Chapter 5: The Earth-Moon System

Portrait of Antoine Henri Becquerel.

The discovery of radioactivity happened by accident. In 1896 French physicist Antoine Henri Becquerel left some photographic plates in a drawer with some uranium-bearing minerals. Later he opened the drawer and found the plates fogged. Being a good scientist, he did not dismiss the event but investigated further. He found that the uranium emitted "rays," which, like X-rays (discovered the previous year), could pass through cardboard. The new "rays" turned out to be not electromagnetic radiation, like ultraviolet light or X-rays, but rather energetic particles emitted by unstable atoms. It might seem that serendipity plays no part in the scientific method, yet a number of important discoveries happened by accident. Examples include X-rays by Roentgen and penicillin by Fleming. The perceptive scientist will pursue unexpected phenomena. Often, they will find a flaw in the equipment. Occasionally, they will make an important discovery. A lesser scientist than Becquerel might have seen the fogged photographs, shrugged, and carried on with his work. Louis Pasteur put it this way: "Fortune favors the prepared mind."

Alpha decay is one example type of radioactive decay, in which an atomic nucleus emits an alpha particle, and thereby transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less. Many other types of decays are possible.

Here is how radioactivity works. A radioactive atom is an unstable atom that spontaneously changes (usually into a more stable form) by emitting one or more particles from its nucleus. The original atom thus becomes either a new element (change in the number of protons in the nucleus) or a new form of the same element, called an isotope (change in the number of neutrons in the nucleus). The original atom is called the parent isotope and the new atom is called the daughter isotope. 

Simulation of many identical atoms undergoing radioactive decay, starting with either four atoms (left) or 400 atoms (right). The number at the top indicates how many half-lives have elapsed. Note the law of large numbers: With more atoms, the overall decay is less random.

The time required for half of the atoms of any original radioactive parent isotope to decay into daughter isotopes is called the half-life of the radioactive element. If a billion atoms of a parent isotope were present in a certain mineral specimen, a half billion would be left after one half-life, a quarter billion after the second half-life, and so on. Sometimes the result of the decay is another radioactive element, so the decay continues. But in every case, the final product of the decay (or chain of decays) is a stable element.

The Shroud of Turin - believed to be the burial cloth of Jesus, now known to be from the 12th century by radioactive carbon dating.

One important example is the use of carbon isotopes to measure the ages of organic materials. Every living thing takes in carbon — animals by the carbon in their food and plants by the carbon dioxide they absorb from the air. Most carbon is in the stable form of carbon-12. However, carbon in the natural environment has one in a million atoms of the isotope carbon-14, which has a half-life of 5,570 years. When living things die, no more carbon-14 is introduced and the existing amount diminishes steadily by the radioactive decay. The remaining fraction of carbon-14 gives a good estimate of how long since the living material died. This is how we know the age of wooden artifacts taken from an Egyptian tomb, for example. In another celebrated case, the Shroud of Turin — reputed to be the burial cloth of Jesus — was shown to be a fraud dating from the 12th century. By using this idea and a different isotope with a longer half-life, we can measure the age of dinosaur fossils.