Evans , Robley Dunglison

(1907–) American physicist

Born in University Place, Nebraska, Evans was educated at the California Institute of Technology where he obtained his PhD in 1932. He went to the Massachusetts Institute of Technology in 1934 and was appointed professor of physics there in 1945.

In 1940 Evans suggested that radioactive potassium–40 could be of use in geologic dating. It is widespread in the Earth's crust and has an exceptionally long halflife of over a thousand million years. It decays to the stable isotope argon–40 and determination of the ratio of ⁴⁰K to ⁴⁰Ar allows estimations of the age of potassium-bearing rocks ranging from 100,000 to about 10 million years. The technique proved to be particularly valuable as it permitted accurate dating beyond the limits of Willard Libby'scarbon–14 technique.