The 1890s was a decade of mysterious rays. Cathode rays were being studied in several laboratories, and, in 1895, Röntgen discovered x rays. In 1896, after hearing of Röntgen’s discovery, the French scientist A. H. Becquerel wondered if mineral crystals that fluoresce after exposure to sunlight were emitting x rays. He put a piece of film in an opaque envelope, then placed a crystal on top and left it in the sun. To his delight, the film in the envelope was exposed.
Becquerel thought he had discovered x rays coming from crystals, but his joy was short lived. He soon found that the film could be exposed equally well simply by being stored in a closed drawer with the crystals. Further investigation showed that the crystal, which happened to be a mineral containing uranium, was spontaneously emitting some new kind of ray. Rather than finding x rays, as he had hoped, Becquerel had discovered what became known as radioactivity.
Ernest Rutherford soon took up the investigation and found not one but three distinct kinds of rays emitted from crystals containing uranium. Not knowing what they were, he named them for their ability to penetrate matter and ionize air. The first, which caused the most ionization and penetrated the least, he called alpha rays. The second, with intermediate penetration and ionization, were beta rays, and the third, with the least ionization but the largest penetration, became gamma rays.
Within a few years, Rutherford was able to show that alpha rays are helium nuclei emitted from the crystal at very high velocities. These became the projectiles that he used in 1909 to probe the structure of the atom. The outcome of that experiment, as you learned in Chapter 37, was Rutherford’s discovery that atoms have a very small, dense nucleus at the center.
Rutherford’s discovery of the nucleus may have settled the question of atomic structure, but it raised many new issues for scientific research. Foremost among them were:
What is nuclear matter? What are its properties?
What holds the nucleus together? Why doesn’t the repulsive electrostatic force blow it apart?
What is the connection between the nucleus and radioactivity?
These questions marked the beginnings of nuclear physics, the study of the properties of the atomic nucleus.