The Source

by James A. Michener

Laboratories determine the Carbon-14 content in a sample by counting the number of Carbon-14 disintegrations per minute per gram of ordinary carbon. Living samples give off 15.3 such disintegrations a minute; those that died in 3535 B.C.E. give off half that number, or 7.65; and those that died in 9035 B.C.E. yield 3.83 disintegrations per minute. Unfortunately, material that died more than 50,000 years ago yields such a diminished rate that present instruments cannot accurately measure the disintegrations, so that dates earlier than 70,000 are largely guesswork, although a similar substance, potassium argon, promises to yield reliable dates back to two million years. Cullinane had submitted his carbon samples to two different laboratories—he had more than forty to choose from, in countries from Australia to Switzerland—so that one result could be checked against the other.