On May 9, 1951, on a coral atoll in the Pacific, scientists ignited what they hoped would be the first man-made thermonuclear reaction, the basis of the hydrogen bomb. A fireball rose 1,800 feet.

But the explosion alone, awe-inspiring though it was, was not enough to convince one eyewitness, Edward Teller, considered the father of the H-bomb, that thermonuclear fusion had indeed occurred. For that he had to wait for the results of a test devised by two young fellow physicists who worked with him at Los Alamos, N.M., where the first atomic bombs had been built.

At 5:30 the next morning, one of those colleagues, Louis Rosen, told Dr. Teller the exhilarating news: yes, fusion had been achieved. Dr. Teller promptly raced out to pay off a $5 bet.

He had bet against himself, wagering with a colleague that fusion would not occur.

Dr. Rosen died on Aug. 15 in Albuquerque at the age of 91; his granddaughter, Ambyr Hardy, said a subdural hematoma was the cause.