X-rays) which possesses a wavelength. That is just another way of saying that there is a regular vibration of some kind associated with the beam. Then there is the crystal in which the atoms are all lined up in regular rows and layers. If we have two beams in which the frequency of vibrations are the same or very nearly the same we have the possi- bility of interference between them. When Signals Fade We have all heard a radio signal fade when the sky wave interferes with the ground wave. Then the two signals coming from the same station but by different paths are vibrating at the same rate but in opposite directions and cancel each other so the receiver detects no sig- nal. (The engineer says they are out of phase) In the diffraction camera the phenomenon is almost the same. We have our signal (the beam) coming from a single station (the source of the beam) and strik- ing the crystalline specimen before we detect it. At the crystal we find that the parts of the beam that strike the atoms of the first layer are reflected but the rest goes through to the second layer and the same thing happens and so on through the crystal. As far as our detector (the photographic plate) is concerned it is receiving a signal from one station but by a large number of paths. As you might ex- pect, the signal is so confused that it almost invariably cancels itself ELECTRON DIFFRACTION PATTERN OB- TAINED FROM A THIN GOLD FILM. PATTERN OF ZINC OXIDE SMOKE PAR- TICLES SHOWS RING CONSTRUCTION. [26 RADIO AGE!