RADIOMETRIC BASICS

The basic idea works like this. All matter is made of incredibly small particles called atoms. Atoms have a core called the nucleus. In the nucleus are even smaller particles called protons and neutrons. (Electrons circle the nucleus.) There are 92 different kinds of atoms in nature, so there are 92 different "elements" -- substances made of these atoms. Different atoms vary only in how many protons and neutrons are in the nucleus.

For reasons unknown, some kinds of atoms are unstable. Their nuclei occasionally come "unglued." The protons and neutrons in the nucleus fly apart and the nucleus actually explodes. Fragments of the nucleus spew outward with speeds up to tens of thousands of miles an hour. The explosion also releases energy. We call such a nuclear explosion, with all the outflying fragments, radioactivity. An atom having an unstable nucleus is said to be radioactive. A radioactive atom is like a bomb. At any time it is liable to blow up, spewing nuclear "shrapnel" all around. The pieces of nuclear "shrapnel" -- the nuclear fragments -- expelled in the break-up of an atomic nucleus are called radioactive emissions.

Since no one understands what makes atoms unstable, no one can predict when one will blow up. But Marie Curie found that we can measure how long it takes for half the atoms of any given type to break up or "decay," an interval of time called the "half life." Most rocks, minerals and all other things have small amounts of radioactive atoms in them. Atoms of one kind of element can bond with atoms of another to form a "compound." Almost all radioactive atoms in nature are therefore bonded into compounds with other atoms which are not radioactive. This means ordinary substances -- the compounds, and mixtures of compounds, we encounter every day -- have small amounts of radioactive atoms as part of their make up.

Radioactive atoms are in the compounds of rocks, the minerals in the rocks, the air, water, the food we eat, even our own bodies! Every second our bodies are bombarded with thousands of fragments from radioactive atoms exploding, decaying, all around us -- some of them within ourselves. This natural radioactivity around and within us is called "background radiation." We cannot not escape background radiation, even by sealing ourselves off from the world, because the air, water and food we need for life is naturally slightly radioactive. Even more to the point, some of the natural compounds in our bodies themselves contain radioactive atoms.

But what about a rock below ground, protected from the world around it? Or a long-dead person -- a mummy -- sealed up in an ancient tomb? These things would be receiving no radioactive atoms -- in air, water or food -- from the outside, so the radioactive atoms originally in them would slowly decay, exploding into fragments that are not radioactive. Eventually, if the thing were old enough, we would be unable to measure any radioactive fragments ("emissions") coming out of them. There would be little or no radioactivity left. And herein lies the entire basis for radiometric dating.

If we could be sure that our sample -- our rock or long-dead mummy -- had been sealed off from the world since it was newly formed (or in the case of the mummy, newly dead), then its remaining level of radioactivity might tell us its age. At least this idea would work if we could compare the present level of radioactivity with whatever the radioactivity was when the sample was new. Measuring the present radioactivity is no problem. Labs around the world routinely carry out such measurements. But how can we know the original level of radioactivity?

It is often assumed if a sample has little radioactivity it must be old -- radioactive atoms have had lots of time to decay, so not much radioactivity remains. It there is more radioactivity in the sample, this means radioactive atoms have not had so much time to decay, and the sample must be younger. To put it another way, if the difference between original ("primordial") radioactivity and the present level of radioactivity is large, the sample must be old. If the primordial and present levels differ only a little, the sample must be younger.