The Making of the Atomic Bomb

Three hundred pounds of irradiated UNH — yellowish crystals like rock salt — arrived from St. Louis by truck on July 27, a Monday:

The UNH was surrounded by a layer of lead bricks. [Truman] Kohman and [Elwin H.] Covey were detailed to unload the shipment and carry it up to our lab on the fourth floor for extraction of the 94²³⁹. The UNH crystals came packaged in small boxes of various sizes, made to fit into the various niches around the cyclotron target. Some of the boxes were made of masonite, but most of them were of quarter inch plywood. Unfortunately, some of the seams and edges had cracked open, allowing crystals of hot [i.e., radioactive] UNH to creep out. We could not get hold of any instrument to measure the radioactivity. I told Kohman and Covey their best protection would be to wear rubber gloves and a lab coat… Although they struggled for half the day to get all the boxes and lead bricks upstairs into the storage area, I think they were conscientious and kept their radiation exposure to a minimum.

While Seaborg's high-spirited crew of young chemists began attempting to extract plutonium 239 from the bulky St. Louis UNH, wrestling with carboys of ether and heavy three-liter separatory funnels held at arm's length from behind lead shields, Cunningham and Werner in narrow Room 405 started toward isolating plutonium as a pure compound. They first measured out a 15-milliliter solution of UNH irradiated earlier that summer in the 60-inch Berkeley cyclotron. They assumed their solution then contained about one microgram of plutonium 239. (Pu239, that is: Seaborg had chosen the abbreviation Pu rather than PI partly to avoid confusion with platinum, Pt, but also “facetiously,” he says, “to create attention” — P.U. the old slang for putrid, something that raises a stink.) Working with their ultramicrochemical equipment — slow, tedious operations via micromanipulator gearing down large motions to microscopically small — on August 15, a Saturday, they mixed the rare earths cerium and lanthanum into their solution as carriers, partially evaporated it and precipitated the carriers and the Pu as fluorides. They dissolved the precipitated crystals in a few drops of sulfuric acid and evaporated the resulting solution to a volume of about one milliliter, a thousandth of a liter, some twenty drops. They checked the larger volume of solution left behind and found essentially no alpha activity, evidence that the alpha-active Pu had crystallized out with the rare earths. That was a day's work and they stored the precipitate solution carefully for Monday and went home.

On Monday, August 17, Cunningham and Werner began by oxidizing their small volume of precipitate to change the oxidation state of its Pu. They repeated the oxidation and reduction cycles on the solution several times. At the end of the day their quartz centrifuge microcone contained a minute drop of liquid that radiated some 57,000 alpha particles per minute. They set it in a steam bath to concentrate it.

On Tuesday the two men transferred the concentrated solution to a shallow platinum dish to prepare to concentrate it further. It began creeping over the sides. Rather than lose it they moved it quickly to the only larger dish at hand, which was contaminated with lanthanum. Their mis-judgment of volume condemned them to another day of repurifying. Upstairs in the attic and on the roof Seaborg's bulk UNH crew stirred large-volume extractions of ether and water. It was hot and heavy work.

Room 405 had a purified concentrate again to process Wednesday morning. It was still contaminated with a potassium compound and with silver. Cunningham and Werner diluted it and precipitated out the silver as a chloride. They added five micrograms of lanthanum and precipitated out the Pu along with the lanthanum carrier. They dissolved the precipitate, oxidized it once more to change over the Pu and precipitated out the lanthanum. That left pure plutonium in solution, one more morning's work to bring down.

Of Thursday, August 20, 1942, Seaborg writes:

Perhaps today was the most exciting and thrilling day I have experienced since coming to the Met Lab. Our microchemists isolated pure element 94 for the first time! This morning Cunningham and Werner set about fuming… yesterday's 94 solution containing about one microgram of 94²³⁹, added hydrofluoric acid whereupon the reduced 94 precipitated as the fluoride… free of carrier material…

This precipitate of 94, which was viewed under the microscope and which was also visible to the naked eye, did not differ visibly from the rare-earth fluorides…

It is the first time that element 94… has been beheld by the eye of man.

By afternoon “a holiday spirit prevailed in our group.” After several hours' exposure to air “the precipitated [plutonium] had taken on a pinkish hue.” Someone photographed Cunningham and Werner at their crowded bench in the narrow, tile-walled room — trim, strong-jawed young men looking weary. The crew upstairs that muscled carboys and lead bricks shuffled in like clumsy shepherds to peer through the microscope at the miracle of the tiny pinkish speck.

In the summer of 1942 Robert Oppenheimer gathered together at Berkeley a small group of theoretical physicists he was amused to call the “luminaries.” Their job was to throw light on the actual design of an atomic bomb.

Hans Bethe, now thirty-six and a highly respected professor of physics at Cornell, had resisted joining the bomb project because he doubted the weapon's feasibility. “I considered… an atomic bomb so remote,” Bethe told a biographer after the war, “that I completely refused to have anything to do with it… Separating isotopes of such a heavy element [as uranium] was clearly a very difficult thing to do, and I thought we would never succeed in any practical way.” But Bethe may well have headed the list of luminaries Oppenheimer wanted to attract. By 1942 the Cornell physicist had established himself as a theoretician of the first rank. His most outstanding contribution, for which he would receive the 1967 Nobel Prize in Physics, was to elucidate the production of energy in stars, identifying a cycle of thermonuclear reactions involving hydrogen, nitrogen and oxygen that is catalyzed by carbon and culminates in the creation of helium. Among other important work during the 1930s Bethe had been principal author of three lengthy review articles on nuclear physics, the first comprehensive survey of the field. Bound together, the three authoritative studies came to be called “Bethe's Bible.”

He had wanted to help oppose Nazism. “After the fall of France,” he says, “I was desperate to do something — to make some contribution to the war effort.” First he developed a basic theory of armor penetration. On the recommendation of Theodor von Karman, whom he consulted at Caltech, he and Edward Teller in 1940 extended and clarified shock-wave theory. In 1942 he joined the Radiation Laboratory at MIT to work on radar. That was where Oppenheimer found him.

Oppenheimer cleared his plan with Lee A. DuBridge, the director of the Rad Lab, then set a senior American theoretician, John H. Van Vleck, professor of physics at Harvard, to snare Bethe for the Berkeley summer study. “The essential point,” he counseled Van Vleck, “is to enlist Bethe's interest, to impress on him the magnitude of the job we have to do… and to try to convince him, too, that our present plans… are the appropriate machinery.” Oppenheimer felt the weight of the work. “Every time I think about our problem a new headache appears,” he told the Harvard professor. “We shall certainly have our hands full.” Van Vleck arranged to meet Bethe conspiratorially in Harvard Yard and succeeded in convincing him he was needed. The prearranged signal to Oppenheimer was a Western Union Kiddygram, an inexpensive standardized telegram with a message like “Brush your teeth.”

Oppenheimer also invited Edward Teller. In 1939 Bethe had married Rose Ewald, the attractive and intelligent daughter of his Stuttgart physics professor Paul Ewald; Edward and Mici Teller, “our best friends in this country,” had attended the New Rochelle wedding. Setting out for Berkeley in early July 1942, the Bethes stopped over in Chicago to pick up the Tellers. Teller showed Bethe Fermi's latest exponential pile. “He had a setup under one of the stands in Stagg Field,” Bethe remembers — “in a squash court — with tremendous stacks of graphite.” A chain reaction that made plutonium would bypass the problem of isotope separation. “I then,” says Bethe, “became convinced that the atomic-bomb project was real, and that it would probably work.”

The other luminaries enlisted for the summer study were Van Vleck, the Swiss-born Stanford theoretician Felix Bloch, Oppenheimer's former student and close collaborator Robert Serber, a young Indiana theoretician named Emil Konopinski and two postdoctoral assistants. Konopinski and Teller had arrived at the Met Lab at about the same time earlier that year. “We were newcomers in the bustling laboratory,” Teller writes in a memoir, “and for a few days we were given no specific jobs.” Teller proposed that he and Konopinski review his calculations that seemed to prove the impossibility of using an atomic bomb to ignite a thermonuclear reaction in deuterium:

Konopinski agreed, and we tackled the job of writing a report to show, once and for all, that it could not be