The Physics of War (36 page)

He was a little disappointed, however, when he met her. She immediately began talking about the letter she had received from Hahn. She finally handed it to him to read. Frisch suggested that the strange outcome of Hahn's experiment might be an error due to contamination, but she argued that Hahn was too good a chemist to allow that. They continued talking about it for some time.

In general, only small particles such as electrons, neutrons, and alpha particles were observed in nuclear reactions. A heavier or slightly lighter nucleus might be produced, but there seemed to be no way that a nucleus with half the atomic weight of uranium would be produced. The only way it could be produced is if the uranium nucleus had somehow broken in half. But that was impossible—the energy required for something like that had to be incredibly large, and the neutrons that hit the nucleus only had a small energy.

Frisch had brought his skis and wanted to do some cross-country skiing while he was there, so off they went, Frisch on skis and Meitner walking in
the snow. They began talking about the two models of the nucleus. Ernest Rutherford had suggested it was a small, rigid ball, but Bohr had recently put forward a new and different model that was quite controversial at the time. He suggested that the nucleus was actually relatively soft and pliable—more like a drop of water.

There was no way Rutherford's model would allow a splitting into two nuclei, each half the size of the original one. Bohr's model, however, might work. They paused and sat down on a fallen tree near the path. Lise pulled out a piece of paper and a pencil from her pocket. She drew a picture of a uranium nucleus, assuming that it was a sphere. What would happen to it if a neutron hit it? If it was like a drop of water, it could change its shape slightly; it could become elongated. Meitner began to calculate the forces on the drop. Cohesive forces held the drop together, so if it eventually broke apart, these forces would have to be overcome. And the cohesive force was related to the surface tension of the drop. The force that might overcome it had to come from the charge of the nucleus. And indeed, the large, unstable uranium nucleus would likely wobble. If so, it would become elongated at first, but as it continued to oscillate it might begin to resemble a dumbbell, and if this happened, the two masses at the ends of the dumbbell would repel one another as a result of their similar charges.
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A wobbling drop that fissions into two smaller drops.

Meitner calculated how much energy would be released if this occurred. She was surprised to find that it would be about 200 million electron volts (an electron volt is the energy an electron gains in passing through a voltage difference of 1 volt). This was not a large amount, but when multiplied by the number of nuclei that would be splitting, it would be very large. But where did this energy come from? Meitner immediately thought about a lecture she had attended many years earlier at which Einstein had given a formula relating mass and energy. She added the masses of the two product nuclei and compared the sum with the mass of uranium. Then she used Einstein's formula to convert the difference in mass to energy. Amazingly, the result was the same: 200 million electron volts. This was obviously not a coincidence. Uranium nuclei had split in half—an amazing discovery if indeed that was what had happened. They decided to publish their results as soon as possible.

Frisch rushed back to Copenhagen. He could hardly wait to tell Bohr. But Bohr was getting ready for a trip to the United States and couldn't spend much time with him. Nevertheless, he was delighted with the news, and he encouraged Frisch and Meitner to publish as soon as possible. Frisch began writing up the paper, but he was stumped by the problem of how to describe the splitting. A friend noted that it was quite similar to the breaking apart of a simple cell in biology, and that was called “fission.” The name “nuclear fission” immediately came to mind, and he used the phrase in the article. It was published five weeks later in the scientific journal
Nature.

By then Hahn had published his result, but Meitner had not yet told him of the interpretation that she and Frisch had developed, so there was no mention of fission in his paper. Meitner, in fact, hesitated for a while before she told Hahn. She wanted to be sure that the paper she and Frisch had written was published first. There was some irony in all of this, however. Hahn was awarded the Nobel Prize in 1944 for his discovery of fission, with no mention of Meitner, even though she was the one who interpreted his result as fission.

A CHAIN REACTION

Bohr could hardly contain his excitement about the new discovery as he sailed to America. Along with coworker Leon Rosen, he tried to work out the details of what might happen during the fission of a uranium nucleus. There was no doubt that a tremendous amount of energy would be released. Could it be used to make a bomb? The possibility worried him. He had promised Frisch that he wouldn't mention the discovery until after Frisch and Meitner had published their results, but he forgot to mention this to Rosen.

Bohr, Rosen, and their group were met in New York by Fermi, Fermi's wife, and John Wheeler, a former student of Bohr's. Bohr said nothing about the discovery, but within a short time he discovered that everyone seemed to know about it. Then he realized he had forgotten to tell Rosen to keep it secret. The secret was now out, so he decided to make an announcement at a Washington conference on theoretical physics that he would be attending within a few days. Many of the world's top physicists were at the meeting, including Hans Bethe, Edward Teller, George Gamow, Harold Urey, Isidor Isaac Rabi, Otto Stern, and Gregory Breit. As expected, everyone was stunned when the news was announced, particularly after Bohr mentioned that a super bomb might be possible using nuclear fission.

When Fermi heard about the discovery, he had mixed emotions; he realized he had come very close to making the discovery himself, and he was annoyed. But at the same time he realized it was a momentous discovery, and it was important to follow up on it as quickly as possible. He immediately set up a simple experiment at Columbia University to verify the result, and he was pleased to see that there was no doubt: uranium nuclei did, indeed, fission.

Everyone was talking about the new discovery, and as Bohr, Wheeler, Fermi, and Leo Szilard got together for dinner the day after the conference they were still tossing around ideas about it. One of the most interesting of these ideas was one that Bohr had casually mentioned at the meeting. He pointed out that if the uranium nucleus split in half, leaving two lighter nuclei, there would have to be some neutrons left over, but he wasn't sure how many. Nevertheless, if there were two or more neutrons coming out of the reaction, each of them might produce a new fission. Furthermore, it was like the old story of the employer offering a new employee a wage of one cent the first day, then doubling the wage each day thereafter. After rejecting it, the employee realizes that he would have been a millionaire within a month. In essence, it doesn't take much doubling before small numbers become huge. And since each of the new fissions would take place in a tiny fraction of a second, an incredible amount of energy would be released very rapidly.
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The possibility was so exciting that Bohr asked Wheeler if he would like to work in collaboration with him to see what was possible, and Wheeler agreed. But they soon found that they would need some additional experimental results, so an experiment was set up at Princeton University to find out how the rate of fission would be affected by the speed or energy of the incoming, or bombarding, neutrons. In particular, they wanted to find out if there was a significant difference between slow and fast neutrons. They began bombarding uranium with extremely energetic neutrons, and, as expected, the higher the energy of
the neutron, the greater the fission rate. But they also got an unexpected result: at very low neutron energies, the rate of fission also increased. In essence, the fission rate was high for slow neutrons and also for very fast neutrons. This seemed a little crazy. Bohr and Wheeler thought about it. The reason for this had to be related to the uranium they were using, which was natural uranium that had come out of the ground.

Fission creating a chain reaction.

To understand why this is important we have to go back to the elements and look more closely at how they are made up. As we saw earlier, they have a certain number of protons and neutrons in their nucleus (we will ignore the electrons because they are irrelevant for this discussion). Furthermore, each element is identified by a mass number (A) (closely related to the atomic weight), and an atomic number (Z). The mass number is equal to the number of protons plus the number of neutrons, while Z is the number of protons in the nucleus, and it's the number of protons in the nucleus that uniquely defines an element. For example, the carbon nucleus has six protons, but it can also have either seven or eight neutrons. This difference in the number of neutrons does not change it into a new element; rather, the different numbers of neutrons identify different isotopes of the same element. And, as it turns out, uranium also has two isotopes that differ in the number of neutrons they contain. They are referred to as U-238 and U-235. Natural uranium is a mixture of these two isotopes.

As Bohr and Wheeler looked closely at the results of the Princeton experiment they realized that the sudden increase in fission as a result of the bombardment with slow neutrons was due to U-235. The increase with fast neutrons was mainly due to U-238. This meant that U-235 fission required less energy than U-238 fission. Thus, U-235 would be much better for a bomb, particularly because secondary neutrons were very slow. The problem was that natural uranium consisted almost entirely of U-238; only 0.7 percent of natural uranium was U-235. And to make things even worse, because they were chemically the same, there was no chemical process that could separate U-235 from U-238. Some sort of physical process, such as diffusion, would have to be used, and it would be difficult to do.

Although they didn't realize it at the time, others were already thinking along the same line. Irene Joliet-Curie and her husband Frederic, in Paris, also realized that a bomb might be possible. Furthermore, Otto Hahn, who was still in Nazi-controlled Germany, would no doubt soon come to the same conclusion. In addition, Werner Heisenberg, one of the brightest and most famous physicists in the world, was also in Germany, along with several other world-renowned physicists. One who was particularly worried about the bomb-making implication of nuclear fission was Leo Szilard.

THE LETTER TO ROOSEVELT

Leo Szilard had come to America several years earlier from Germany. He was Jewish, and when Hitler came to power he knew his days in Germany were numbered. In 1933 he went to England, and later moved on to the United States. Interestingly, about this time he had already begun to think about the possibility of a super bomb. He told Fermi about his worry, but Fermi didn't take him seriously; at this stage Fermi was not yet convinced that a bomb could be built. Disappointed in Fermi's response, Szilard decided to do something about it on his own.
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He knew that one of the largest known deposits of uranium in the world was in the Belgian Congo. And as soon as German scientists realized how important uranium was, they would rush to buy up as much of it as possible. Szilard had to stop them. He remembered that Einstein was a personal friend of Belgium's queen. He immediately phoned Einstein, who was now at Princeton's Institute for Advanced Study, but he was told that Einstein was at his summer home on Long Island.

Szilard acquired Einstein's address, but he now had a problem. He had never learned to drive a car, so he had to get his friend Isidor Isaac Rabi to drive
him. After some trouble, they finally found their destination and were greeted by Einstein. Szilard told him the news, and Einstein was surprised; he had heard nothing about the new discoveries, but he was immediately concerned. He knew that if the Germans produced such a bomb they would likely use it, and it worried him. Szilard told him about the uranium deposits in the Belgian Congo and suggested that he write a letter to Elizabeth, the queen of Belgium. Einstein was reluctant to bother her, but he offered to write a letter to a friend who was in the Belgian cabinet.