Physicist Helped Create Atomic Bomb, Confirmed Fusion in Test

Louis Rosen, 91, a physicist who helped build the first atom bomb and several years later informed Edward Teller that Teller's thermonuclear device had worked, died of a subdural hematoma Aug. 20 in Los Alamos, N.M.

Few scientists had longer or more productive careers than Dr. Rosen or were more deeply involved in the major issues of their discipline. During 65 years as a scientist and administrator at the Los Alamos laboratory where the atom bomb was designed and built, Dr. Rosen was known as an unassuming and much loved figure. He provided advice and guidance until days before his death.

At Los Alamos, he was also recognized as architect and godfather of a major atom-smashing device, a linear accelerator that was a half-mile long and formed the focus of a research center.

Starting in 1972, it gave a river of protons a then-unprecedented push of 800 million electron volts to tease out directly -- or indirectly (through production of mesons) -- secrets from the atomic nucleus. Mesons are subatomic particles, and his accelerator was known as a "meson factory."

It was not done to satisfy scientific curiosity alone. "We are simply running out of conventional organic sources of energy," Dr. Rosen said decades ago. In seeking support for basic research, he said that nuclear technology would be needed for energy and that nuclear science would make it as safe as possible.

Dr. Rosen was born June 10, 1918, in New York and grew up in the Catskill Mountain region north of New York City. The family "was very poor," his granddaughter said, and when it was time for college, in the midst of the Great Depression, Dr. Rosen exchanged his pennies for postage stamps and wrote to state universities, far and wide.

"Whichever was the first to offer a scholarship" would be his destination, said the granddaughter, Ambyr Hardy of Long Beach, Calif. It turned out to be the University of Alabama. After obtaining bachelor's and master's degrees there, he received a doctorate in physics at Penn State. Then it was on to Los Alamos, where he joined many of the scientific titans of his time in the World War II effort to build the atom bomb.

A few years later, in 1951, a test was conducted at a Pacific Ocean atoll of the system devised by Teller for achieving nuclear fusion and ultimately the hydrogen bomb. The explosion was enormous. But in itself the blast did not prove that fusion had occurred.

Dr. Rosen and a colleague were in charge of providing the first confirmation. Their test took time. Teller was said to be downcast, uncertain, until he wakened the next morning.

According to a published account attributed to Dr. Rosen, the test results -- positive -- were just becoming visible when "in came Edward." Teller was given a quick look. Then he dashed out, "moving very fast."

Among Dr. Rosen's gifts were what the Journal of Nuclear Medicine once called "legendary abilities" to obtain government funds for science. The journal quoted him: "My wife used to say that my trips to Washington were not worthwhile unless they produced at least $2 million for every $200 I spent in travel."

His wife, the former Mary Terry, died in 2004. Their son, Terry Rosen, died in 2003.

In addition his granddaughter, survivors include a grandson; four great-grandchildren; and a brother, Bernard, a longtime Washington area resident who recently moved to San Francisco.