TUCSON, Ariz. - Willis E. Lamb Jr., a Nobel prize-winning physicist whose work on the electron structure of the hydrogen atom revolutionized the quantum theory of matter, has died. He was 94.

Lamb died May 15 in a Tucson hospital from complications of a gallstone disorder, according to an announcement from the University of Arizona, where he was professor emeritus of physics and optical sciences.

Lamb worked as a physicist at various universities from the late 1930s until retiring from the University of Arizona in 2002.

Lamb was awarded the Nobel prize for physics in 1955 for research he conducted while working at Columbia University's Columbia Radiation Laboratory. He was working on defense-related research into microwave sources for radar when he became interested in the properties of the hydrogen atom.

He designed and built a device in 1947 with Columbia graduate student R.C. Retherford to study the effect of microwave radiation on the hydrogen atom, according to a University of Arizona biography. That led to measurements that showed a change in the amount of energy emitted from the hydrogen atom in different states that became known as the "Lamb shift."

"Clearly he was a brilliant and serious scientist," his wife, Elsie Wattson Lamb, said in a statement released by the university. "But he was also deeply human."

Born July 12, 1913, in Los Angeles, Lamb attended the University of California, Berkeley as an undergraduate and graduate student. His doctoral thesis was overseen by J. Robert Oppenheimer, who went on to lead the U.S. effort to develop the atomic bomb during World War II.

Lamb married his first wife, historian Ursula Schaefer, in 1939. She died in 1996. A second marriage ended in divorce. He married Elsie Wattson earlier this year. *