Scientist and historian Louis Brown, 75, who worked at the Carnegie Institution for more than 30 years, died of a heart attack Sept. 25 as he and his wife left an afternoon performance of "Madame Butterfly" at the Metropolitan Opera in New York City.

Dr. Brown, a Washington resident, was best known for his work in physics and geochemistry, and for building instruments to explore nuclear phenomena and make isotopic measurements. He spent most of his career at Carnegie's Department of Terrestrial Magnetism, which studies the physical earth and planetary and astronomical sciences.

Although he retired in 1994, he still went to the laboratory every day, walking to the institution's Broad Branch Road campus from his Connecticut Avenue apartment, his brother said. When he died, Dr. Brown was building a mass analyzer to study the isotopic composition of meteorites.

Although Dr. Brown wrote dozens of scientific papers, colleagues said he took the most pleasure from writing history. He recently completed a history of the past 100 years of science at his Carnegie department; the work will be published this fall by Cambridge University Press.

His 1999 book, "A Radar History of World War II," was called "a compendious and scholarly history" by Physics Today. Historian Richard Rhodes said it was "a great book, of permanent value: powerful . . . and freighted with deep insight into science and human affairs. It will remain for all time the definitive history of the invention and application of radar during the Second World War."

Dr. Brown was born in San Angelo, Tex., and graduated from St. Mary's University in 1950. He joined the Army, serving in Germany as an artillery lieutenant. After his tour, he attended the University of Texas, where he received a doctorate in physics in 1958.

He began his career at Carnegie, working on the Carnegie Van de Graaff accelerator project, an American-Swiss collaboration to study nuclear interactions and beams of heavy ions.

For the next 15 years, Dr. Brown directed a program in nuclear physics at the Carnegie Institution in collaboration with the University of Basel. That work led to his exploration of the isotopic composition of rocks and soils, which required the design and fabrication of novel ion detectors for mass spectrometry. Dr. Brown helped develop and use the method of the accelerator mass spectrometry with members of the Tandem Laboratory of the University of Pennsylvania.

He played a pivotal role in showing that oceanic sediments are carried deep into the earth and contribute to the molten rock that erupts from volcanoes in island arcs, such as those that cluster around the Pacific Ocean.

Dr. Brown was a fellow of the American Physical Society and received the Amerbach Prize from the University of Basel in 1963.

He loved the opera from his college days, and frequented the Washington theater scene. He also taught himself Spanish.

Survivors include his wife of 52 years, Lore Elisabeth Frick Brown, of the District; and his brother, Michael Brown, also of the District.

Louis Brown, shown in 1993, worked at Carnegie for more than 30 years. A historian, he also wrote the lauded book "A Radar History of World War II."