“In the first place, this method works only in certain cases of suspected forgery. Over the last 50 or 100 years, a number of paintings have turned up that seemed, even to the best art experts, to be several hundred years old. Some of these were genuine, and some were painted by forgers who could not resist the high prices paid for works of art. The National Gallery of Art, in Washington, D. C., thinking that there might be a way of detecting these forgeries, gave its support to a group of scientists who developed a method for this purpose.
“To understand how the method works, you need to know a little about how radioactive atoms disintegrate to form atoms of other elements. In this case we are interested in the natural radioactivity that occurs in certain rocks. As a matter of fact, in almost all rocks in the earth’s crust there is a certain small quantity of uranium.”
“I thought uranium was rare,” interrupted Bill.
“It is, but we’re talking about such small quantities that its difficult for scientists using the most sensitive equipment to detect it. The uranium in the rock decays to another radioactive element and that one decays to another, and another, and another, and so forth, in a series of elements that results in lead, which is not radioactive. In this series are two radioactive elements, radium and a radioactive isotope of lead, that help us to date paintings. To understand this, we must first understand how radioactive elements decay.
“All radioactive elements have what is known as a ‘half-life’; that is, in a certain period of time, half of the element disintegrates to another form. In another equal period of time, half of what is left disintegrates, and then half again, and so on. In the case of the uranium, which starts the series I am describing, the half-life is over 4,000,000,000 years. Because of its long half-life there is plenty of uranium around and will be for a long, long time. On the other hand, radium, which I mentioned a moment ago, has a half-life of only 1600 years. In 1600 years, half of it would be gone, and in another 1600 years half of that would be gone, and so on.
“The radioactive lead that we’re interested in has a half-life of only 22 years. This means that if you start with a small quantity of this radioactive isotope of lead, which is called lead-210,[1] then in only a few hundred years it would have disappeared. However, in rock, where there is uranium, the uranium keeps feeding the elements following it in the series, so that as fast as they decay they are reproduced by the element before them.”
The Uranium Series. In this simplified diagram, the double vertical arrows represent alpha radioactivity and the single slanted arrows represent beta radioactivity. The times shown on the arrows are the half-lives for each step.
Uranium-238 ⇓α4½ billion years Thorium-234 ↓β24 days Protoactimum-234 ↓β1⅕ minutes Uranium-234 ⇓α¼ million years Thorium-230 ⇓α80 thousand years Radium-226 ⇓α1600 years Radon-222 ⇓α3⅘ days Polonium-218 ⇓α3 minutes Lead-214 ↓β27 minutes Bismuth-214 ↓β20 minutes Polonium-214 ⇓αless than one second Lead-210 ↓β22 years Bismuth-210 ↓β5 days Polonium-210 ⇓α138 days Lead-206 (Not Radioactive)
“I don’t quite understand how that works,” said Harley. “What do you mean ‘it keeps feeding it’?”