Hall, a native of New York City, died Nov. 1 of cancer in Cambridge, England, his wife, Joan, said today.
A Soviet cable declassified by the U.S. National Security Agency in 1995 identified Hall and his Harvard roommate, Saville Sax, as Soviet informants. The FBI had questioned Hall and Sax in 1951, but did not press charges for lack of evidence. Sax died in 1980.
At the time the cable was published, Hall was at the end of a distinguished career at Cambridge University, where he had was a pioneer in developing biological X-ray microanalysis.
Hall was 19 when he was recruited to work on the atomic bomb project at Los Alamos, N.M.
His spying was detailed in a 1997 book, "Bombshell: The Secret Story of American's Unknown Spy Conspiracy," by Joseph Albright and Marcia Kunstel.
In a statement to Albright and Kunstel in 1997, Hall said he had been "immature, inexperienced and far too sure of myself" when he was working at Los Alamos.
"During 1944 I was worried about the dangers of an American monopoly of atomic weapons if there should be a postwar depression," he added. "To help prevent that monopoly, I contemplated a brief encounter with a Soviet agent, just to inform them of the existence of the A-bomb project. I anticipated a very limited contact. With any luck it might easily have turned out that way, but it was not to be."
Hall publicly confirmed his activities last year, said his daughter, Ruth Hall.
"I don't call it an admission. I call it an achievement," she said. "He wasn't a spy in the sense of someone who goes into it for glory or money."
It was long believed that David Greenglass and Klaus Fuchs, both convicted of spying, gave the Soviets the first information on the "implosion principle" developed at Los Alamos as a new way to ignite an atomic bomb.
Albright and Kunstel contended that Hall divulged it first, though some evidence suggests Fuchs was giving more detailed information to the Soviets at the same time.
In late 1944, according to the book, Hall passed a description of the implosion principle to Saville Sax, who took it to their Soviet control officer in New York.
Robert McQueen, an FBI agent who interrogated Hall in 1951, told The Washington Post in 1996 that he "was convinced that Hall was guilty, but I could never develop enough evidence to prosecute him."
Last year, Hall told the British Broadcasting Corp. how the FBI interrogation ended.
"I reached for my coat, I think, and I just got up and walked out. And step by step, waiting for the handcuffs to be put on before I walked into the elevator, expecting to be collared before I got on the elevator. But I called for the lift, and it came, and I went in, and I got in and went downstairs, and walked out onto the street. And they didn't come," he said.
Hall is survived by his wife, two daughters and three grandchildren.
© Copyright 1999 The Associated Press