The Discovery of Fission
German chemist and Nobel laureate Otto Hahn and Austrian-Swedish physicist Lise Meitner are credited with roles in discovering nuclear fission, the process of splitting an atom into two or more smaller parts. By the 1930s, Hahn and Meitner had experimented with radioactive materials for some time. After Meitner left Germany in 1938, Hahn and German chemist Fritz Strassmann made a breakthrough in their experimentation methods that enabled them to collect physical data showing that an atom of uranium would “burst” under certain conditions. They announced their finding in 1939. Shortly thereafter, Meitner and British physicist Otto Robert Frisch published a paper that provided a theoretical explanation for the process, and named it “fission.” This article by Hahn, which appeared in Scientific American in 1958, describes how, through a number of twists and turns in the road, he and his colleagues discovered the fission process in uranium.
The Discovery of Fission
In 1939 Otto Hahn and Fritz Strassmann announced that uranium nuclei "burst" when they are bombarded with neutrons. How they came to this conclusion is a classic example of the nature of science
By Otto Hahn
The editors of Scientific American have asked me to set down my personal recollections of the discovery of uranium fission. This I am very pleased to do.
The story must go back to the year 1904, for that was the year in which I was "transmuted" from an organic chemist into a radiochemist and thus began to learn about radioactive elements. Armed with a letter of recommendation from the professor with whom I had taken my doctor's degree in Germany, I went to London in 1904 with two objectives in mind: namely, to learn English and to study with Sir William Ramsay, the discoverer of the inert gases. Sir William, who had just become interested in radium, gave me a dish with a barium salt which he said contained approximately 10 milligrams of radium, and he asked me if I would purify the radium by the method of Marie Curie. The barium salt, as it happened, came from a mineral which contained not only radium but also a good deal of thorium, and while working with the material I discovered a previously unknown radioactive substance. I called it radiothorium, "a new element." (Actually it was an isotope [an atom with the same atomic number as another atom but a different atomic mass] of thorium, but the existence of isotopes of elements was not yet known.)
Ramsay urged me to give up organic chemistry and devote myself to the study of radium. Accordingly, with a view to learning more about the field, in the fall of 1905 I went to Montreal to study with the then already famous Professor Ernest Rutherford. There an amicable argument about the stability of radiothorium with Rutherford's well-known associate, B. B. Boltwood of Yale University, led me to the hypothesis that another radioactive element, rather long-lived, must exist in the periodic table between thorium and radiothorium. And in fact, after returning to Germany to work at the University of Berlin, I found this substance, which I called mesothorium.
When I attempted to separate mesothorium from radium, however, it became obvious that this new "element" was practically indistinguishable chemically from radium. I first tried the separation method that Madame Curie had worked out for the purification of radium. In this method, after radium has been precipitated from the mineral with barium as the carrier substance, the radium is separated from the barium by fractional crystallization of the chlorides or bromides of the two elements: the radium is concentrated in the first fraction of crystals that comes out. I applied this crystallization treatment to the mixture of mesothorium and radium I had obtained from a thorium-uranium mineral. But mesothorium and radium refused to crystallize in separate fractions. And try as I would, with every conceivable method of separation, my efforts to segregate mesothorium from radium came to naught. I decided that the two substances were remarkably alike, but I did not have the courage to label them chemically identical.
With the discovery of "isotopy" a few years later by Frederick Soddy, also a former student of Rutherford and Ramsay, everything became clear. Mesothorium was now identifiable as an isotope of radium, and radiothorium as an isotope of thorium. By this time I had learned also that radiothorium was a decay product from mesothorium. The long-lived mesothorium slowly changes, without detectable emission of radiation, to a shorter-lived form which I called mesothorium 2; the latter is an isotope of the radioactive element actinium. In turn, mesothorium 2, with a half-life of 6.2 hours, decays by beta-particle emission to radiothorium.
Some of this information, as we shall see, is directly relevant to the subject of uranium fission. And in the course of these studies I was acquiring what was to be of the greatest help to Fritz Strassmann and myself later in discovering the fission process: namely, a thorough familiarity with methods of separating radioactive substances.
In the fall of 1907 an important event in my chronicle took place: Lise Meitner, the Austrian physicist, joined me at Berlin. She had come from Vienna to attend Max Planck's lectures in theoretical physics, and, having free time to spare, she visited my laboratory to work with me. What had been intended as a temporary stay in Berlin developed into a collaboration of more than 30 years—an association which was only terminated in the summer of 1938 through the actions of the Nazi regime. It is perhaps needless to say that the friendship has continued.
The working facilities in the overcrowded Chemical Institute of the University of Berlin were then very modest. Our laboratory was an unoccupied woodworking shop. But when, in 1911, Kaiser Wilhelm II inaugurated his Society for the Advancement of Science, we were given a radiochemical department in the first institute set up by the Society—the Kaiser Wilhelm Institute of Chemistry. Over the years this department developed into two large departments, one for radiochemistry under my direction and one for nuclear physics under Dr. Meitner.
In 1917 we discovered element 91, which we named protactinium. It was our work on the chemical properties of this substance that gave rise later to our keen interest in investigating the irradiation of uranium with neutrons. This part of the story, which leads directly to the discovery of fission, begins in the year 1932 in Rutherford's laboratory at the University of Cambridge.
James Chadwick's discovery of the neutron in that year not only explained the phenomenon of isotopy but also gave physicists a new and extraordinarily effective means of producing artificial transmutation of elements. Since neutrons carry no electric charge, they easily enter the nucleus of an atom. The first to point out the great value of neutrons for initiating nuclear reactions was Enrico Fermi in Italy. He and his co-workers—Edoardo Amaldi, Oscar D'Agostino, Franco Rasetti and Emilio Segrè—bombarded practically all the elements of the periodic table with neutrons and produced a great number of artificial, radioactive isotopes.
Extending their experiments all the way up to uranium, the heaviest natural element (atomic number 92), Fermi's group found that uranium, too, gave rise to new substances, some of which decayed very rapidly. Usually the capture of a neutron by a nucleus produces an unstable isotope which emits a beta particle [an electron or a positron] and is thus transformed into the next higher element. Since Fermi's products from uranium emitted beta particles, he and his colleagues made the plausible assumption that the transmutation produced short-lived isotopes of uranium which then, by beta-decay, gave rise to elements beyond uranium—element 93 and possibly even 94.
Since no such elements occur in nature, Fermi's conclusion was widely disputed. Some physicists suggested that his best-identified new substance—a material with a half-life of 13 minutes—was actually an isotope of protactinium, rather than a "transuranic" element. Naturally Dr. Meitner and I were keenly interested, for we knew a good deal about the chemical properties of protactinium, our discovery. We decided to repeat Fermi's experiments to try to determine whether his substance was or was not protactinium.
Fermi's products were of course much too small in amount to be detected in any way except with a Geiger-Müller counter. But fortunately we had a convenient means of making a chemical test. I had myself discovered a beta-emitting isotope of protactinium, derived from uranium. This isotope, with a half-life of 6.7 hours, gave us a definite marker, or indicator, of the presence of protactinium in small amounts. Accordingly we added a certain amount of our protactinium isotope to the "transuranic" products obtained by Fermi's procedure, and then applied chemical precipitations to the mixture. Very little of the protactinium (less than one thousandth) came out with Fermi's newly discovered substances. This proved unequivocally that his substances could not be protactinium. We were also able to show with equal certainty that they were not isotopes of thorium or actinium. That they might be lighter elements seemed entirely out of the question. Ida Noddack did suggest, to be sure, that we could not be certain they were transuranic elements unless we excluded all the other elements of the periodic table as possibilities, but this thought was considered to be wholly incompatible with the laws of atomic physics. To split heavy atomic nuclei into lighter ones was then considered impossible. Thus our experiments appeared to establish the correctness of Fermi's assertion that he had detected "transuranic elements." Actually, as we were to learn later, all of the radioactive substances detected in these early experiments were fission products, not transuranic elements.
Over the course of years of work Lise Meitner and I, subsequently joined by Strassmann, found a great number of radioactive transmutation products, all of which we had to regard as elements beyond uranium. They could be arranged in a regular series, for in chemical properties they corresponded to known elements lower in the periodic table: namely, rhenium, osmium, iridium and platinum.
There was one product which, unlike Fermi's extremely short-lived "uranium isotopes," had a half-life of 23 minutes. Its life was sufficiently long for us to establish chemically that it was in fact an isotope of uranium. Since it emitted a beta particle, it was evident that this isotope must become an isotope of element 93, which we called eka-rhenium. We looked for the new element, but were unable to detect it. If we had not been convinced that we had already identified two other isotopes of element 93—an erroneous assumption, as it turned out—we would have prepared stronger samples of the material and made a determined effort to find the disintegration product of our 23-minute uranium isotope. We should then have had the pleasure of discovering element 93. Later Edwin M. McMillan and Philip H. Abelson in the U. S. identified an isotope of element 93 with a half-life of 2.3 days, and they named the element neptunium. Subsequently, when we ourselves obtained a stronger neutron beam for irradiating uranium, we had no trouble in detecting neptunium.
On the main road toward the discovery of fission, the next step in this curious chronicle of near-discovery was taken by Irène Joliot-Curie and P. Savitch in France. They were greatly puzzled by an artificially produced substance of 3.5 hours' half-life which they first took for an isotope of thorium, later for actinium, and finally for a "transuranic element" strongly resembling lanthanum. They believed that they had succeeded in separating the substance from lanthanum by fractional crystallization, but apparently they were misled by irrelevant separations in their mixture. Actually their substance was undoubtedly lanthanum itself: had Madame Joliot-Curie and Savitch recognized this, they would have been on the verge of discovering fission.
In a paper published by the French Academy of Sciences in 1938, Madame Joliot-Curie and Savitch concluded: "It appears, therefore, that this substance can only be a transuranic element, with properties very different from those of the other known transuranic elements." They considered various ways of fitting the element into a transuranic series, but the possibilities they discussed seemed to us highly unlikely. Strassmann and I decided to look further into their results. (Professor Meitner had gone to Stockholm, having been forced by the Hitler regime to leave Germany in July, 1938.)
We found that after the "transuranic elements" had been precipitated and removed, the solution still contained some radioactive products. Experiments in chemical separation of these substances now gave a remarkable result. When we used barium as the carrier, three radioactive isotopes, with different half-lives, came down with the barium. We were certain that these could not be accidental impurities, because our barium precipitates were extraordinarily pure. At Strassmann's suggestion, the carrier we used was barium chloride, which is deposited from strong hydrochloric acid in beautiful small crystals that are free of any trace of adsorbed impurities.
Now the precipitates had to be either barium or radium, which is chemically similar to barium. There was nothing in the knowledge of nuclear physics at the time to suggest that barium could possibly be produced as a result of the irradiation of uranium with neutrons. Therefore we could only conclude that the products must be isotopes of radium.
Still, it was a strange affair to be producing radium from uranium under the conditions of our experiments. In order to reach radium (element 88) from uranium (92) we had to assume that the parent element decayed by emitting two alpha particles. [An alpha particle is two protons and two neutrons bound together.] But we had irradiated the uranium with low-energy (thermal) neutrons, and slow neutrons had never before been observed to produce alpha-particle transmutations, nor was it possible to understand how they could do so. In this case irradiation with slow neutrons was apparently yielding more intense nuclear reactions than had ever been produced by bombardment of a substance with fast neutrons.
We continued with our experiments and were able to distinguish no fewer than four isotopes of "radium" produced artificially from uranium. We called them radium I, II, III and IV. Their half-lives were less than one minute (an estimate), 14 minutes, 86 minutes and approximately 300 hours, respectively.
Now since we were dealing with very weak preparations, we undertook to separate the radium isotopes from the carrier substance, barium, to obtain thinner layers of material, so that their radioactivity could be more easily measured. We used the method of fractional crystallization with which, 30 years earlier, I had separated the radium isotope mesothorium from barium.
Our attempts to concentrate our artificial radium isotopes by this method came to nothing. We then tried a more effective process that we had developed, using chromates of the substances instead of their chlorides or bromides. All in vain.
It next occurred to us that the "radium" might conceivably be failing to separate from the barium because it was present only in extremely small amounts—so few atoms that they could be detected only with the Geiger-Müller counter. To test this possibility, we put some natural, definitely identified radium through the same experiments. We meticulously purified this radium and diluted it to intensities as weak as those of the artificial "radium" isotopes in the aforementioned experiments. But this time the definitely known radium, in spite of its small amount, separated from barium upon crystallization as one should expect.
Finally we turned to "indicator" experiments such as I had used to check whether Fermi's product was protactinium. We mixed one of our supposed artificial radium isotopes with a known natural isotope of radium and then tried to separate the mixture from barium by fractionation as before. The experiments and their results were rather involved, and I shall not attempt to describe them here in detail. But the findings were quite clear: in each case most of the natural radium in the mixture separated out, but the artificial "radium" (e.g., Ra III or Ra IV) stayed with the barium in toto, according to the radioactivity measurements. In short, our artificial "radium" could not be separated from barium for the simple reason that it was barium!
In January, 1939, we published an account of these "experiments that are at variance with all previous experiences in nuclear physics." In interpreting the experiments we expressed ourselves very cautiously, partly because the series of tests had not yet been quite finished—they took several weeks. But our caution was not due to any mistrust of our results. Indeed, we already had a strong check of our conclusion, for we had identified a decay product of one of our "radium" isotopes as lanthanum, which meant that the parent had to be not radium but barium.
Our overcautiousness stemmed primarily from the fact that as chemists we hesitated to announce a revolutionary discovery in physics. Nevertheless we did speak of the "bursting" of uranium, as we called the surprising process that had yielded barium, far down in the periodic table. In this first paper we also speculated on what the other partner of the splitting of the uranium atom might be. Not being physicists, we thought of uranium's atomic weight (238) rather than the number of its protons (92). Subtracting the atomic weight of barium (137) from that of uranium, we guessed at 101 as the atomic weight of the other fragment. This could be an isotope of technetium or of one of the medium-heavy metals.
Immediately after our paper appeared, Meitner and Otto R. Frisch came out independently with their historic publication showing how Niels Bohr's model of the atom could explain the cleavage of a heavy nucleus into two nuclei of medium size. Meitner and Frisch named the process "fission." Subtracting the nuclear charge of barium (56) from that of uranium (92), they identified the other fission product as element 36, the inert gas krypton. And indeed, Strassmann and I had actually detected this element, or rather, its decay product, strontium, among the products of the neutron irradiation of uranium. Now in quick succession we discovered other fission products, determined their chemical properties and traced their subsequent transformations. We also found that thorium, like uranium, underwent fission when bombarded by fast neutrons.
This, in broad outline, is the story of the discovery of fission. The fact that the process is accompanied by the release of enormous amounts of energy was soon ascertained by Meitner and Frisch, and, independently, by John R. Dunning, Fermi and co-workers in the U. S. and by Frédéric Joliot-Curie in France. Strassmann and I conjectured, but did not prove experimentally, that neutrons were liberated in the process. This was later proved by Joliot-Curie, H. von Halban and L. Kowarski. The liberation of neutrons of course made it possible to harness the vast amount of energy released by the fission of uranium in the form of a "chain reaction," and this possibility was pointed out as early as mid-1939 by, among other workers, S. Flügge of the Kaiser Wilhelm Institute of Chemistry.
Strassmann and I did not concern ourselves with the harnessing of the energy of fission. During the war we continued, together with two or three co-workers, to investigate the complex fission processes and to publish our findings. By the beginning of 1945 we had made up a table containing approximately 100 fission products and their transformations. But the science of fission was then a classified subject in the U. S., and our results were not published there until November, 1946.
Source: Reprinted with permission. Copyright © February 1958 by Scientific American, Inc. All rights reserved.
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The term fission was first used by the German physicists Lise Meitner and Otto Frisch in 1939 to describe the disintegration of a heavy nucleus into two lighter nuclei of approximately equal size. The conclusion that such an unusual nuclear reaction can in fact occur was the culmination of a truly dramatic episode in the history of science, and it set in motion an extremely intense and productive period of investigation.
The story of the discovery of nuclear fission actually began with the discovery of the neutron in 1932 by James Chadwick in England (see above). Shortly thereafter, Enrico Fermi and his associates in Italy undertook an extensive investigation of the nuclear reactions produced by the bombardment of various elements with this uncharged particle. In particular, these workers observed (1934) that at least four different radioactive species resulted from the bombardment of uranium with slow neutrons. These newly discovered species emitted beta particles and were thought to be isotopes of unstable "transuranium elements" of atomic numbers 93, 94, and perhaps higher. There was, of course, intense interest in examining the properties of these elements, and many radiochemists participated in the studies. The results of these investigations, however, were extremely perplexing, and confusion persisted until 1939 when Otto Hahn and Fritz Strassmann in Germany, following a clue provided by Irène Joliot-Curie and Pavle Savic in France (1938), proved definitely that the so-called transuranic elements were in fact radioisotopes of barium, lanthanum, and other elements in the middle of the periodic table.
That lighter elements could be formed by bombarding heavy nuclei with neutrons had been suggested earlier (notably by the German chemist Walter Noddack in 1934), but the idea was not given serious consideration because it entailed such a broad departure from the accepted views of nuclear physics and was unsupported by clear chemical evidence. Armed with the unequivocal results of Hahn and Strassmann, however, Meitner and Frisch invoked the recently formulated liquid-drop model of the nucleus (see above) to give a qualitative theoretical interpretation of the fission process and called attention to the large energy release that should accompany it. There was almost immediate confirmation of this reaction in dozens of laboratories throughout the world, and within a year more than 100 papers describing most of the important features of the process were published. These experiments confirmed the formation of extremely energetic heavy particles and extended the chemical identification of the products.
The chemical evidence that was so vital in leading Hahn and Strassmann to the discovery of nuclear fission was obtained by the application of carrier and tracer techniques. Since invisible amounts of the radioactive species were formed, their chemical identity had to be deduced from the manner in which they followed known carrier elements, present in macroscopic quantity, through various chemical operations. Known radioactive species were also added as tracers and their behaviour was compared with that of the unknown species to aid in the identification of the latter. Over the years, these radiochemical techniques have been used to isolate and identify some 34 elements from zinc (atomic number 30) to gadolinium (atomic number 64) that are formed as fission products. The wide range of radioactivities produced in fission makes this reaction a rich source of tracers for chemical, biologic, and industrial use.
Although the early experiments involved the fission of ordinary uranium with slow neutrons, it was rapidly established that the rare isotope uranium-235 was responsible for this phenomenon. The more abundant isotope uranium-238 could be made to undergo fission only by fast neutrons with energy exceeding 1 MeV. The nuclei of other heavy elements, such as thorium and protactinium, also were shown to be fissionable with fast neutrons; and other particles, such as fast protons, deuterons, and alphas, along with gamma rays, proved to be effective in inducing the reaction.
In 1939, Frédéric Joliot-Curie, Hans von Halban, and Lew Kowarski found that several neutrons were emitted in the fission of uranium-235, and this discovery led to the possibility of a self-sustaining chain reaction. Fermi and his coworkers recognized the enormous potential of such a reaction if it could be controlled. On Dec. 2, 1942, they succeeded in doing so, operating the world's first nuclear reactor. Known as a "pile," this device consisted of an array of uranium and graphite blocks and was built on the campus of the University of Chicago.
The secret Manhattan Project, established not long after the United States entered World War II, developed the atomic bomb. Once the war had ended, efforts were made to develop new reactor types for large-scale power generation, giving birth to the nuclear power industry.
The fission process may be best understood through a consideration of the structure and stability of nuclear matter. Nuclei consist of nucleons (neutrons and protons), the total number of which is equal to the mass number of the nucleus. The actual mass of a nucleus is always less than the sum of the masses of the free neutrons and protons that constitute it, the difference being the mass equivalent of the energy of formation of the nucleus from its constituents. The conversion of mass to energy follows Einstein's equation, E = mc2, where E is the energy equivalent of a mass, m, and c is the velocity of light. This difference is known as the mass defect and is a measure of the total binding energy (and, hence, the stability) of the nucleus. This binding energy is released during the formation of a nucleus from its constituent nucleons and would have to be supplied to the nucleus to decompose it into its individual nucleon components.
A curve illustrating the average binding energy per nucleon as a function of the nuclear mass number is shown in Figure 1
Figure 1: The average binding energy per nucleon as a function of the mass number, A (see...
. The largest binding energy (highest stability) occurs near mass number 56--the mass region of the element iron.Figure
indicates that any nucleus heavier than mass number 56 would become a more stable system by breaking into lighter nuclei of higher binding energy with the difference in binding energy being released in the process. (It should be noted that nuclei lighter than mass number 56 can gain in stability by fusing to produce a heavier nucleus of greater mass defect--again, with the release of the energy equivalent of the mass difference. It is the fusion of the lightest nuclei that provides the energy released by the Sun and constitutes the basis of the hydrogen, or fusion, bomb. Efforts to harness fusion reaction for power production are being actively pursued. [See nuclear fusion.])
would indicate that all matter should seek its most stable configuration, becoming nuclei of mass number near 56. However, this does not happen, because barriers to such a spontaneous conversion are provided by other factors. A good qualitative understanding of the nucleus is achieved by treating it as analogous to a uniformly charged liquid drop. The strong attractive nuclear force between pairs of nucleons is of short range and acts only between the closest neighbours. Since nucleons near the surface of the drop have fewer close neighbours than those in the interior, a surface tension is developed, and the nuclear drop assumes a spherical shape in order to minimize this surface energy. (The smallest surface area enclosing a given volume is provided by a sphere.) The protons in the nucleus exert a long-range, repulsive (Coulomb) force on each other due to their positive charge. As the number of nucleons in a nucleus increases beyond about 40, the number of protons must be diluted with an excess of neutrons to maintain relative stability.
If the nucleus is excited by some stimulus and begins to oscillate (i.e., deform from its spherical shape), the surface forces will increase and tend to restore it to a sphere, where the surface tension is at a minimum. On the other hand, the Coulomb repulsion decreases as the drop deforms and the protons are positioned farther apart. These opposing tendencies set up a barrier in the potential energy of the system, as indicated in Figure 2
rises initially with elongation, since the strong, short-range nuclear force that gives rise to the surface tension increases. The Coulomb repulsion between protons decreases faster with elongation than the surface tension increases, and the two are in balance at point B, which represents the height of the barrier to fission. (This point is called the "saddle point" because, in a three-dimensional view of the potential energy surface, the shape of the pass over the barrier resembles a saddle.) Beyond point B, the Coulomb repulsion between the protons drives the nucleus into further elongation until at some point, S (the scission point), the nucleus breaks in two. Qualitatively, at least, the fission process is thus seen to be a consequence of the Coulomb repulsion between protons. Further discussion of the potential energy in fission is provided below.
The height and shape of the fission barrier are dependent on the particular nucleus being considered. Fission can be induced by exciting the nucleus to an energy equal to or greater than that of the barrier. This can be done by gamma-ray excitation (photofission) or through excitation of the nucleus by the capture of a neutron, proton, or other particle (particle-induced fission). The binding energy of a particular nucleon to a nucleus will depend on--in addition to the factors considered above--the odd-even character of the nucleus. Thus, if a neutron is added to a nucleus having an odd number of neutrons, an even number of neutrons will result, and the binding energy will be greater than for the addition of a neutron that makes the total number of neutrons odd. This "pairing energy" accounts in part for the difference in behaviour of nuclides in which fission can be induced with slow (low-energy) neutrons and those that require fast (higher-energy) neutrons. Although the heavy elements are unstable with respect to fission, the reaction takes place to an appreciable extent only if sufficient energy of activation is available to surmount the fission barrier. Most nuclei that are fissionable with slow neutrons contain an odd number of neutrons (e.g., uranium-233, uranium-235, or plutonium-239), whereas most of those requiring fast neutrons (e.g., thorium-232 or uranium-238) have an even number. The addition of a neutron in the former case liberates sufficient binding energy to induce fission. In the latter case, the binding energy is less and may be insufficient to surmount the barrier and induce fission. Additional energy must then be supplied in the form of the kinetic energy of the incident neutron. (In the case of thorium-232 or uranium-238, a neutron having about 1 MeV of kinetic energy is required.)
The laws of quantum mechanics deal with the probability of a system such as a nucleus or atom being in any of its possible states or configurations at any given time. A fissionable system (uranium-238, for example) in its ground state (i.e., at its lowest excitation energy and with an elongation small enough that it is confined inside the fission barrier) has a small but finite probability of being in the energetically favoured configuration of two fission fragments. In effect, when this occurs, the system has penetrated the barrier by the process of quantum mechanical tunneling. This process is called spontaneous fission because it does not involve any outside influences. In the case of uranium-238, the process has a very low probability, requiring more than 1015 years for half of the material to be transformed (its so-called half-life) by this reaction. On the other hand, the probability for spontaneous fission increases dramatically for the heaviest nuclides known and becomes the dominant mode of decay for some--those having half-lives of only fractions of a second. In fact, spontaneous fission becomes the limiting factor that may prevent the formation of still heavier (super-heavy) nuclei.
In nuclear fission the nucleus of an atom breaks up into two lighter nuclei. The process may take place spontaneously in some cases or may be induced by the excitation of the nucleus with a variety of particles (e.g., neutrons, protons, deuterons, or alpha particles) or with electromagnetic radiation in the form of gamma rays. In the fission process, a large quantity of energy is released, radioactive products are formed, and several neutrons are emitted. These neutrons can induce fission in a nearby nucleus of fissionable material and release more neutrons that can repeat the sequence, causing a chain reaction in which a large number of nuclei undergo fission and an enormous amount of energy is released. If controlled in a nuclear reactor, such a chain reaction can provide power for society's benefit. If uncontrolled, as in the case of the so-called atomic bomb, it can lead to an explosion of awesome destructive force.
The discovery of nuclear fission has opened a new era--the "Atomic Age." The potential of nuclear fission for good or evil and the risk/benefit ratio of its applications have not only provided the basis of many sociological, political, economic, and scientific advances but grave concerns as well. Even from a purely scientific perspective, the process of nuclear fission has given rise to many puzzles and complexities, and a complete theoretical explanation is still not at hand.