Otto Hahn led the group that established uranium fission, an essential step toward the development of both nuclear power and nuclear weapons.

When Adolf Hitler seized power in Germany on January 30, 1933, Otto Hahn did not think Hitler’s regime would last. Hitler was still in power ten years later, however, and Hahn refused to work on military research for Hitler. In this the world was fortunate, for military research in Hahn’s field led to the atomic bomb. [kw]Hahn Splits the Uranium Atom (Dec., 1938)
[kw]Splits the Uranium Atom, Hahn (Dec., 1938)
[kw]Uranium Atom, Hahn Splits the (Dec., 1938)
[kw]Atom, Hahn Splits the Uranium (Dec., 1938)
Atoms;splitting
Uranium;fission
Nuclear fission
[g]Germany;Dec., 1938: Hahn Splits the Uranium Atom[09870]
[c]Science and technology;Dec., 1938: Hahn Splits the Uranium Atom[09870]
[c]Physics;Dec., 1938: Hahn Splits the Uranium Atom[09870]
[c]Chemistry;Dec., 1938: Hahn Splits the Uranium Atom[09870]
Hahn, Otto
Meitner, Lise
Strassmann, Fritz
Fermi, Enrico
Joliot-Curie, Irène
Joliot, Frédéric
Savić, Pavle Petar
Frisch, Otto Robert

Hahn was a superb experimentalist; specifically, he was a radiochemist. He headed the department of radioactivity of the Kaiser Wilhelm Institute for Chemistry in Berlin and had done so since the department’s inception in 1912. Together with a colleague, Lise Meitner, he discovered a new element that they named protactinium. He enjoyed a modest fame that gave him some protection against Hitler’s followers.

Otto Hahn.

(The Nobel Foundation)

Much of Hahn’s work lay in discovering the chain of decay products of the naturally occurring radioactive elements. Uranium, for example, is a natural element that is radioactive; that is, it emits radiation. Upon so doing, it changes or decays into a “daughter” element, thorium. Thorium, in turn, decays to radium, which decays to radon, and so forth, with the ultimate end product being a stable form of lead. As one element decays into another, three types of radiation may be emitted. British physicist Ernest Rutherford, Rutherford, Ernest under whom Hahn once studied, named the three types for the first three letters of the Greek alphabet: alpha, beta, and gamma. Uranium, thorium, and radium all emit alpha particles Alpha particles when they decay. In 1909, Rutherford showed that alpha particles are actually helium nuclei, and, in fact, this is where the earth’s helium Helium originates. The helium used to fill balloons comes from gas wells drilled in the ground; it is found there because natural radioactive elements in the dirt and rocks emit alpha particles as they decay. Helium itself is stable and is not radioactive.

In trying to understand radioactive decay, Radioactive decay some scientists pictured a uranium nucleus as composed of swarms of alpha particles. They imagined that alpha decay occurred when one of the alpha particles managed to break free and leave the swarm. In the uranium decay chain, there are eight steps in which alpha particles are emitted; there are six steps in which beta particles are emitted. Beta particles are the familiar electrons, but in this case they are emitted by the nucleus. One normally thinks of the nucleus as a ball of neutrons and protons, and it is not clear where the electron comes from. To envision a very crude model, suppose that a neutron is a proton (positive charge) and an electron (negative charge) somehow crammed together. In beta decay, Beta decay a neutron must change into a proton and an electron. Thus, when the beta particle (electron) is ejected from the nucleus, the nucleus is left with one more proton (positive charge) than it formerly had.

Going beyond natural radioactivity, artificial radioactivity Radioactivity;artificial was discovered in 1934 by Irène Joliot-Curie and her husband, Frédéric Joliot. They showed that alpha particles striking the nuclei of light elements, such as aluminum, could make those elements radioactive, but alphas were ineffective at making heavy elements radioactive because alpha particles are composed of two protons bound together with two neutrons; heavy elements are composed of a large number of protons (92 for uranium) and a larger number of neutrons. The large positive charge of the heavy elements repels the positive charge of the alpha particle and keeps it from penetrating the nucleus.

When alpha particles were allowed to fall on the light element beryllium, a strange new particle was emitted. In 1932, James Chadwick Chadwick, James showed that this new particle had zero charge; consequently, it was named the neutron. Neutrons The neutron made a wonderful nuclear bullet because it was not repelled by the positively charged nuclei as the alpha particle had been. It became popular to bombard various elements with neutrons and look for interesting results. Enrico Fermi did exactly that. What usually occurred was that the target nucleus absorbed the incoming neutron, and then beta decayed into the element having one more proton. In a fateful experiment, Fermi bombarded uranium with neutrons. He expected to make a new element beyond uranium, a “transuranic” element. Transuranic elements He made trace amounts of the elements that would come to be called neptunium and plutonium.

Because Hahn and Meitner were experts on the chemistry of the heavy radioactive elements, they repeated Fermi’s experiment to analyze the results more carefully. Fritz Strassmann, a young German analytic chemist, joined their group in 1935. By early 1938, they had found ten different radioactive elements where Fermi had found only a few. Supposing the main ones to be beta decay daughters, Hahn, Meitner, and Strassmann proposed four transuranic elements, but they did so with reservations. Although they had seen evidence of their new elements, they had not been able to isolate them cleanly.

Pavle Petar Savić, a chemist from Yugoslavia, was now working with Joliot-Curie. They claimed to have found thorium after bombarding uranium with neutrons. Hahn’s group thought it unlikely that one neutron could knock an alpha particle out of uranium and change it to thorium. They carefully checked for thorium but could find none. Savić and Joliot-Curie then claimed to have found a radioactive element that they could not separate from lanthanum. In a similar vein, Hahn’s group found a radioactive product that they supposed was a type of radium, but they could not chemically separate it from barium.

In the waning weeks of 1938, Hahn and Strassmann conducted a remarkable series of complex experiments. The experiments were difficult because the researchers had to work with small samples. Hahn’s group had only a weak neutron source; consequently, some of the daughter nuclei were produced only by the thousands. (It can be compared with the million billion atoms to be found in a pencil dot.) With Hahn’s expertise, they used techniques such as fractional crystallization, wherein relatively pure crystals of various substances crystallize at different temperatures from a hot solution.

Hahn had pioneered the technique of tracing a substance by its radioactivity, in which case one does not need weighable quantities of the target substance. Daughter atoms are often created “glowing hot,” which makes them stand out. Such nuclei are said to be in an excited state. Even if they could be seen with the naked eye, these nuclei would not look hot because they don’t give off normal heat as they cool or “de-excite.” Instead, they emit packets of energy called gamma rays. Gamma rays Hahn and Strassmann had sensitive Geiger-Müller counters with which to detect these gamma rays. From the number of rays detected, they could estimate the number of atoms in their sample.

In time, Hahn and Strassmann firmly established that the “radium” they thought they had formed, and that they had been unable to separate from barium, was in fact radioactive barium. As impossible as it seemed to them, a single neutron caused the uranium nucleus to split into two roughly equal parts. Their scientific paper announcing that humankind had split the atom was published on December 22, 1938.

Hahn asked Meitner to develop a theory that would make sense of his results. Meitner and her nephew Otto Robert Frisch became absorbed in the problem for some time. It seemed impossible to them that the nucleus should be brittle and seemingly split into two parts as if a neutron struck it along a cleavage line. In fact, Frisch recalled that Niels Bohr Bohr, Niels had suggested that the nucleus was not brittle, but was more like a drop of liquid. Frisch visualized the nucleus as a liquid drop struck by a neutron, causing it to vibrate. Oscillations could build up that would break the original drop into two drops. Because these smaller drops would both be positively charged, they would repel each other with tremendous force. Borrowing a term used to describe the splitting of cells in biology, Frisch later named the process “fission.” Because of the strength of the electrical repulsion, the fission fragments should fly apart with millions of times the energy involved in chemical reactions. It took Frisch only a few hours to set up an experiment in his laboratory to detect the energetic fission fragments.

Another characteristic of fission was soon discovered: Each fission releases two or three neutrons. These neutrons can cause other fissions that cause still more fissions in what is called a chain reaction. If this reaction is controlled, one can build a nuclear reactor and use it for an energy source. If the reaction is allowed to proceed without control, a bomb can turn itself to incandescent vapor in less than one millionth of a second.

Hahn’s work thus formed an essential link in the chain that led to both nuclear power and nuclear weapons. In 1944, Hahn received the Nobel Prize in Chemistry for his discovery of nuclear fission. Nobel Prize recipients;Otto Hahn[Hahn] As the power of the nucleus became available, some predicted that nuclear power would be so cheap, it would be provided free as a government service. Others saw in nuclear weapons a way to end World War II and to make future wars unthinkable. It can be argued that the use of the atomic bomb on Japan in 1945 ultimately saved many lives by making an Allied invasion of mainland Japan unnecessary. There is no doubt that the terrifying prospect of nuclear war was of prime importance in keeping the superpowers from directly attacking each other during the years of the Cold War.

Hahn’s own life can be seen as a summary of the societal impact of his work. During World War I, Hahn worked with Fritz Haber Haber, Fritz to use poison gas as a weapon. When Hahn expressed reservations, Haber argued that gas could bring an early end to the war and thereby save lives. Later, Hahn was so upset when he saw the agony of Russian soldiers who had been gassed that he attempted to use his own respirator to aid them. During World War II, although Hahn avoided direct military work, he did not actively speak out against government policies, but during the 1950’s he became politically active and quite outspoken against the misuse of nuclear power and against the stockpiling of nuclear weapons. Atoms;splitting
Uranium;fission
Nuclear fission

• Hahn, Otto. New Atoms: Progress and Some Memories. New York: Elsevier, 1950. Collection of papers about the discovery of fission and related topics accessible to lay readers. Includes Hahn’s Nobel lecture as well as some of his personal reminiscences from the history of natural radioactivity.
• _______. Otto Hahn: A Scientific Autobiography. Edited and translated by Willy Ley. New York: Charles Scribner’s Sons, 1966. Autobiography discusses Hahn’s life in science and features three of his key scientific papers on discovering fission as well as biographical notes on other scientists. Includes photographs, a chronology of Hahn’s life, a comprehensive bibliography of Hahn’s publications, and many helpful footnotes by the editor/translator.
• Hoffmann, Klaus. Otto Hahn: Achievement and Responsibility. New York: Springer-Verlag, 2001. Biographical work focuses on Hahn’s contributions to science as well as his views on social and scientific responsibility. Includes index.
• Irving, David. The German Atomic Bomb. New York: Simon & Schuster, 1967. Gripping account of the efforts of German scientists and the German government to develop the atomic bomb includes detailed discussion of Hahn’s discovery of fission. Also addresses why the Germans failed despite their early start over the Allies’ bomb programs.
• Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. Overview of the scientific achievements of the twentieth century. Chapter 3 discusses the early days of nuclear physics, including Hahn’s work. Includes many illustrations and index.
• Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. Comprehensive, highly detailed account of the making and use of the American atomic bomb, intended for general readers. Includes the story of Hahn’s discovery of fission and places it within the context of the worldwide efforts then under way to develop the atomic bomb. Features extensive bibliography.
• Shea, William R., ed. Otto Hahn and the Rise of Nuclear Physics. Boston: D. Reidel, 1983. Collection of copiously referenced papers on Hahn’s discoveries of nuclear fission, nuclear isomerism, radiothorium (thorium-228), and related matters. Includes a fine chapter on Hahn and social responsibility. Accessible to the general reader, but somewhat technical at times.

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Rutherford Discovers the Proton

Chadwick Discovers the Neutron

Cockcroft and Walton Split the Atom