Once the neutron was discovered, a new era began in studying the nucleus. Rutherford had bombarded nuclei with a particles to discover the nucleus. In ground-breaking work, the Joliot-Curies bombarded aluminum with a particles and discovered that the aluminum was converted into a radioactive form of phos­phorus, the first proof of artificial transmutation (converting one element into another) and artificial radioactivity. The a particle was absorbed by the nucleus of aluminum (Z = 13) and converted it to phosphorus (Z = 15).

Enrico Fermi recognized that there was a huge advantage in using neutrons to study artificial transmutation because they have no charge, so they are not repelled by the protons in the nucleus. He began a systematic series of experi­ments irradiating all of the elements with neutrons produced by the same kind of neutron source that Chadwick had used to discover neutrons, namely a par­ticle irradiation of beryllium. He and his colleagues irradiated 60 elements and found that 40 of them became radioactive (10). When they irradiated uranium, the heaviest element known, they found that the neutron was absorbed by the nucleus and subsequently negative p particles were emitted with a half-life of 13 minutes. According to the rules of p decay, this means that a neutron changed into a proton, changing the element from uranium with a Z of 92 to a new element with a Z of 93. If true, this would mean they had created an entirely unknown ele­ment, since uranium is the highest atomic number element that occurs naturally. These artificially made elements are known as transuranics. Unfortunately, Fermi got a little ahead of himself in interpreting the experiment as the creation of a new transuranic element (it wasn’t), though it is now known that transuranics can indeed be produced by capturing neutrons.

Lise Meitner and Otto Hahn, working at the Kaiser Wilhelm Institute in Berlin, began bombarding uranium with neutrons, and they found a veritable zoo of dif­ferent radioactive species decaying by p decay with different half-lives ranging from 10 seconds to 23 minutes. Lise Meitner, an Austrian of Jewish ancestry, had to leave Germany in 1938 to avoid being arrested by the Nazis. She was able to escape to Denmark with the help of Niels Bohr—who helped many physicists escape Germany at this time—and subsequently went to Stockholm, Sweden. In the meantime, Hahn—who was probably the world’s best radiochemist—and Fritz Strassman continued trying to identify the radioactive elements that were producing the varying lifetimes after neutron bombardment of uranium. He pre­cipitated different elements out of the radioactive material and finally concluded that an element with the same chemical properties as barium had radioactivity (9, 10). But this seemed impossible! Uranium has a Z of 92 while barium has a Z of 56. This could not possibly happen by p decay. What was going on? Hahn was a chemist, not a physicist, so he wrote to Meitner in Sweden to ask what physics could explain these results.

Meitner had a nephew, Otto Frisch, who was working with Bohr in Copenhagen. For Christmas 1938 they were both invited to stay in Kungalv, Sweden, with a friend of Meitner’s. They were walking in the snow, talking about Hahn’s unbeliev­able results, when Meitner had a revelation! She recalled that Bohr had explained the nucleus as a liquid drop, held together by the short-range strong force that acted similarly to the surface tension holding a drop of water together. If a neu­tron entered the nucleus, it would be like perturbing a drop of water, causing it to jiggle. As the nucleus jiggled, if it became elongated, the repulsive force due to the charge of the protons would be able to overcome the strong force holding it together, distorting it into a dumbbell shape and finally splitting it into two sepa­rate pieces. This would explain Hahn’s results because barium (Z = 56) would be one piece and the other piece would have to have an atomic number of 36, which would be krypton.

Meitner and Frisch did some quick calculations and determined that it was indeed possible for a large nucleus to split into two pieces, and they would carry away about 200 million electron volts (MeV) of energy.6 They showed that the energy came from the decrease in mass of the two pieces of the nucleus com­pared to the original uranium nucleus, according to Einstein’s prescription that E = mc2 (12). When Frisch went back to Copenhagen and told Bohr, he imme­diately understood and exclaimed “Oh, what idiots we have been! Oh, but this is wonderful! This is just as it must be!” (10). Frisch asked a biologist friend what he called the division of bacteria and he said “binary fission”; Frisch shortened it to just “fission,” and that became the new name for the splitting of the nucleus.

Everything suddenly became clear. The different lifetimes that Fermi and Hahn and others found when they bombarded uranium with neutrons were from dif­ferent pieces of the nucleus that split. A nucleus of uranium could split in many different ways, as long as the pieces conserved the number of nucleons and energy was conserved. The different pieces, called fission products, would then undergo p decay to convert excess neutrons into protons to make a more stable nucleus. Each of the many pieces would have different decay half-lives. Fermi had not discovered transuranic elements after all—he had discovered fission but hadn’t recognized it.

A new world began. Soon after Frisch told him the news about fission, Bohr traveled to the United States, and the word spread like wildfire among physicists, many of whom had emigrated from Europe to the United States because of the war. A Hungarian physicist, Leo Szilard, was in New York at the time, and he heard about the discovery. He immediately realized that the fission fragments would have excess neutrons, and if enough neutrons are emitted for each ura­nium nucleus that decays, it would be possible for those neutrons to cause fission in other uranium nuclei, leading to a chain reaction. And if a chain reaction could occur, there was the possibility that a bomb could be made. But no one really knew if that were possible. Because of fears that Germany would develop a bomb if it were possible, the United States soon began a crash program to determine whether an atomic bomb was possible and, if so, to build it before Germany could. The history of that effort, the Manhattan Project, is compellingly told by Richard Rhodes in his book The Making of the Atomic Bomb (10).

This book is about nuclear power, not atomic bombs. There are fundamen­tal differences in the two processes, but the physics for nuclear power had to be developed before atomic bombs could be built. Enrico Fermi was driven out of Italy because of the rise of Mussolini and the fascists who allied with Hitler in Germany. Fermi came to the United States and became a professor at the University of Chicago, where he joined the secret effort to build the bomb. Fermi had made a critical discovery when he was bombarding various elements with neutrons back in Italy. His lab had measured different intensities of radioactivity when they irradiated samples on a marble table (only in Italy could this happen!) or a wooden table. What could cause this? Fermi, for some reason, decided to put paraffin in front of his neutron source before irradiating a sample and found that the radioactivity increased dramatically. He realized that the paraffin, a rich source of hydrogen, was slowing down the neutrons so they could more easily be captured by the uranium nucleus. The wooden table, having more hydrogen than marble, also slowed neutrons down more than the marble table, so the radioactiv­ity was greater. The slow neutrons were more effective in causing fission, though he did not know that at the time.

It was later discovered that natural uranium is actually a mixture of two dif­ferent isotopes: 238U accounts for 99.3% and 235U accounts for 0.7%.7 Bohr once again had the critical insight. He realized that 238U had a high probability, called a cross section, of capturing slow neutrons of about 25 eV without fissioning. This would make a new isotope of uranium, 2’92U, which p-decays to the transuranic element neptunium-239 (239Np). This is what Fermi thought he was observing in his experiments on neutron bombardment of uranium. Neptunium-239 rapidly undergoes p decay to form another transuranic element, plutonium-239 C^jPu). 235U, on the other hand, has a high probability of fissioning when it captures very slow neutrons called “thermal” neutrons. 238U required very high energy neutrons to fission, but neutrons quickly lose their energy in collisions so they could not sustain a chain reaction. So, the first requirement for fission to occur is to slow down neutrons with a material called a moderator. The other critical requirement for a chain reaction is that the fission of a 235U nucleus must produce more than one neutron on average that can be captured by another 235U nucleus and cause it to fission. This would cause a self-sustaining controlled fission reaction.

Fermi had to build a reactor to prove that a fission chain reaction really was feasible, since no one knew whether it could be done. The reactor was built under top secret conditions on a rackets court under the stands of Stagg Field at the University of Chicago. The first decision was what to use to slow down the neutrons. Water is a good moderator, but it can absorb some neutrons. Since they were going to use natural uranium, it was essential that neutrons not be absorbed by the moderator. They decided to use carbon, which would slow down the neutrons with little absorption, and it could be made into graphite blocks to construct the reactor. The next decision was how to arrange the uranium in the graphite moderator. It takes a critical mass of uranium packed within a critical volume to undergo a chain reaction. If there is not enough uranium or it is in too large a volume, then the neutrons that are produced do not cause a self-sustaining fission reaction. Fermi decided to make graphite blocks with recesses in them to hold cylinders of pressed uranium oxide. The blocks with the fuel were arranged in a slightly oval pile with blocks of pure graphite inter­spersed. The first name for a reactor was a “pile.” As the pile was built by adding blocks of graphite and blocks with uranium, there came a point where it was approaching a critical mass.

There is an obvious problem here: How do you shut it down when it goes critical? Cadmium is an element that strongly absorbs slow neutrons, so cad­mium sheets were nailed to flat wooden strips, which could be inserted into the pile through slots. When these control rods were inserted, the reactor could not go critical. On December 2, 1942, in freezing cold in the unheated rackets court, the final construction of the pile was completed, and the control rods were slowly withdrawn while detectors measured the neutron intensity. At 3:49 p. m. the neutron counters began clicking with increasing speed—the first chain reaction created by humans had begun! After four and a half minutes, Fermi shut down the reactor by lowering the control rods (10). Thus began the era of nuclear power. The principles that Fermi and others used in that reactor, Chicago Pile #1 or CP-1, are still the principles used in every nuclear reactor in the world, though most of them use water for a moderator and use uranium that is enriched to about 3-4% of 235U so that it fissions more efficiently and has a smaller critical mass.

To make a bomb, at least two neutrons would need to be produced for each fissioning uranium nucleus and captured by other uranium nuclei to get a geo­metric progression of fissioning nuclei (one fission causes 2 fissions, which cause 4 fissions, which cause 8 fissions, and so on). It turns out that the fission of 235U produces on average two and a half neutrons, so there are plenty to cre­ate a chain reaction. However, to efficiently capture these excess neutrons and have an extremely fast chain reaction requires that the natural uranium be highly enriched to over 90% 235U. It is physically impossible for a power reactor to ever become a fission bomb because a power reactor does not have the highly enriched 235U that is necessary for a bomb.