Richard Brooke Roberts, 69, a retired biophysicist with the Carnegie Institution here who also did work in nuclear physics, died Friday at Suburban Hospital in Bethesda after a heart attack.

Dr. Roberts joined the institution in 1937 as a fellow. He served in the Department of Terrestrial Magnetism and from 1953 to 1963 was chairman of its biophysical section. He retired from the institution in 1975.

He was a member of a small group of American physicists who confirmed the discovery of uranium fission in 1939. He also was a principal contributor to the discovery of delayed neutrons. Control and atomic reactors is based on these neutrons.

Dr. Roberts participated in some of the first experiments involving radio-active tracers. He worked with Dr. Lewis B. Flexner in determining rates of placental transfer of radioactive sodium from the maternal to the fetal circulations.

During World War II, Dr. Roberts worked at the Johns Hopkins Applied Physics Laboratory. He earlier had conducted experiments to show that certain electronic vacuum tubes could remain functional after being fired from an artillery weapon.

This was the first proof of potential feasibility of a proximity fuze, a weapon that became a crucial factor in winning the war in both Europe and the Pacific.

The proximity fuze was brought to successful prototype in about two years and Dr. Roberts participated in all its major steps. He also helped supervise large-scale production of the weapon.

His contributions brought him the Presidential Medal for Merit in 1947.

After the war, Dr. Roberts became active in molecular biology. With a small group of the Department of Terrestrial Magnetism, he discovered the major chemicals synthetic mechanisms by which cell duplication occurs. Later, his interest turned toward neuroscience and the understanding of memory and the chemicals influencing it.

In 1961, Dr. Roberts was voted a member of the National Academy of Sciences, which recognized him as an authority on sciences dealing with the detection of hidden nuclear explosions, and at the same time on ways to muffle such explosions in shock-absorbing caves.

Dr. Roberts was born in Titusville, Pa. He was a graduate of Princeton University, where he also received a master's degree and a doctorate in physics.

He was a former president of the Biophysical Society and a member of the American Physical Society, the American Association for the Advancement of Science and the Cosmos Club.

Dr. Roberts, who lived in Washington, belonged to the Columbia and the Kenwood country clubs and the Society of the Cincinnati.

He is survived by his wife, Josephine, of Washington; three children, Richard F., of Los Angeles, Julie Haley of San Francisco, and Edward Thomas, of Bethesda; three stepchildren, Byron T. Edwards of Tacoma, Wash., Josephine T. Bullard of Greenwich, Conn., and Marcus C. Rice of Charlottesville, Va.; two brothers, Thomas and Walter, of Princeton, N.J., and 14 grandchildren.