by Barry Parker
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.10
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 G
ermany. 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.11 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.
They talked about other things they could do. A letter to the White House was suggested. Szilard knew, however, that a letter signed by him would be ignored whereas a letter from Einstein would be taken seriously. Einstein agreed to sign a letter written by Szilard. The next problem was getting the letter to Roosevelt; it had to be delivered to him directly to have any impact. Szilard remembered an acquaintance by the name of Alexander Sachs, who sometimes visited Roosevelt. Szilard gave him the letter on August 15, 1939, and Sachs agreed to deliver it.
Germany was on the verge of attacking Poland at this time, however, and Roosevelt was particularly busy. Sachs tried several times but was unable get an appointment. He finally succeeded in October 1939. Roosevelt agreed that action was needed, and he authorized the creation of an advisory committee on uranium. The committee had its first meeting on October 21, at which six thousand dollars was budgeted for conducting experiments on neutrons. Szilard was disappointed with the small amount of money, but at least it was a first step.
Several problems would have to be overcome, however, before they could build a bomb. First of all, the uranium would have to be purified. At that time uranium had no known uses and very little of it had been mined, and the small amount that had been produced was not pure. Furthermore, Bohr and Fermi had showed that it was U-235 that was of most interest, and there was only a small amount of it in natural uranium. The U-235 would have to be separated out. And perhaps of most interest, some sort of device would have to be built to slow down the fission process so that it could be controlled. Such a device would be needed in order to determine whether a bomb would be possible. This device was eventually called a nuclear reactor. And for a controlled reaction, a moderator would be needed (a material that would absorb some of the neutrons coming out of the reaction). Two moderators were known: heavy water and graphite. Heavy water was expensive, so graphite seemed to be the better choice.
THE WAR BEGINS
In September 1939 Germany invaded Poland and World War II began. Because of Hitler's policies regarding Jews, many Jews had fled Germany, including Albert Einstein and other top physicists. But Werner Heisenberg, who had just won a Nobel Prize, was not Jewish, and he had no interest in leaving. He had been offered positions at several major universities in the United States, but he had turned them down. As a German, he felt compelled to serve his country in its time of need. And indeed, by the time the war started, the Nazi government had also heard about the possibility of a super bomb. In fact, a group of Germany's leading scientist (the few that were left) had been recruited to study uranium fission. The group was referred to as the Uranverein (Uranium Society), and one of the most prominent members was Otto Hahn, the man who had discovered fission.12 Members of the Uranverein were reluctant at first to invite Heisenberg because he was a theorist, and the building of a bomb needed experimentalists. Furthermore, he had been friends with many Jewish scientists, including Einstein, so he was not considered desirable. Interestingly, Hahn was a rather reluctant member of the group, and in the end he made almost no contribution to it. He continued to raise objections against the project, sure that it would never be possible. The Uranverein finally decided to invite Heisenberg into the group in late September 1939 to help solve what seemed to be insurmountable problems. Soon he was leader of the group.
Heisenberg realized that the first step had to be the building of a nuclear reactor: a slowed-down bomb. And the best moderator was heavy water, or deuterium. But deuterium was not plentiful in Germany; however, it was being produced in a plant in Vemork, Norway. Germany had not yet invaded Norway, so would have to purchase the deuterium it needed. The Vemork plant was perched high above the fjords in a remote region of Norway, about 150 miles from Oslo.
The Germans approached the owner of the Vemork plant and offered to buy all its available heavy water. The Norwegians were surprised by the offer and wondered why the Germans would need so much. When they were not given an answer they refused to sell. About the same time, the Joliet-Curies in Paris had arrived at the same conclusion as the Americans and Germans. For a bomb, a reactor would be needed, and it would need heavy water as a moderator. The French government therefore also sent a representative to Vemork, and when he told Norwegian officials what the heavy water was needed for they promised him all they had at no cost.
Then, in April 1940, Germany invaded Norway, changing the situation. The German army raided the plant immediately and was disappointed to find that all the heavy water had been shipped to France. So the Germans immediately ordered an acceleration in production and insisted that everything that was produced was to be shipped to Berlin.
In early June 1940, Germany also invaded France, and when Paris fell on June 14, the Uranverein physicists immediately went to the Joliet-Curie laboratory, expecting to find heavy water and uranium. Joliet-Curie claimed that it had been loaded onto a ship that had been sunk, and the Germans accepted the answer. The materials had actually been shipped to England.
By now the Germans had made considerable progress. One of the Uranverein group had calculated that uranium would have to be enriched so that it contained 70 percent more U-235 than U-238 before it could be used in a reaction. They had also discovered that when U-238 captured a neutron it formed U-239, which was unstable and radioactively decayed in twenty-three minutes. The new element, which was unnamed, might be fissionable according to Carl Friedrich von Weizsäcker, one of the group members. This meant that if they could build a reactor they might have a way of producing another element that could be used for a bomb.
In the summer of 1940 a new building at the Kaiser Wilhelm Institute in Berlin was designated for the exclusive use of the Uranverein. It was next door to the Institute of Physics and was called the Virus House. Germany now had a good supply of uranium, ample heavy water, and several important developments from the Joliot-Curie lab. But the members of the Uranverein were soon stumped by the difficulty of separating U-235 from natural uranium.
MEANWHILE IN ENGLAND
In the meantime, Otto Frisch had not sat still; he had immigrated to England and was now working with Rudolf Peierls at the University of Birmingham. Peierls had previously worked for Fermi at the University of Rome. Frisch and Peierls collaborated on the problem of how much U-235 would be needed for a bomb. This was later referred to as the critical size. To their surprise, their calculations indicated that a very large amount was needed. Despite the setback, their work showed that a bomb was indeed p
ossible. In fact, they concluded in their report that what was needed was two pieces of U-235 that were smaller than the critical size. When the two pieces were brought together, they would immediately explode, but they could be handled safely as long as they were below the critical size. Because of their work, British officials decided to set up an organization called the MAUD Committee for atomic research in early 1941. No one was quite sure where the name came from. It is not an acronym, and it appeared to have come from a letter sent by Meitner to an English friend. She ended the letter with the name Maud, and for a while people thought it was some sort of code. It turned out that it wasn't.
In July 1941 the MAUD Committee issued two reports.13 The first stated that it had been determined that a bomb could now be built using approximately twenty-five pounds of enriched uranium, and it would have the destructive effect of eighteen hundred tons of TNT. It recommended that work begin immediately and that it should be conducted in collaboration with the United States. At the time the United States had many more resources than England for building such a bomb. In addition, a new committee codenamed Tube Alloys was set up in conjunction with Canada for further developments of nuclear weapons.
In June 1940 the MAUD report was sent to Vannevar Bush, the head of the National Defense Research Committee in the United States. The results were reported to Roosevelt, and there was a considerable amount of discussion about the possibility of a bomb, but little action.