4 - The search for structure

Summary

Rutherford scattering

Shortly after the discovery of the radioactivity of uranium by Becquerel in 1896 [155] and its ability to ionize gases, Rutherford [156] began a study of the rate of discharge of a parallel plate capacitor in gas by placing successive layers of thin aluminum foil over the surface of a layer of uranium oxide on one plate. He concluded that “the uranium radiation is complex, and that there are present at least two distinct types of radiation: one that is readily absorbed which will be termed for convenience the α radiation, and the other of a more penetrative character, which will be termed the β radiation.” In 1906, Rutherford [157] observed that α particles from the decay of radium scattered, i.e. deviated from their original direction of motion, when passing through a thin sheet of mica, but did not scatter in vacuum. He made this observation by passing α particles through narrow slits and making an image on a photographic plate. In vacuum, the edges of the image were sharp while the image of α particles that passed through the mica was broadened and showed diffuse edges. This observation was controversial because it was not expected that α particles would scatter [158]: “Since the atom is the seat of intense electrical forces, the β particle in passing through matter should be much more easily deflected from its path than the massive α particle.”