Archaeologists can figure out how old a substance is by radiocarbon dating, but to do that they need to know what the substance is—and that’s not always clear. Radioactive material comes to the rescue again!
Scientists have discovered or manufactured quite a few new elements by hurling neutrons at old elements. Along the way they learned some things. One of the main things they learned is that there are two major ways a neutron of just the right energy can affect an element. First, the neutron can gift the nucleus of the element with some energy, which the element immediately re-gifts to the world in the form of a gamma ray. That’s not the end of the process. With the addition of a neutron, the nucleus is unstable—or radioactive. Sooner or later it will decay, maybe once, maybe several times. Each time it does so it will shoot out more gamma rays, and often a beta particle such as a positron or electron as well.
The timing of each of these events depends on the properties of the nucleus. Some elements will shoot out a gamma ray once and be done with it. Other nuclei will shoot out the gamma ray and then, depending on the half-life of the nucleus, send out another gamma ray in a few seconds, a few minutes, or another year.
Even if the half-life of an element is a few years, if you shoot hundreds of thousands of neutrons into hundreds of thousands of atoms, a few will decay right away. It’s only the rate of decay that varies, and that’s the give-away. Scientists know the timing and the number of gamma rays given off by each element, and each isotope of each element. To identify the chemical composition of a substance, they bombard it with neutrons and then take note of when and how the newly radioactive substance decays. This is called neutron activation analysis, and its a boon to archaeologists, who often find artifacts but rarely find any record of how the artifacts were made. A tiny sample of the substance, and a whole lot of neutrons, and they can pin down the chemical composition of their discovery.