The Making of the Atomic Bomb

students (probably 12 years old), persuading him that it was most interesting and important that he should prepare some neat paper cylinders in which we could irradiate our stuff.

The next letter that went to the Ricerca Scientifica (and in summary form to Nature) reported artificially induced radioactivity in iron, silicon, phosphorus, chlorine, vanadium, copper, arsenic, silver, tellurium, iodine, chromium, barium, sodium, magnesium, titanium, zinc, selenium, antimony, bromine and lanthanum. By then they had established a routine: they irradiated substances at one end of the second floor and tested them under the Geiger counters at the other end, down a long hall. That shielded the counters from stray radiation from the neutron source. But it also meant, whenever the half-life of an induced radioactivity was short, that someone had to run down the hall. “Amaldi and Fermi prided themselves on being the fastest runners,” Laura Fermi notes, “and theirs was the task of speeding short-lived substances from one end of the corridor to the other. They always raced, and Enrico claims that he could run faster than Edoardo. But he is not a good loser.” A dignified Spaniard showed up one day to confer with “His Excellency Fermi.” Rome's young professor of theoretical physics, a dirty lab coat flying out behind him, nearly knocked the visitor down.

They came, finally, to uranium. They had roughly classified the effects they were seeing. Light elements generally transmuted to lighter elements by ejecting either a proton or an alpha particle. But the electrical barrier around the nucleus works against exits as well as entrances, and that barrier increases in strength with increasing atomic number. So heavy elements got heavier, not lighter: they captured the bombarding neutron, threw off its binding energy by emitting gamma radiation, and thus, with the addition of the neutron's mass, but with no added or subtracted charge, became a heavier isotope of themselves. Which then decayed by the delayed emission of a negative beta ray to an element with one more unit of atomic number. Uranium did the same; after a delay it emitted a beta electron. That should mean, Fermi realized, that bombarding uranium with neutrons was producing first a heavier isotope, uranium 239, and then a new, man-made transuranic element, atomic number 93, something never seen on earth before.

It was necessary to purify their uranium sample (uranium nitrate in solution, a light yellow liquid) of the obscuring beta activity its natural decay products gave off. (Uranium decays naturally through a series of fourteen complex steps down the periodic table to thorium, protactinium, radium, radon, polonium and bismuth to lead.) Trabacchi in his generosity had by then even lent the group a young chemist, Oscar D'Agostino, fresh from training in radiochemistry on the Rue Pierre Curie; D'Agostino accomplished the laborious purification in early May. They were using stronger sources now, up to 800 millicuries of radon, about a million neutrons per second. Irradiating the uranium nitrate gave “a very intense effect with several periods [of half-lives]: one period of about 1 minute, another of 13 minutes besides longer periods not yet exactly determined” — thus their May 10 report.

These several induced radioactivities were all beta emitters. They made whatever atom was emitting them heavier by one atomic number. It seemed to follow, then, that they were transmutations up the periodic table into the uncharted new region of man-made elements. To confirm that stunning possibility Fermi needed to demonstrate with chemical separations that the neutron bombardment was not unaccountably creating elements lighter than uranium. The one-minute half-life was too short to work with, so he concentrated on the thirteen-minute substance. D'Agostino diluted the irradiated uranium nitrate with 50 percent nitric acid, dissolved into the acid a small amount of manganese salt and set the solution to boil. By adding sodium chlorate to the boiling solution he precipitated crystals of manganese dioxide. When he filtered the crystals from the solution the radioactivity went with the manganese, much as the radioactivity the Jo-liot-Curies had induced in aluminum went off with the hydrogen gas. If the radioactivity could be precipitated out of the uranium solution along with a manganese carrier, then it must not be uranium anymore.

By adding other carriers and precipitating other compounds D'Agostino proved that the thirteen-minute substance was neither protactinium (91), thorium (90), actinium (89), radium (88), bismuth (83) nor lead (82). Its behavior excluded elements 87 (then known as ekacesium), and 86 (radon). Element 85 was unknown. Perhaps because the half-lives were different, Fermi made no attempt to check polonium (84). But he felt he had been sufficiently thorough. “This negative evidence about the identity of the 13 min-activity from a large number of heavy elements,” he reported cautiously in Nature in June, “suggests the possibility that the atomic number of the element may be greater than 92.”

Corbino injudiciously announced “a new element” at the annual convocation, the King of Italy in attendance, that closed the academic year, which set the press baying and gave Fermi a few sleepless nights. Having so splendidly accomplished Szilard's “rather boring task,” the weary physicist was happy to depart after that with his wife and their small daughter Nella for a summer lecture tour sponsored by the Italian government through Argentina, Uruguay and Brazil.

Leo Szilard had emerged from his bath that spring of 1934 to pursue his favorite causes, not yet joined, of releasing the energy of the nucleus and of saving the world. In a late-April memorandum condemning the recent Japanese occupation of Manchuria he seemed to look ahead to a far future: “The discoveries of scientists,” he wrote, “have given weapons to mankind which may destroy our present civilization if we do not succeed in avoiding further wars.” He probably meant military aircraft; the horrors of strategic bombing and even its potential for deterrence through a balance of terror were much bruited at mid-decade. But almost certainly he was also thinking of atomic bombs.

Several weeks earlier, looking for a patron, he had sent Sir Hugo Hirst, the founder of the British General Electric Company, a copy of the first chapter of The World Set Free. “Of course,” he wrote Sir Hugo with a touch of bitterness, still brooding on Rutherford's prediction, “all this is moonshine, but I have reason to believe that in so far as the industrial applications of the present discoveries in physics are concerned, the forecast of the writers may prove to be more accurate than the forecast of the scientists. The physicists have conclusive arguments as to why we cannot create at present new sources of energy for industrial purposes; I am not so sure whether they do not miss the point.”

That Szilard saw beyond “energy for industrial purposes” to the possibility of weapons of war is evident in his next patent amendments, dated June 28 and July 4, 1934. Previously he had described “the transmutation of chemical elements”; now he added “the liberation of nuclear energy for power production and other purposes through nuclear transmutation.” He proposed for the first time “a chain reaction in which particles which carry no positive charge and the mass of which is approximately equal to the proton mass or a multiple thereof [i.e., neutrons] form the links of the chain.” He described the essential features of what came to be known as a “critical mass” — the volume of a chain-reacting substance necessary to make the chain reaction self-sustaining. He saw that the critical mass could be reduced by surrounding a sphere of chain-reacting substance with “some cheap heavy material, for instance lead,” that would reflect neutrons back into the sphere, the basic concept for what came to be known (by analogy with the mud tamped into drill holes to confine conventional explosives) as “tamper.” And he understood what would happen if he assembled a critical mass, spelling out the results simply on the fourth page of his application:

If the thickness is larger than the critical value… I can produce an explosion.

As if to mark in some distant inhuman ledger the end of one age and the beginning of another, Marie Sklodowska Curie, born in Warsaw, Poland, on November 7, 1867, died that day of Szilard's filing, July 4, 1934, in Savoy. Einstein's was the best eulogy: “Marie Curie is,” he said, “of all celebrated beings, the only one whom fame has not corrupted.”

There is nothing in the documentary record to indicate that Szilard was yet thinking of uranium. His June amendment describes a possible chain reaction using light, silvery beryllium, element number 4 on the periodic table.

To study that metal Szilard needed access to a laboratory and a source of radiation. The beryllium nucleus was so lightly bound he suspected he could knock neutrons out of it not only with alpha particles or neutrons but even with gamma rays or high-energy X rays. Radium emitted gamma rays and radium was available conveniently at the nearest large hospital. So Szilard, an unusually practical visionary, dropped in to see the director of the physics department at the medical college of St. Bartholomew's Hospital. Couldn't he use St. Bart's radium, “which was not much in use in summertime,” for experiments? Something of value to medicine might emerge. The director thought he could if he teamed up with someone on the staff. “There was a very nice young Englishman, Mr. [T. A.] Chalmers, who was game, and so we teamed up and for the next two months we did experiments.”