The Making of the Atomic Bomb

KINDLY DISREGARD MY RECENT LETTER STOP WRITING

The secret patent had revived.

Naturwissenschaften reached Paris about January 16. One of Frederic Joliot's associates recalls that “in a rather moving meeting [Joliot] made a report on this result to Madame Joliot and myself after having locked himself in for a few days and not talked to anybody.” The Joliot-Curies were once again appalled to find they had barely missed a major discovery. In the next few days Joliot independently deduced the large energy release and considered the possibility of a chain reaction, as Szilard had thought he might. He tried to track down the neutrons from fission first, found that approach difficult, then set up an experiment somewhat like Frisch's. He detected fission fragments on January 26.

* * *

The newest building on the DTM grounds was the Atomic Physics Observatory, the working contents of which had just been brought on line two weeks before: a new 5 MV pressure Van de Graaff generator that Tuve, Roberts and their colleagues had built for $51,000 to extend their studies in the structure of the nucleus. The Van de Graaff was named for the Alabama-born physicist who invented it, but Tuve was the first — in 1932 — to put it to practical use in experiment. It was essentially a monumental static-electricity generator, an insulated motor- driven pulley belt that picked up ions from discharge needles in its metal base, carried them up through an insulated support cylinder into a smooth metal storage sphere and deposited them on the sphere. As ions accumulated the sphere's voltage increased. The voltage could then be discharged as a spark — Van de Graaffs discharging lightning-bolt sparks have been staples of mad-scientist movies — or drawn off to power an accelerator tube. The new machine was built inside a pear-shaped pressure tank, as large as the tank of a water tower, that helped reduce accidental sparking.

When Tuve had first proposed the Van de Graaff to the zoning board of the prosperous Chevy Chase neighborhood the board had turned him down. Smashing atoms smacked of industrial process and the neighborhood had its property values to consider. Tuve noted the popularity of the Naval Observatory, across Connecticut Avenue a few miles west, and rechristened his project the Atomic Physics Observatory, which it was. As the APO it won approval.

Roberts and Hafstad chose to work with the APO. They had intended to use the old 1 MV Van de Graaff in the building next door to make neutrons for their splitter experiment, but that machine's ion-source filament was burned out. Although the APO's vacuum accelerator tube leaked, finding the leak looked to be less tedious than replacing the filament. In fact it needed two days. Hafstad went off Friday night on a ski weekend and another young Tuve protegd, R. C. Meyer, took his place.

Roberts' laboratory notebook entries summarize Saturday's work:

Sat 4:30 PM

Set up ionization chamber to try to detect

U²³⁸₉₂ + n > Ba^?₅₆ + Kr^? ₅₆

Neutrons from Li + D [accelerated deuterium nuclei bombarding lithium]

…

With uranium lined I. C. observed

?'s [approximately] 1–2 mm and occasional 35 mm kicks (Ba + Kr?)

The APO's target room was a small circular basement accessible down a steel ladder, a chilly kiva that smelled pleasantly of oil. As soon as Roberts saw the “tremendous pulses corresponding to very large energy release” he and Meyer ran every test they could think of. “We promptly tried the effect of paraffin (for slow neutrons) and then cadmium to remove the slow neutrons. We also tried all the other heavy elements available [to determine if they would split] and saw the same [i.e., fission] with thorium.” Having made that original discovery (Frisch had made it independently in Copenhagen before them) they stopped to eat. “I told Tuve after supper and he immediately called Bohr and Fermi and they came out Saturday night.”

Not only Bohr and Fermi came, in heavy, dark, pin-striped three-piece suits, Fermi swarthy with a day's growth of beard, but also Tuve; Rosen-feld; Teller; Erik Bohr, handsome in a heavy overcoat over a decorative Danish sweater; Gregory Breit, owlish in spectacles; and John A. Fleming, the conservative director of the DTM, who had the presence of mind to bring along a photographer. All except Teller posed in the target room with Meyer and Roberts for a historic photograph. The box of the ionization chamber in the foreground is stacked with disks of paraffin; Bohr holds the stub of an after-dinner cigar; Fermi's grin reveals the gap between his front teeth left by a baby tooth he shed late; Roberts looks into the camera weary but satisfied. Fermi had been amazed at the ionization pulses on the oscilloscope and had insisted they check for equipment malfunctions: he had never seen such pulses in Rome (they were captured by the aluminum foil Amaldi had wrapped around his uranium to block its alpha background). Bohr was still fretting. “I had to stand and look at the first [sic] experiment,” he wrote Margrethe, “without knowing certainly if Frisch had done the same experiment and sent a note to Nature.” Back at Princeton on Sunday he learned from other family letters that Frisch had. “There followed,” Roberts concludes, “several days of excitement, press releases and phone calls.”

Science reporter Thomas Henry had attended the conference; his story appeared in the Washington Evening Star on Saturday afternoon. The Associated Press picked it up. Shortened, it earned a place on an inside page of the Sunday New York Times. Dunning may have seen it there; he finally wired Fermi news that morning of the Columbia experiment. As Herbert Anderson remembers it, “Fermi… rushed back to Columbia and straightaway called me into his office. My notebook lists the experiments he felt we should do right away. The date was January 29, 1939.” They had already agreed, says Anderson, that “I would teach him Americana, and he would teach me physics.” Both lessons began in earnest.

The San Francisco Chronicle picked up the wire-service story. Luis W. Alvarez, Ernest Lawrence's tall, ice-blond protege, a future Nobelist whose father was a prominent Mayo Clinic physician, read it at Berkeley sitting in a barber chair in Stevens Union having his hair cut. “So [I told] the barber to stop cutting my hair and I got right out of that barber chair and ran as fast as I could to the Radiation Lab… where my student Phil Abelson… had been [trying to identify] what transuranium elements were produced when neutrons hit uranium; he was so close to discovering fission that it was almost pitiful.” Abelson still remembers the painful moment: “About 9:30 a.m. I heard the sound of running footsteps outside, and immediately afterward Alvarez burst into the laboratory… When [he] told me the news, I almost went numb as I realized that I had come close but had missed a great discovery… For nearly 24 hours I remained numb, not functioning very well. The next morning I was back to normal with a plan to proceed.” By the end of the day Abelson found iodine as a decay product of tellurium from uranium irradiation, another way the nucleus could split (i.e., tellurium 52 + zirconium 40 = U 92).

Alvarez wired Gamow for details, learned of the Frisch experiment, then tracked down Oppenheimer:

I remember telling Robert Oppenheimer that we were going to look for [ionization pulses from fission] and he said, “That's impossible” and gave a lot of theoretical reasons why fission couldn't really happen. When I invited him over to look at the oscilloscope later, when we saw the big pulses, I would say that in less than fifteen minutes Robert had decided that this was indeed a real effect and… he had decided that some neutrons would probably boil off in the reaction, and that you could make bombs and generate power, all inside of a few minutes… It was amazing to see how rapidly his mind worked, and he came to the right conclusions.

The following Saturday Oppenheimer discussed the discovery in a letter to a friend at Caltech, outlining all the experiments Alvarez and others had accomplished during the week and speculating on applications:

The U business is unbelievable. We first saw it in the papers, wired for more dope, and have had a lot of reports since… In how many ways does the U come apart? At random, as one might guess, or only in certain ways? And most of all, are there many neutrons that come off during the splitting, or from the excited pieces? If there are, then a 10 cm cube of U deuteride (one would need the D [deuterium, heavy hydrogen] to slow them without