Carbon-14 is a radioactive isotope of carbon having a mass number of 14 and a half-life of approximately 5,700 years. It occurs naturally, arising from cosmic rays, and is used as a tracer in studies of metabolism and in radiocarbon dating—a method of determining the age of carbonaceous, once-living material. Carbon-14 was discovered by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory, but its existence had been predicted six years earlier by whom? Discuss