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Atom-Bomb Scientist Tells His Story

Chemist Don Hornig kept his hand on the switch to stop the test

IN the spring of 1944, my life became a mystery novel. I was working at the High Explosives Research Laboratory at the Woods Hole (Mass.) Oceanographic Institute when I got a call from Harvard Prof. George Kistiakowsky, a scientist whom I greatly admired. He was working at a new laboratory, he said, and needed me badly.

That was all I needed to say yes. I was advised to leave as soon as possible, without telling anyone, and report to 109 East Palace Ave., Santa Fe, N.M., where I would be given further instructions.

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At Santa Fe, I was directed to Los Alamos, a former boys' school on a mesa at the base of the Jemez Mountains. I found that I was to join Kistiakowsky and others under J. Robert Oppenheimer working on the Manhattan Project, the crash program to build an atomic bomb. I was 23 years old, a physical chemist of limited experience with a newly acquired PhD who had done research on shock waves produced by large explosions.

To maintain secrecy, the project's overall director, Maj. Gen. Leslie Groves, insisted on strict compartmentalization: Scientists would have access to information only on a need-to-know basis.

Oppenheimer persuaded Groves that the only hope of resolving the enormous scientific and technical problems facing his group was to let scientists discuss these challenges with one another.

When I arrived in May 1944, two ways to create a critical mass were being considered. The first used a high velocity gun to shoot two sub-critical chunks of enriched uranium (U-235) together. This would not work with plutonium, though, which requires much faster assembly to avoid a premature explosion. The solution proposed was ''implosion'': Surround a subcritical shell of plutonium with explosive charges (called ''lenses'') shaped to focus the detonation waves toward the center of the plutonium shell and drive all segments together.

Only enough U-235 would be available for one bomb. But if the implosion concept worked, several such bombs might be ready by the end of 1945.

ENOUGH was known about the gun method to justify going ahead without a full test. But the implosion method was still in a preliminary stage; among its problems was that of a firing system that could initiate all the explosive lenses at once. The only idea for doing so was an explosive switch, but that could not be tested without destroying the firing mechanism. Oppenheimer raised the problem in June 1944 at one of his seminars.

After listening to the discussion, I suggested developing triggered spark gaps to act as very rapid switches. They might make it possible to initiate the explosive lenses within a fraction of a microsecond. Oppenheimer told me to get a few people together and work on the idea. By the end of the summer, we had accumulated enough evidence to say that this method was feasible. After much debate and many misgivings, we were given a go-ahead in October to incorporate triggered spark gaps into a firing unit for ''the gadget,'' as the bomb was called.

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Many months of work remained before the firing mechanism would be ready. The ''X-unit'' had to initiate at 32 points with astonishing simultaneity. If it did not, the explosive yield would be low.

The full-scale test of the bomb was planned for pre-dawn Mon., July 16, the day before President Harry Truman was to meet with Joseph Stalin and Winston Churchill at Potsdam, Germany. By then, we felt we were ready, but many involved in the project doubted that the complicated implosion-bomb system would work effectively and with a large enough explosive yield. Several days before the firing date, the doubts were exacerbated when heavy clouds passed over the desert test area near Alamogordo, N.M., and an electrical surge set off the X-unit. It would have set off a live bomb.

On the Sunday before the test, shortly before 9 p.m., Oppenheimer decided someone should be in the tower to baby-sit the bomb because of the possibility of sabotage. Maybe because I was the youngest, I got the job. In the darkness, amid heavy rain, lightning, and strong winds, I climbed the ladder to the top of the 100-foot tower. Pulling out a paperback and sitting under a 60-watt bulb, I read to keep my mind off the lightning and the bomb. I stopped frequently to count the seconds between the sound of a thunder clap and the lightning flash, and tried not to think of what might happen if the tower got a direct hit and the gadget went off. At least I would never know about it. I was never happier than when the phone rang just before midnight, and Kistiakowsky told me to come down.

Shortly before daybreak at a concrete bunker 10,000 yards south of the tower, my final task was to watch the control panels to assure that all the condenser banks reached the 15,000 volts needed to initiate the explosion. My right hand was poised over a control switch that would stop the test if something went wrong. At 5:29:45 a.m. the gadget went off. Through the open door, looking away from the tower at the mountains, I saw a brilliant flash, then ran outside to see a great fireball rapidly rising, with peach, green, and rosy red colors, gradually transforming into a mushroom cloud.

My first reaction, having not slept for 48 hours, was ''Boy, am I tired.'' My second was ''We sure opened a can of worms.'' Nobody knew where this would lead, but I had no regrets. If it ended the war without the tremendous casualties an invasion of Japan would cause, it was worth it.

* After a career as a chemistry professor at Brown and Princeton universities, Donald F. Hornig became a science adviser to Presidents Eisenhower, Kennedy, and Johnson. Later he was president of Brown and concluded his career at Harvard.