How They Built the Bomb
Fifty years ago, as war in the Pacific raged, scientists in the American desert knocked on the door of the Atomic Age
A sense of gloom as heavy as the sky overhead pervaded the small group gathered in a crude, concrete-reinforced wooden shelter on the desert of south-central New Mexico in the predawn blackness of Monday, July 16, 1945. Ten thousand yards to the north, in a cab atop a 100-foot steel tower rested a compact device called "the gadget" - the world's first atomic bomb. Everything had been in readiness for the scheduled 4 a.m. firing when the fickle weather had broken its promise. Dripping skies and unfavorable winds now threatened to cancel the long-awaited test.
Amid the desert underbrush not far from the control shelter, two men stood silently, engulfed in the urgency of the moment. Under ordinary circumstances, they would have had little in common. Prof. J. Robert Oppenheimer, gentle, aesthetic director of the Los Alamos scientific laboratory, was a theoretical physicist and, incidentally, a deep student of literature. Maj. Gen. Leslie Groves, shrewd, hardheaded boss of the Manhattan Project, the highly secret wartime crash program for developing the atom bomb, was an Army engineer, a get-it-done-and-let-the-heads-roll professional soldier with a construction man's inbred skepticism toward egghead scientists.
Yet events of the past three years had drawn these two men into a genuine friendship. As they waited in the early-morning darkness, the heavyset general would sometimes rest his hand with a fatherly gesture on the drooping shoulders of the tall but slightly built scientist. Then they both would look up into the drizzle to see if they could glimpse a few stars or find any other indication of a break in the weather.
Oppenheimer and Groves knew that on the very next day, 6,000 miles away in Potsdam, Germany, the President of the United States was scheduled to sit down with British Prime Minister Winston Churchill and Soviet Premier Joseph Stalin to work out a surrender ultimatum to Japan. And President Truman had been told by Secretary of War Henry Stimson that word was expected by nightfall whether or not an atomic bomb could be the new superweapon that might quickly end the war with Japan.
The weather was only one of dozens of uncertainties that troubled Oppenheimer, Groves, and their colleagues. The scientists felt confident in the correctness of the complex theories they had translated, bit by bit, into the gadget resting atop the tower. But they were dealing with so many untested factors that, in a betting pool, their guesses as to the yield of the device varied from zero to an explosive force equivalent to 18,000 tons of TNT.
Prof. George Kistiakowsky, one of the world's foremost authorities on explosives, considered his guess of 1,400 tons highly optimistic. Many of the laboratory scientists picked figures of only a few hundred tons, which would hardly qualify the atomic bomb as a superweapon or justify the expenditure of the $2 billion it cost.
Since 1939, scientific work toward an atomic bomb had been progressing with utmost secrecy under the leadership of Enrico Fermi, Leo Szilard, Harold Urey, John Wheeler, Eugene Wigner, Arthur Compton, Ernest Lawrence, Niels Bohr, and Oppenheimer.
A vital breakthrough had occured Dec. 2, 1942, at the University of Chicago's "Metallurgical Laboratory" - a security cover name for the atomic-research team working there - when physicists under Fermi produced the world's first controlled nuclear chain reaction.
Soon afterward, Oppenheimer and the vanguard of scientists at the hastily established Los Alamos laboratory had begun assaulting a mountain of theoretical and technical unknowns. And at Oak Ridge, Tenn., and Hanford, Wash., the industry teams of Union Carbide and Carbon, Tennessee Eastman, and duPont had been working all-out to develop engineering processes that would somehow deliver the large amounts of uranium-235 and plutonium required if any bombs were to be made in time to help end the war.
During the 12 months prior to July 1945, a Los Alamos group under Harvard Prof. Kenneth Bainbridge had been working on a desolate New Mexico desert plateau, preparing for the full-scale test of the bomb. Located on a remote section of the Alamogordo Air Base, the site was shielded on the east by the 8,600-foot-high Oscura Mountains and on the south by the San Andres range. The nearest highway was 20 miles away. The closest towns of any size, Socorro and Carrizozo, were 40 miles distant. Alamogordo was 60 miles to the southeast, and Albuquerque was 110 miles to the north.
Oppenheimer, who delighted in thinking up code names, had selected "Trinity" for the test operation. The lab director claimed the word had no special significance: At the time a cover name was needed he happened to be reading a John Donne sonnet that included the line, "Batter my heart, three person's of God...."
By the second week in July, with the crucial test only days away, Bainbridge and his team were working around the clock. The entire Trinity area had become a huge outdoor laboratory with scientists battling the heat, wind, and sand.
Hundreds of miles of wire and cable had been strung or buried, and scores of bunkers had been built to protect measuring equipment. Emergency plans had been drawn up to evacuate participants and residents in nearby ranches or towns in case of dangerous fallout or some other, unforeseen hazard. Counterintelligence agents had prepared a decoy news release. Should the noise and flash from the bomb prove too spectacular to be kept secret, it would be announced that an ammunition dump had accidentally exploded at the Alamogordo Air Base.
With each successive step in the unfolding drama, tension mounted.
Wednesday, July 11
Fat Man and Little Boy
Oppenheimer shuffled through a pile of papers in his Los Alamos office, then gave parting instructions to his secretary, Anne Wilson. Several times Oppenheimer interrupted his secretary's questions with a brusqueness uncharacteristic of him. But in the past few weeks he had become almost a different person, withdrawn and abrupt, chain-smoking cigarettes instead of drawing leisurely on a pipe, and dropping from an already sparse 130 pounds to a gaunt 116.
After a final word to Ms. Wilson, Oppenheimer went home and stuffed a few clothes into a bag. He looked in on his sleeping six-month-old daughter Toni and gave three-year-old Peter a big hug and a kiss. Kitty Oppenheimer, who had combed their back yard for hours until she had finally found a four-leaf clover, solemnly gave the good-luck token to her husband as she saw him to the door.
In a Los Alamos laboratory, physicists Robert Bacher, Phil Morrison, Marshall Holloway, and Louis Slotin made final adjustments on the specially machined pieces of plutonium that would make up the nuclear core.
In another laboratory, Kistiakowsky supervised the complicated assembly of the bulky nonnuclear portion of the bomb. Each piece, as it was put into a subassembly, was measured and X-rayed for structural defects. Every step was logged and photographed.
The bomb, nicknamed "Fat Man," was an ingenious device based primarily on ideas furnished by scientists Seth Neddermeyer, John von Neumann, J. L. Tuck, Hans Bethe, and Robert Christy. Its secret was a method of making a subcritical amount of plutonium become supercritical and explode by "squeezing" it together (implosion). The small core of plutonium was surrounded by a sphere of high explosive, shaped to focus the shock wave inward. Around the high explosive were electric detonators.
If the high explosive could be detonated at all points with the almost unbelievable simultaneity of less than a millionth of a second, the plutonium would be compressed into a supercritical mass. If the bomb did not blow itself apart too soon, the resulting chain reaction might release the equivalent of thousands of tons of explosive force.
In addition to the Fat Man bomb, the Los Alamos scientists had developed another, less-complicated atomic bomb using uranium-235 at its core. Nicknamed "Little Boy," the nine-foot-long, four-ton bomb had been sent to Tinian Island in the Pacific early in July after its firing mechanism had been tested at Los Alamos using inert materials in place of the nuclear core.
There could be no full-scale test of Little Boy because only enough uranium-235 was available to prepare the one gadget, and it would be many months before sufficient U-235 would be available for a second bomb. The Fat Man bomb, however, was expected to use a small amount of plutonium with greater efficiency. And the production schedule of plutonium assured that a second bomb like the one to be tested at Trinity could be ready by early August, and another, if needed, a few weeks later.
Thursday, July 12
A bomb in the back seat
At 3 a.m. Phil Morrison and biophysicist Paul Aebersold, of the radiation-monitoring team, met at the heavily guarded nuclear-materials vault. Morrison cautiously loaded the pieces of the core into a special carrying case. The case was placed on the back seat of a sedan, between Morrison and Aebersold, with a security officer in the front seat. Another car of security men drove ahead of them. When they reached the Trinity site, 200 road miles south of Los Alamos, the case was taken to an abandoned ranch house where a makeshift assembly laboratory had been prepared.
Friday, July 13
On to Trinity
At one minute after midnight, the well-guarded truck containing the nonnuclear assembly of the bomb began its 200-mile trip from Los Alamos. The bomb assembly arrived at Trinity before noon and was gently deposited at the base of the 100-foot tower. At the ranch house, the nuclear core was assembled by Bacher, Morrison, Holloway, and Slotin and carefully checked.
Saturday, July 14
A critical glitch
Early in the morning, Bacher and Holloway drove to the tower with the nuclear core. Army Sgts. Leo Jercinovic and Al Van Vessem opened up part of the high-explosive assembly. The nuclear core was suspended by a chain from the tower hoist and was slowly lowered into the bomb.
Bacher watched the needles on instruments that had been set up to record any increase of radioactivity. Holloway leaned over the bomb, his head inside it, and gave arm signals to the hoist operator. The nuclear core was inched carefully downward.
Suddenly, it stuck!
What had gone wrong? Bacher, Holloway, Oppenheimer, and Kistiakowsky held a hurried conference. The parts had been measured and checked several times and had fitted exactly during the dry runs. Perhaps the nuclear materials, ordinarily stored next to cold metal, had warmed and expanded slightly in the short time since leaving Los Alamos.
Bacher suggested the core be left in contact with the colder bomb assembly to see if the core would cool and contract. After about five minutes, Holloway went back to the bomb and signaled for the core to be lowered again. It clicked into place.
After the bomb was hauled into the tower cab 100 feet off the ground, the detonators were put into position. Then the firing unit was attached. For safety reasons, and because additional checks on experiments had to be made, the bomb was not finally armed. Instead, wires carrying signals from the control point stayed hooked into a dummy firing unit that had been in use for all the rehearsals.
Sunday, July 15
From drizzle to downpour
General Groves arrived at Trinity base camp, 10 miles south of ground zero. With him were a number of V.I.P. observers, including Vannevar Bush, wartime head of the Office of Scientific Research and Development, and James Conant, chairman of the National Defense Research Committee.
In midafternoon, a group of scientists left Los Alamos in two buses, bound for an observation point 20 miles northwest of Trinity. At the test site, men hustled about in the 90-degree F. heat, making final checks of instruments and circuits. The sky was overcast, but meteorologist Jack Hubbard was hopeful that the unusual summer weather front would soon break up.
Shortly after 5 o'clock Bainbridge called a halt to the testing of circuits. Donald Hornig, a 25-year-old-scientist from Harvard, climbed the tower, unscrewed the cables from the dummy firing unit, and attached them to the nuclear bomb. This was the last step. Now all that stood between Fat Man and its moment of truth was the closing of two switches.
At base camp, all the suppertime conversation was centered on the weather, growing worse by the minute. A light drizzle turned into a downpour accompanied by occasional flashes of lightning. Oppenheimer decided that Hornig should go back to the top of the tower and make sure everything was all right.
Hornig found everything in order. In the dim light of a single 60-watt bulb that hung over the gadget, he pulled out of his pocket a paperback, "Desert Island Decameron." But with lightning flashing nearby, Hornig found it difficult to concentrate on his reading.
At each flash he stopped reading and audibly counted the seconds, trying to judge how close the bolt had been to the tower. He knew that a week before, during a dry run with the practice firing unit in place, a lightning bolt had activated the firing circuit. No harm had been done other than to close the firing circuit prematurely and spoil the practice.
Now Hornig tried not to think about what might happen if lightning hit the live gadget.
*Part II will appear tomorrow. Robert Cahn won a Pulitzer Prize for a Christian Science Monitor series on America's national parks.This article is adapted from one he wrote for The Saturday Evening Post that appeared July 16, 1960.