Cowan died Friday at his home in Los Alamos. He was 92.
Friends confirmed his death to The Associated Press, saying it was the result of a fall at his home. Cowan was in good health and was planning to travel and continue working with the nonprofit science institute that he helped found in 1984.
"It's very sudden, very unexpected. An enormous loss," said close friend and institute co-founder David Pines. "The world is diminished for all of us who knew him."
Cowan worked at Los Alamos National Laboratory for nearly 40 years. He started in 1949 as a scientist and went on to serve as a director of chemistry and as associate lab director of research.
After doing graduate studies at Princeton, Cowan continued his nuclear research as part of the top-secret Manhattan Project that developed the atomic bomb. According to the Santa Fe Institute, Cowan was a troubleshooter for the effort at various research sites around the country and was among the few people who had knowledge of the bomb's separate components.
Cowan arrived at Los Alamos National Laboratory in 1949 and within weeks began directing efforts to turn up radioactive fallout in samples that were collected near the Soviet border. What Cowan and his team detected indicated the Soviets were in possession of a nuclear bomb.
Cowan was considered one of the world's experts on nuclear weapons diagnostics by 1956, according to a biography from the lab.
"We can truly say that our country lost a true hero today," former U.S. Sen. Pete Domenici, R-N.M., said in a statement. "He cannot be replaced, but let's hope he left footprints for us to follow."
Cowan was appointed to the White House Science Council during the Reagan administration.
It was during one of his meetings with the council that he looked around the room and thought about the need to educate the next generation of scientists to ensure the government would continue to have a valuable cadre of advisers.
Conversations about the formation of the Santa Fe Institute followed, some of them being held in the director's conference room on the fourth floor at Los Alamos Lab.
"He was a superb judge of people," said Pines. "He had a real instinct for who was a promising scientist and who was not and this was invaluable to him as he became a manager at Los Alamos."
Bill Enloe, chief executive of Los Alamos National Bank, which was founded by Cowan, said the chemist had a unique ability to lead people.
"It was not by intimidation or by position. It was because what he said made so much sense," Enloe said. "He accomplished a great deal because people were anxious to help and work with him."
Enloe ticked off a list of Cowan's accomplishments that ranged from his scientific accolades and the start of the scientific think tank to the early childhood development programs in New Mexico that he helped influence.
Then there was Cowan's love of travel, food, wine and music. He sat on the board of the Santa Fe Opera and was the first treasurer of the opera's foundation.
Pines recalled the story Cowan had told him about his role in helping preserve the opera, a venue that today draws thousands of visitors from around the world to its unique outdoor stage.
"He managed to get a loan for them from the bank that tied them over," Pines said. "Otherwise the Santa Fe Opera would have gone under many, many years ago."
Officials at the Santa Fe Opera downplayed the suggestion that the organization was ever on the financial ropes, but they said Cowan was a terrific asset to the opera while he served on the board.
Friends used words such as intelligent and practical to describe Cowan, who lived in the same modest home on Los Alamos' 42nd Street since first moving there with his wife decades ago. His wife, Helen "Satch" Dunham, was also a chemist. She died last year and the couple had no children.
Cowan was a philanthropist, having given most of his wealth to charitable causes that he was passionate about, Enloe said.
"He had a large impact on a lot of people," he said.
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