Seth H. Neddermeyer, 80, a physicist who helped develop the atomic bomb and later conducted research on cosmic rays that helped lead to the discovery of subatomic particals called muons, died Jan. 29 in Seattle. He had Parkinson's disease.

Dr. Neddermeyer was largely responsible for developing the implosion trigger, the device that enabled the atomic bomb to be detonated. He was the 1983 recipient of the U.S. Department of Energy's prestigious Enrico Fermi Award.

He was a native of Richmond, Mich. He received a bachelor's degree in physics at Stanford University in 1929 and a doctorate in physics at the California Institute of Technology. He served on the physics faculty at Cal Tech from 1935 to 1941.

For the next two years, he worked for the National Bureau of Standards' national defense research committee. In 1943, nuclear physicist J. Robert Oppenheimer tapped him for the Manhattan Project in Los Alamos, where the early atomic bombs were built.

In 1946, he left the government to become a physics professor at the University of Washington, where he taught until retiring as professor emeritus in 1973.

In a 1983 interview, he said, "I get so overwhelmed by a feeling of terrible guilt when I think about the history of the bomb. I'm terribly worried now about the current world situation. What the hell can we do about it?"