790 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



(Davisson records that elastic scattering had previously been observed only 

 with electrons having initially an energy of 12 electron-volts or less). A 

 collector is posted at a place where it collects such electrons as are scattered 

 in a direction making some chosen angle 6 with the direction exactly oppo- 

 site to that of the original or "primary" beam. There may be inelastically- 

 scattered or secondary electrons which travel toward the collector: its 

 potential is so adjusted as to prevent the access of these. 



The collector is moved from place to place so as to occupy successively 

 positions corresponding to many values of the angle d. It is always in the 

 same plane passing through the primary beam, and so the curve of number- 

 of-scattered-electrons (per unit solid angle) plotted against is a cross- 

 section of a three-dimensional scattering pattern; but, for obvious reasons 

 of symmetry, the three-dimensional pattern is just the two-dimensional 

 pattern rotated around the axis which is provided by the primary beam. 

 This two-dimensional pattern is what I have called the polycrystalline 

 scattering-pattern. It is a curve plotted, in polar coordinates or in Cartesian, 

 against B over a range of this angle which extends from —90° to +90°; 

 but the part of the curve which runs from 6 = —90° to 0=0° is the 

 mirror-image of the other part, and either by itself suffices. The curve can- 

 not be plotted in the immediate vicinity of ^ = 0°, because the source of 

 the electrons gets in the way. 



The first published report of such an experiment is to be found, under 

 the names of Davisson and C. H. Kunsman, in Science of November 1921; 

 in that same November Davisson presented the work before the American 

 Physical Society. The metal was nickel, and the pattern had two most 

 remarkable features. These were sharp and prominent peaks; one inferred 

 from the trend of the curve in the neighborhood of ^ = 0"^ and presumably 

 pointing in exactly that direction, consisting therefore of electrons which 

 had been turned clear around through 180 degrees; the other pointing in a 

 direction which depended on the speed of the electrons, and for 200-volt 

 electrons was at 70°. 



Any physicist who hears of experiments on scattering is likely to think 

 of the scattering-experiments performed by Rutherford now more than 

 forty years ago, which established the nuclear atom-model. These were 

 measurements of the scattering-pattern of alpha-particles, and this does not 

 look in the least like the curve observed by Davisson and Kunsman: it 

 shows no peaks at all. Alpha-particles, however, are seven thousand times 

 as massive as electrons: they are deflected in the nuclear fields, and so great 

 is the momentum of an alpha-particle that it does not suffer any perceptible 

 deflection unless and until it gets so close to a nucleus that there are no 

 electrons at all between the nucleus and itself. But with so light a particle 



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