Dr. Peter King, 74, a retired scientist at the Office of Naval Research who was mainly responsible for development of the system that in 1949 detected the first explosion of an atomic bomb by the Soviet Union, died of cancer Feb. 25 at his home in Fairfax.

Dr. King was able to detect the Soviet atomic explosion through a chemical analysis of fissionable material from the explosion that was found in rainwater collected in Kodiak, Alaska, and Washington state. His discovery enabled President Harry S Truman to make a dramatic announcement to the world on Sept. 23, 1949, that the Russians had exploded a nuclear device less than a month earlier -- before the Russians announced it themselves.

But Dr. King's role in the detection of Russia's breaking of the U.S. nuclear monopoly remained unknown to the public until 1960 when he was given a Distinguished Civilian Service Award, the Navy's highest civilian decoration, at a ceremony at the Pentagon. The accompanying citation credited Dr. King with identifying the fissionable material from the Soviet blast and fixing the approximate time of the explosion.

The United States was not conducting nuclear testing at the time of the Russian blast -- Aug. 29, 1949 -- so the scientists were able to conclude that the nuclear device could have been detonated only by the Russians.

Dr. King was born in Boston, attended the University of Notre Dame and graduated from the University of California at Los Angeles. He joined the Naval Research Laboratory in 1939 and later went to the Office of Naval Research. He earned a doctorate in chemistry at Catholic University in 1942.

He became superintendent of the chemistry division and associate director of research for materials in 1956. In these capacities, he directed research programs in applied chemistry and physics. In 1964, he became chief scientist at the Office of Naval Research branch in London, and he remained there for two years before returning to become chief scientist at the Office of Naval Research here. He retired in 1972.

In retirement, Dr. King was an amateur silversmith, and he also established and operated a blacksmith shop at his farm in Fulton County, Pa.

He was a member of the Cosmos Club and the Palaver Club of Washington and the St. Louis Catholic Church in Fairfax.

He is survived by his wife, Elizabeth, of Fairfax; three daughters, Pamela True of Oakland, Calif., Elizabeth Goforth of Parks, Ariz., and Victoria King of Burlington, Vt.; three sons, Peter Jr. and Stephen, both of Hardwick, Mass., and John of Reston; a brother Hugh, and two sisters, Audrey Devine and Janet King, all of Los Angeles, and six grandchildren.