Click on this image to see the details more clearly. In ultra-brief form it embodies knowledge that led to a couple dozen Nobel prizes from 1901 to the 1930s.In each small chart, the horizontal axis is the Proton number (the Atomic Number), Z, and the vertical axis is the Neutron number, N. Atomic Mass, A, is Z+N. Therefore, in the lower left Uranium Series, The starting point is U238: Z=92, N=146 and A=238.
A careful look reveals that three of the four series begin with or pass through a Uranium isotope, and all four begin with or pass through a Thorium isotope. All pass through at least Radium, Radon, Polonium, and Lead isotopes, as well as a few others. Three end at the stable isotopes of Lead, 206, 207 and 208, while the fourth ends at Thallium 205. In an alpha-beta decay scheme, one of those four isotopes must be the end result.
These are the end result. Where did we learn all this? That is the subject of Radioactivity: A History of a Mysterious Science by Marjorie C. Malley. Beginning with the discovery of "Uranium rays" that affected photo emulsions in 1896, scientists labored to learn, step by step, of the different "rays" and "emanations" of uranium, thorium and their decay products, which were initially given names like "Uranium X" and "Mesothorium".
A marvelous feature of the book is to immerse us in the time, using the terms current as the discoveries were being made. As the various radioactive "rays" were discovered, there were periods of years during which, for example, the alpha "ray" was misunderstood because, while its bending by a magnet could not at first be discerned, it was stopped by paper, unlike the x-rays to which it was being compared. Eventually the trichotomy was discerned: alpha equals fast-moving He++, beta equals electron, and gamma equals extra-powerful x-ray. Then the fun began! If heavy atoms "radiated" by emitting helium, the helium particle (this was before the neutron was known) must be a building block of atomic nuclei. Only much later were protons and neutrons found.
Before isotopes were discerned (I hesitate to say, discovered), confusion reigned. We now know that "Radium" referred to Ra226, while Actinium X, Mesothorium I and Thorium X referred to other isotopes of Radium. They could not be chemically separated, and it was only by measuring the atomic mass of radioactively-produced substances that they could be told apart until the invention of the mass spectrograph in 1919.
Radioactivity discoveries helped elucidate quantum theory. For example, the energy of emitted alpha particles was related to total intensity, or inversely related to half life. For uranium, for example, we find that isotopes with half lives of a minute or a few minutes have alpha energies near 7MeV, half lives of a few days go with energies near 6MeV, half-lives of years to thousands of years go with energies near 5MeV, and half-lives in the millions- to billions of years imply alpha energies of 4.5 MeV or less. The phenomenon of quantum tunneling and exponential statistics, coupled with Heisenberg's Uncertainty Principle, eventually explained such patterns.
Throughout the book, we learn of the persons who worked all this out, of their labors, theories, missteps and discoveries. Of course, the Curies are the most famous, and Roentgen and Becquerel and Rutherford not far behind. The roll call of famous pre-1920 physicists and chemists numbers in the dozens, and many received Nobel Prizes. Marie and Irène Curie stand out as the only mother and daughter to both receive a Nobel Prize, while the two William Braggs, father and son, received a joint Prize in 1915.
The dangers of handling radioactive materials were slow to be realized. Several persons induced radium burns in their skin, but longer-term effects went unknown for decades. As late as 1960 children touring uranium-producing facilities were given vials or capsules containing yellowcake, or pure U3O8. Both my mother and I received such souvenirs. In my case, I carried the capsule in my pocket for a few weeks before putting it in a dresser drawer. Later I gave it to a geology professor for use as a standard material in his radiation lab (by 1971 it was almost impossible to obtain uranium compounds). I don't know if carrying a strong gamma-emitter like that had anything to do with the cancer I had forty years later; it occurred at a location in my colon next to the pocket in which I carried the capsule at age 12.
By the time the Manhattan Project and similar European efforts were undertaken, the decay chains illustrated above were well known, and Radioactivity as a discipline had been subsumed into Nuclear Physics and Particle Physics. The focus of study turned to those few isotopes that could be stimulated to fission, the basis of nuclear power plants and atomic bombs. In the public mind these overshadow other advances such as nuclear medicine and radionuclide imaging and therapy, and even the tiny speck of an alpha-emitter in a smoke detector (It is a tiny enough amount that even if you dug it out and ate it, you'd suffer little harm).
We live in the post-Atomic age. Nuclear power plants are on the wane, particularly after a few melt-downs in recent years and the disaster last year in Japan. Yet much of modern life would be unthinkable without the discoveries of more than a century ago, when the understanding of how atoms worked was turned upside down, starting with a few pieces of photographic film fogged by proximity to uranium ore.
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