Bohr arrived at Gamow’s home late in the afternoon the day before the conference began. An hour or so later, Geo called me in agitation: “Bohr has gone crazy. He says uranium splits.” That was all of Geo’s message. Within half an hour, I realized what Bohr was talking about. If the uranium nucleus (the heaviest of the naturally occurring elements) were to split, it could split in a variety of ways. That would account for the many simultaneously produced radioactivities.
[Lise] Meitner’s question had been answered, the tool [Leo] Szilard had wished for was now available, and Nazi Germany might well develop a devastating new weapon. My sleep that night was uneasy.
The subject of the conference was low-temperature physics and superconductivity, at that time an unexplained phenomenon. But Bohr was Bohr, and news is news. So Geo opened the conference by announcing (this time politely) that Bohr had something to say. Bohr then described the work in Nazi Germany, the conclusion that fission had occurred, and the decisive confirmation of fission in Copenhagen.
Fission was an amazing discovery. Hahn had promptly written to his friend Lise Meitner, an Austrian Jew who had been forced to leave her position at the Kaiser Wilhelm Institute only a few months before. Meitner, together with her nephew-collaborator Otto Frisch, who was in Copenhagen, immediately designed an experiment to verify the news. If uranium split in two, the fragments would move apart at high speed and lose many electrons. The highly charged fragments would deposit an unusual amount of energy in a Geiger counter (charged-particle detector). Meitner and Frisch discussed their plan with Bohr before he left Copenhagen and wired the successful result of their experiment to him on board ship. Thus he arrived in New York full of the news. Shortly afterward, he came to Washington.
Yet for all that the news was amazing, the discussion that followed Bohr’s announcement was remarkably subdued. After a few minutes of general comments, my neighbor said to me: “Perhaps we should not discuss this. Clearly something obvious has been said, and it is equally clear that the consequences will be far from obvious.” That seemed to be the tacit consensus, for we promptly returned to low-temperature physics.
That evening, Merle Tuve invited the conference participants to visit the Department of Terrestrial Magnetism. There we watched him and his collaborators demonstrate the fission of uranium in a Geiger counter. Tuve, after Bohr’s announcement, had rushed back to his laboratory and reproduced the Meitner-Frisch experiment in a few hours.
That the secret of fission had eluded everybody for all those years amazed me far more than the demonstration. In one of his experiments, Fermi had bombarded uranium with neutrons to observe the alpha particles that picked up extra energy from the neutrons. Because he carried out the experiment in a Geiger counter, the highly energetic fission fragments would have been unmistakable. But Fermi was a very careful experimenter. He covered his uranium with a thin sheet of inert material to stop the normal alpha particles (without the extra energy) in which he was not interested. That sheet also stopped the fission products, which had a short range but extremely high energy-density. Had Fermi forgotten to cover his sample even once, fission would have been discovered years earlier.
Physicist Paul Scherrer in Zurich had an even closer encounter with the discovery. He bombarded thorium (another of Szilard’s favorite substances) with neutrons and saw the fission fragments that Meitner and Frisch had identified. But Scherrer wouldn’t believe his eyes. He thought his Geiger counter was malfunctioning. What wasn’t expected wasn’t seen!
In 1939, I did not realize how fortunate it was that those slight changes in an experiment in Rome or Zurich did not occur. If fission had been discovered in 1933, work on the topic in Germany and the Soviet Union—two nations that took the military applications of science seriously—would have been well advanced by 1939. Under different conditions, the United States probably would not have been the first nation to possess nuclear explosives. Fermi, Scherrer, and Szilard, in their different ways, had a profound and beneficent influence on history.
U.S. Department of Energy
Edward Teller worked at Los Alamos during the Manhattan Project and later was the “father of the hydrogen bomb.”
“I had come close but had missed a great discovery”
A graduate student under Ernest O. Lawrence at the University of California, Berkeley, Philip Abelson was assigned to work on the cyclotron, the prototype for the isotope separation facilities known as “calutrons” (a neologism from “California” and “cyclotron”) built at Oak Ridge, Tennessee. Because of his expertise in nuclear physics, which was then “an amateur sport,” he was quickly enlisted to work on the Manhattan Project. His most agonizing experience was learning about Lise Meitner and Otto Frisch’s discovery of fission.
From “A Graduate Student with Ernest O. Lawrence”
BY PHILIP ABELSON
Nuclear physics is now a mature science with an associated complex technology. But in the 1930s, it was an amateur sport largely played by a few score graduate students. I was one of them.
When I arrived at the Radiation Laboratory of the University of California in Berkeley in August 1935 to study under Ernest O. Lawrence, the game was already under way. Lawrence had developed the cyclotron into a powerful research tool—in many ways the best one in the world—and he had attracted to his laboratory about a score of enthusiastic graduate students and postdoctoral fellows.
I had my undergraduate training in chemistry at what is now Washington State University and received my B.S. degree in 1933. While I loved chemistry and have always enjoyed working in it, I was attracted to physics by Paul Anderson, who was head of the physics department at Washington State. He gave me a teaching assistantship in physics, and in the course of two years I earned a master’s degree in that subject. Anderson and S. T. Stephenson intervened in my behalf to place me at Berkeley. They were effective: during the dark depression days of 1935 I was the only entering out-of-state graduate student to receive a teaching assistantship in the physics department at Berkeley.
On arrival I was immediately put to work. A teaching assistant’s duties required about 15 hours a week in tutorials, laboratories and grading papers. That was easy. The all-demanding task was work on the cyclotron. At that time the instrument was scheduled to be operated 15 hours a day, 7 days a week. Breakdowns were frequent. The vacuum system was put together with beeswax and rosin. Leaks had to be found, and homemade electronic components fixed. I was scheduled to serve 30 hours a week on the cyclotron crew, but in emergencies—and there were many of those—I was expected to work day and night. I was also expected to take the usual graduate courses, study for them and for preliminary comprehensive oral exams and attend seminars while carrying out thesis research.
Despite all the demands and some downright drudgery, the Radiation Laboratory was an exciting place. The cyclotron produced a beam of 20 microamperes at an energy of 5.5 million electron volts (MeV). The periodic table of elements was available for exploration and for pioneering applications of radioactive tracers. With time, the cyclotron was improved. By 1937 the vacuum system had been changed and the energy raised to 8.0 MeV with a beam of 60 microamperes. I had the task of finding and repairing the leaks in a new all-metal vacuum chamber. By that time I was a veteran of the vacuum wars and in comparatively quick order discovered 22 leaks of successively descending magnitude.
During the next two years, I conducted a number of irradiations of uranium with neutrons and followed the radioactive decay. The decay curves were extremely complex, even more so than those in the literature. A better method for identification of the irradiation products was obviously desirable. In 1937 [Luis] Alvarez demonstrated the existence of a K-capture process in gallium-67 followed by emission of characteristic zinc X-rays. This stimulated me to look for X-rays in the products formed by neutron irradiation of uranium. In March 1938, I found X-rays associated with a three-day sulfide precipitable product. The X-rays had an absorption coefficient not too far out of line with the
interpretation that they might be L-X-rays of a transuranic element.
At that time our equipment for detecting and measuring radiation was all homemade. There were a few Geiger counters with associated scalars. Most of the laboratory staff depended on the simple Lauritsen electroscope for their measurements. The air-filled electroscope was not sensitive to X-rays. I adapted an instrument for use with methyl bromide and subsequently built an ionization chamber also filled with this gas. These tools were useful. But steeped in the belief that I was seeking to identify L-X-ray, I knew that I must have a sharper tool.
About that time Bozorth and Haworth of Bell Laboratories published a paper on a bent rock salt crystal spectrograph. Bozorth kindly furnished a 2cm x 2cm x 5cm specimen of the sodium chloride crystal, and I built a spectrograph. I leaved the crystal with a razor blade to obtain a specimen 2mm x 2cm x 5cm which I bent into an arc. (Rock salt crystal can readily be molded in a saturated salt solution.) The bent crystal was attached to a wooden holder with wax, and the combination was mounted on a wooden board. Some pieces of lead were used to shield the X-ray film detector from direct exposure to electrons and X-rays from the source. Altogether the spectrograph cost perhaps 20 cents for materials plus about four hours of a graduate student’s time, which in those days was worth about another 80 cents.
I tested my spectrograph first with X-rays from a conventional tube and then with X-rays from gallium-67. The tests showed that the spectrograph functioned satisfactorily and that identification of the “transuranic” X-rays was in principle feasible. The sole impediment was intensity. This problem could be overcome if I could extract the activity from a large enough sample of uranium after a prolonged bombardment. What was required was about 5 kilograms of uranium.
In those days the laboratory had little money, especially for graduate students. The major funds went into running and improving the cyclotron. I desperately wanted the uranium. How to get it? My stipend was $60 a month. One day I received a letter from my parents enclosing money to buy a new suit. My wife and I went to San Francisco to get it; but before we arrived at the store I saw the sign of Braun Knecht and Heiman, vendors of chemicals. The money for the suit was diverted to the purchase of uranium oxide. Alas, when I began to conduct experiments with the material, I found it contained every kind of impurity, such as soluble silica which formed an unmanageable gel in acid solution.
Ultimately, I freed the uranium of interfering impurities and was set to identify the “transuranic” X-ray when news of uranium fission broke in late January 1939.
My memories of the day that news of uranium fission came to the Berkeley Radiation Laboratory are vivid. That morning, as a member of the cyclotron crew, I was at the control console operating the machine. About 9:30 A.M. I heard the sound of running footsteps outside, and immediately afterward Alvarez burst into the laboratory. He had been in a barbershop near the campus having a haircut when he spotted an item in a newspaper that caused him to jump out of the barber’s chair and head for the laboratory on the run. He had learned that Hahn and Strassmann had identified barium as a product of uranium irradiation. Furthermore, Meitner and Frisch had explained this astounding discovery on the basis of fissioning of the heavy uranium nucleus into two fragments of roughly equal size—a process attended with the release of a large amount of energy.
When Alvarez told me the news, I almost went numb as I realized that I had come close but had missed a great discovery. During that day, other members of the laboratory, including Alvarez, prepared experiments to check on the validity of the fission process, for example, by measuring the energy liberated in a linear amplifier when uranium was exposed to neutrons.
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 that day, I was able to identify the “transuranic” X-ray as being a characteristic X-ray of iodine and in another day showed that the iodine was formed by decay of a radioactive tellurium isotope.
On February 3, 1939, I sent a letter describing this work to the Physical Review. It was published February 15. As a contributor then, I admired the speed with which the editor, John T. Tate of the University of Minnesota, put my letter into print. As an editor now, I admire the brevity of the communication which is one of the shortest I have seen. It went as follows:
CLEAVAGE OF THE URANIUM NUCLEUS
We have been studying what seemed to be L X-rays from the seventy-two-hour “transuranic” element. These have now been shown by critical absorption measurements to be iodine K X-rays. The seventy-two-hour period is definitely due to tellurium as shown by chemical test, and its daughter substance of two-and-a-half-hour half-life is separated quantitatively as iodine. This seems to be an unambiguous and independent proof of Hahn’s hypothesis of the cleavage of the uranium nucleus.
—PHILIP ABELSON
University of California,
Berkeley, California,
February 3, 1939
In May 1939, I received my Ph.D. degree, and life settled back to a more sedate pace as I began seeking a position elsewhere. In July, I accepted an offer from the Carnegie Institution of Washington to join the department of terrestrial magnetism with Merle Tuve, Lawrence Hafstad, Richard Roberts, Norman Heydenberg and George Green. Roberts, Green and I were to build a 60-inch cyclotron at the department. I was to join the department in early September. While I was traveling east by train, the news broke of Hitler’s invasion of Poland. The remainder of the journey was somber, for I wondered as did many others whether sooner or later the United States would be involved in a world war.
PAINTING THE CYCLOTRON
In 1935, I went to Berkeley as an out-of-state teaching assistant from Washington. The moment I got down to Berkeley, I went to the cyclotron library and happened to meet Ernest Lawrence. Right away he decided to put me to work—painting the cyclotron. He had some battleship gray paint and a brush and I started to paint. And much to my surprise, pretty soon Ernest Lawrence was painting, too. So between the two of us, we painted the cyclotron.
—PHILIP ABELSON
Courtesy of AIP Emilio Segre Visual Archives
Physicists Ernest O. Lawrence (right) and M. Stanley Livingston (left) stand next to a cyclotron at the University of California, Berkeley.
Enlisting Einstein
Two Hungarian physicists who took refuge in the United States, Leo Szilard and Eugene Wigner, felt a pressing need to warn people that Nazi Germany was developing a nuclear weapon. The following excerpt describes how the two of them persuaded Albert Einstein to sign his famous letter of August 2, 1939, to President Franklin D. Roosevelt and their fateful mission, which depended in part on the directions of a seven-year-old boy and the intervention of a Wall Street financier.
From Genius in the Shadows
BY WILLIAM LANOUETTE
When Eugene Wigner came to New York from Princeton in early July, Szilard showered him with the calculations he had made for the carbon-uranium lattice. Wigner was quick to see that this might work. They were also quick to link this approach—the closest yet to a workable chain reaction—with recent news from Europe that German military expansion could easily overrun Belgium, whose colony in the Congo was then the world’s principal uranium source.
Wigner wanted to alert the Belgian government and suggested they seek advice from their former professor in Berlin, Albert Einstein. Wigner occasionally saw Einstein around the Princeton campus; to Szilard, who had worked closely with him in the 1920s and early 1930s, Einstein had reverted from colleague and counselor to a famous but remote scientist. Einstein knew the Belgian monarchs well—in his unpretentious way calling her “Queen” and addressing the royal couple as the “Kings”—so perhaps, Szilard suggested, he might alert the queen of the Belgians about the perilous importance of the Congo’s uranium. The two agreed that it was worth a try, and from his Princeton office they learned that Einstein was then at a cottage in Peconic, Long Island, owned by a friend named Dr. Moore.<
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Early on the morning of Wednesday, July 12, a clear and hot day, Wigner drove up to the King’s Crown Hotel in his 1936 Dodge coupe, and Szilard climbed in. The two drove out of New York across the new Triborough Bridge, passing the New York World’s Fair, whose theme “Building the World of Tomorrow” was symbolized by a 700-foot-high trylon, a tapered column rising to a point, representing “the finite,” and a 200-foot perisphere globe, representing “the infinite.”
Had Szilard and Wigner thought about it, their own drive that day had more to do with the “World of Tomorrow” than anything they passed on the fairgrounds. But their thoughts were fixed on finding Einstein’s cottage, a task demanding all their attention. First the two Hungarians confused the Indian names in their directions and drove to Patchogue, on Long Island’s south shore, instead of to Cutchogue, on the north. This detour cost them two hours, and once in Peconic, they drove around the tiny resort town asking vacationers in shorts and bathing suits the way to Dr. Moore’s cottage. No one seemed to know.
“Let’s give it up and go home,” Szilard said impatiently. “Perhaps fate intended it. We should probably be making a frightful mistake by enlisting Einstein’s help in applying to any public authorities in a matter like this. Once a government gets hold of something, it never lets go.…”
“But it’s our duty to take this step,” Wigner insisted, and he continued to drive slowly along the village’s winding roads.
“How would it be if we simply asked where around here Einstein lives?” Szilard said. “After all, every child knows him.” A sunburned boy of about seven was standing at a corner toying with a fishing rod when Szilard leaned out of the car window and asked, “Do you know where Einstein lives?”
The Manhattan Project Page 4