Radioactivity was discovered by Marie Curie at the end of the nineteenth century. In 1902 Frederick Soddy (who later discovered isotopes) and Ernst Rutherford jointly discovered that uranium and thorium radio actively decayed at a constant rate. (It always takes exactly the same amount of time for exactly half of the radio active atoms in a sample to decay. It's called a half-life.)
They also discovered that these two radioactive elements fissioned(radioactively decayed) into other elements in a fixed sequence; they always fissiioned in the same way into the same elements. The stage was set for someone to figure out how to use this new information.
Bertram Boltwood was born in 1870 in Amherst, Massachusetts. He studied physics (and later taught physics) at Yale University. While doing research in 1905, Boltwood noticed that when he analyzed the composition of minerals containing uranium or thorium, he always found lead.
Thinking that this find might be significant, he studied 43 mineral samples and ranked them by their estimated age. The amount of lead in these samples always increased as the samples grew older, just as the amount of uranium in them decreased. Boltwood concluded that the ra dioactive decay series starting with uranium ended by creating lead, which was not radioactive. (Uranium eventually decayed into lead.)
He studied the same process with thorium minerals and found the same result.
Boltwood surmised that, if uranium and thorium decayed at fixed, known rates, then he should be able to use the amount of lead and the amount of either of these radioactive materials in a rock sample to de termine how old the rock is, that is, how long it had been since the radioactive decay process in that rock began. In his test samples, he used a Geiger coun-
ter to estimate how many at oms of uranium decayed per minute and an early mass spectrometer to determine how much of each trace element existed in the rock sample.
Knowing how much lead and uranium currently existed in the sample, knowing how fast the uranium de cayed, and knowing the half-life of that particular uranium isotope, Boltwood could then calculate how long ra dioactive decay had been occurring in that rock.
This would tell him how old the rock was. In 1907 Boltwood published his calculations for the ages of 10 mineral samples. In every case they were startlingly old, showing that these rock samples (and the earth) were
tens, and even hundreds of times older than previously thought. Boltwood estimated the age of Earth at over 2.2 billion years (low based on present knowledge, but well over 10 times older than any previous estimate).
In 1947 American chemist Willard Libby realized that the recently discovered carbon isotope, carbon-14, could be used to date plant and animal remains in the same way that uranium was used to date rocks. Libby's carbon-14 dating accurately dated plant tissue back to 45,000 years and has been used to date paper samples as well as plant tissue.
