While J.J.Thomson (1856-1940) and the discovery of his cathode rays (electrons)(Sec. 3.6) were major contributors to the study of the inner atom. The age of nuclear science is generally dated from two "accidental discoveries" that occurred just before the turn of the century. In 1895 Wilhelm von Roentgen (1845-1923), a German scientist, discovered the existence of X rays; and the following year Henri Becquerel (1852-1908) (Sec. 3.8) discovered natural radioactivity.
Roentgen had been working in a dark room with a gas discharge (or cathode ray) tube covered with black paper. He noticed that when he passed an electric current through the tube, a glow appeared on a small screen coated with barium platinocyanide lying a few feet away. What caused the glow? There was no apparent contact between the electric current, the cathode rays (electrons), and the screen. Yet something was able to pass from the current, through the glass walls of the discharge tube, through the black paper, and across several feet of air to create a fluorescence on the screen. Roentgen concluded that it must be some kind of invisible ray. Since the existence of such a ray was unknown to science, Roentgen called it an X ray after the algebraic symbol for an unknown quantity. From numerous experiments with such discharge tubes, Roentgen observed that the glass wall of the tubes fluoresced with a yellow-green glow where the cathode rays (electrons) struck the glass. He finally concluded that X rays are produced when cathode rays (electrons) strike the glass wall. These X rays, in turn, cause certain chemical substances to fluoresce. Today scientists know that Roentgen's invisible X rays, like visible light, are electromagnetic radiation. Though electromagnetic radiation includes rays and waves of many different wavelengths, from short to long, our eyes can detect only the wavelengths of visible light rays. Because X rays have short wavelengths, they cannot be seen.
The wavelengths of X rays are not only shorter than those of visible light rays but they also have a great deal more energy. X rays are usually produced when electrons in the inner shells of atoms are dislodged. In Roentgen's experiments, X rays were produced by the cathode rays (electrons) striking the walls of the discharge tube. Later on, the cathode rays were focused on a metal target placed inside the tube. This arrangement produced a more intense source of X rays. Today it is known that when X rays strike uranium salts and certain other compounds, such as zinc sulfide or barium platinocyanide, visible light is emitted. The screen Roentgen used in his experiments was covered with the latter compound; it fluoresced when struck by X rays. Since the glass of the tube contained small amounts of uranium salts, it too glowed.
Henri Becquerel, the French physicist, discovered that ores and compounds containing uranium had a strange effect on photographic film. Becquerel, like his physicist father, was interested in studying fluorescence. In 1895, Becquerel was studying the fluorescence of uranium compounds. In one of his experiments, he placed crystals of potassium uranyl sulfate, K2SO4(UO2)SO4, on photographic film that was wrapped in dark paper. The crystals on the wrapped film were exposed to sunlight. The film was darkened directly beneath the crystal sample. Becquerel interpreted this to be a result of the penetration of the dark paper by the fluorescence of the uranium compound. In a control experiment, sunlight itself caused no darkening of the film, since it could not penetrate the dark paper wrapping. This proved that there was a difference between the energy of the sunlight and the energy of the radiation emitted by the uranium compound. It was concluded that since the radiation emitted by the uranium did penetrate the paper wrapping, it had greater energy than the sunlight.
Becquerel was convinced that the darkening of the film was due to fluorescence. During February 1896, he made ready another, similar experiment. For several days, the cloudy winter weather in northern France prevented the sun from shining through, so the experiment could not be completed. On March 1, Becquerel decided to use new sets of uranium compounds and photographic film. Fortunately, he developed the earlier films, which had been kept with the crystal samples in a drawer. Becquerel expected at best a faint darkening under the crystals, since exposure to room light did produce some fluorescence. To his amazement, the spots were as dark as if sunlight had been striking the uranium. Becquerel correctly interpreted this to mean that the uranium was spontaneously emitting rays without the stimulation of external light. It was that emission that had caused the observed darkening of the photographic plate.
He repeated the experiment with other substances containing uranium, such as pitchblende, a mineral ore containing oxides of uranium. Pitchblende affected a photographic plate even more than other uranium-containing substances did. Becquerel suspected some unknown element in the ore to be the cause. He asked Madame Marie Sklodowska Curie (1867-1934), a science teacher and research assistant in Paris, to undertake the isolation of this unknown element. With her husband, Pierre Currie (1859-1906), Madame Curie began the search for the unknown element-an element that, like uranium, exhibited the phenomenon of spontaneous emission of high-energy radiation. She named that phenomenon RADIOACTIVITY.
5.2 POLONIUM AND RADIUM
The Curies hypothesized that the greater radioactivity of some uranium minerals indicated the existence of a substance in these minerals that is more radioactive than uranium. They began an intense chemical separation and analysis of these minerals. The story of the Curies' search is one of the most interesting and inspiring in all the history of science. Perseverance, dedication, and intelligence finally brought success. In 1898, they isolated a new radioactive chemical element. In honor of Poland, Madame Curie's native land, the new element was named polonium. Later that same year, the Curies isolated from tons of pitchblende a minute amount of another new element. This element, which they named radium, was over 300,000 times more radioactive than uranium.
Pitchblende ore, which contains the elements uranium, polonium and radium is mined in Canada, Colorado, and Germany. If possible, obtain a piece of pitchblende, or other radioactive source, and place it near a charged electroscope. It is not necessary for the radioactive source to touch the electroscope; just hold the ore near, and watch the leaves. The leaves will become discharged, and fall. Why? Perhaps the pitchblende emits some sort of ray. Substances which have the same properties as uranium, polonium and radium are called radioactive substances, and the whole phenomenon is called radioactivity.
5.3 NATURE OF RADIATION
Three kinds of radiation can originate from naturally radioactive materials. Two of these radiations are composed of particles, and the third is composed of rays, or beams, of quanta. The particles have been designated as alpha and beta particles, and the rays as gamma rays. Ernest Rutherford (1871-1937), a New Zealand-born physicist, (Sec. 3.9 & 3.11) demonstrated that alpha particles are positively charged particles with a mass greater than that of hydrogen. Since it was found that helium gas is present in mineral deposits of radioactive substances,Rutherford believed alpha rays are composed of doubly charged helium ions, He+2.
In 1909, two of Rutherford's students, Johannes Hans Wilhelm Geiger (1882-1945) and Ernest Marsden, carefully investigated the passage of a beam of alpha particles through very thin gold foil. The vast majority of the alpha particles passed straight through the foil, as expected. However, a small number were scattered through large angles from the beam direction. About one in every 8000 alpha particles was deflected by 90o.
When Rutherford was told of this, he was astonished. His reaction to the news was, "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Since most of the alpha particles passed straight through the gold foil, there had to be a very large amount of empty space in the atoms of gold. If the atoms were solid little particles, as John Dalton (1766-1844) had suggested, all the alpha particles would have been deflected. Rutherford then suggested that since only a few alpha particles were deflected, only a very tiny part of the gold atom was involved. That tiny piece of the atom had to be very dense and charged in order to account for the deflection of positively charged alpha particles. The very light and singly charged electrons could not cause such large deflections.
Within two years of the alpha-particle-scattering experiments, Rutherford postulated the explanation: The atom must have a very small central core in which all the positive charge and most of the mass of the atom is concentrated. This small, positively charged, heavy center later became known as the atomic nucleus. Rutherford published this theory in the Journal of the Literary and Philosophical Society of Manchester, where, a little more than a century before, Dalton had announced his concept of the atom.
Rutherford's assumption that alpha particles are helium nuclei and consists of two protons and two neutrons, was proved correct. When emitted from the nucleus of an atom, alpha particles interact with air molecules and travel only a few inches. They have the greatest ability to cause ionization (by knocking off electrons) of the atoms through which they pass.
Becquerel proved that beta rays consist of negative particles identical in charge and in mass to the electron. Beta particles are extremely energetic electrons (cathode rays) originating in the nucleus of the atom. They have a negative charge and a very small mass. Positive particles, called positrons, also exist. They have the same mass as an electron but have a positive charge. The velocity of beta particles varies. They travel faster and farther than alpha particles, interact less with air molecules, and cause much less ionization.
Gamma radiation has no electrical charge. It is electromagnetic radiation with a wavelength shorter than that of X rays. Some consider gamma rays as very high energy X rays and by far the most penetrating, but causing the least ionization. As with all electromagnetic radiation, gamma radiation has no measurable mass and travels at the speed of light.
5.4 RADIOACTIVE DISINTEGRATION
In 1900 William Crookes (1832-1919), an English scientist, (Sec. 3.6) was studying the radioactive properties of the salt, uranium nitrate. He found that when ammonium carbonate was added to an aqueous solution of the salt, a precipitate formed. When an excess of ammonium carbonate was then added to the precipitate, most of the precipitate redissolved but a small quantity remained. First, Crookes separated the small quantity of precipitate by filtration and evaporated the liquid filtrate. What remained after evaporation was a large amount of solid residue that contained most of the uranium salt. He then compared the effect on a photographic film of this large amount of residue and the small amount of precipitate. The results were startling. The residue from the filtrate, containing essentially all of the uranium, exposed the film very slightly while the precipitate exposed it intensely. From this Crookes assumed that radioactivity was not a property of uranium itself but of some impurity associated with it. He named this impurity uranium X. The conclusion was natural enough, but it was incorrect, as you will see.
A year later Becquerel performed a similar experiment. But he allowed the substances which had separated from each other - the active precipitate and the less active residue - to stand for a period of 18 months. At the end of that time he found that the uranium salt residue had completely regained its radioactivity while the small amount of precipitate was no longer radioactive. The mysterious substance which made up the precipitate - a substance which was so very active for a time and then appeared to lose its radioactivity altogether - was Crookes' uranium X.
In 1902 Ernst Rutherford and Frederick Soddy (1877-1956), another English scientist, used the same method of precipitation and filtration to separate an extremely active precipitate from the element thorium. This substance, which behaved much like uranium X, they named thorium X. They noticed that as the thorium X eventually began to lose its radioactivity, the thorium started to regain radioactivity a exactly the same rate.
This experiment and others eventually led Rutherford and Soddy to propose the theory of radioactive disintegration. According to this theory, atoms of radioactive elements are unstable and undergo spontaneous disintegration by emitting radiation. This disintegration produces atoms of new elements which may themelves be radioactive and hence undergo further disintegration.
5.5 RADIOACTIVE DISINTEGRATION SERIES
All elements with atomic numbers larger than that of bismuth have one or more isotopes which are radioactive. A few elements of low atomic number, such as potassium and rubidium, have naturally occurring isotopes which are radioactive.
The naturally occurring radioisotopes of heavier elements belong to chains of successive disintegration's or "decays," and all the species in one chain constitute a radioactive family or series. Three of these series include most of the natural radioactive elements of the Periodic Table. These are the uranium series, the actinium series, and the thorium series. Each series is characterized by a parent (first member) of long half-life and a series of decay processes which ultimately lead to a stable end product. With the three natural series, the end products are isotopes of lead: Pb206 in the uranium series, Pb207 in the actinium series, and Pb208 in the thorium series.
Successive transformations in the natural disintegration series take place in a manner which is described by the displacement laws originally formulated by Rutherford, Soddy, and Fajans.
A fourth radioactive series was discovered during World War II. This series is called the neptunium series after its member of longest half-life, and it was discovered through the production of its members by artificial means. The end product of the series is an isotope of bismuth, Bi209. Both the parent and end product of this chain have been detected in uranium ores in recent years.
The four radioactive series are referred to as:
1. 4n thorium series
2. 4n + 1 neptunium series
3. 4n + 2 uranium series
4. 4n + 3 actinium series
5.6 DETECTING RADIATION
The radiation from radioactive materials will ionize atoms by knocking off electrons. This property is useful in identifying radiation. A radiation detecting device can be made by taking a tube containing a gas, enclosing it in a sheet of metal foil, and running a wire through the center of the tube. To the tube a voltage source and a light are connected in series. The light will not shine, since there is an open circuit across the tube. If the gas in the tube is ionized, however, the ionized gas will conduct a current, and the light will shine.
When a particle from a radioactive atom passes through the tube, it ionizes the gas, and the light shines. The detector can be connected to a number of different instruments other than a light, such as a buzzer or a meter. Using appropriate electronic attachments, a similar detector can be made which counts both the number and kinds of particles entering the tube. The detecting devices used today are much more complex than the one just described. They usually have attachments which in addition to counting the number of particles passing through, can also determine the energy of the particles.
5.7 ENERGY AND NUCLEAR DECAY
The rays produced by radioactive materials are actually a mixture of several particles and quanta of energy. The particles and quanta are released from the nuclei of radioactive atoms in the process of spontaneous nuclear decay. We say the decay is spontaneous, because we have no control over it.
The release of energy accompanying a series of nuclear changes is very large. In fact, the amount of energy released is so great that it cannot be accounted for by the ordinary chemical properties of atoms. Albert Einstein (1879-1955) was the first to advance a theory explaining the origin of this energy. One consequence of Einstein's theory is the well known equivalence of mass and energy, which he expressed in the equation:
E is the energy (in ergs) released. m is the mass (in grams) of matter involved. c is a constant, the speed of light (in centimeters per second).
The predicted energy-mass change is so small in normal chemical reactions that it is not measurable. However, we are all familiar with the nuclear weapons which involve a mass-energy interconversion. The immense forces released by fission bombs are the result of the splitting of the nuclei of uranium or plutonium atoms.
The nucleus of the uranium-238 atom contains 92 positive protons and 146 uncharged neutrons. These particles are bound tightly in the nucleus, even though the protons all have a positive charge and repel each other, and the neutrons have no charge. Acting alone, the electrostatic attraction involved in bonding and van der Waals forces should allow the uncharged neutrons to float away. Since protons all have the same charge, electrostatic forces should cause them to fly apart. Gravitational force, which keeps us at the surface of the earth and keeps the moon in orbit, is not strong enough to hold the nucleus together, particularly a large uranium nucleus.
What force, then, holds the nucleus together? The present hypothesis of nuclear structure is that both protons and neutrons are made of simpler particles, one of which is the meson. There are several kinds of mesons. It has been proposed that the mesons hold the nucleus together through "trading" or "sharing" among neutrons and protons. The meson hypothesis seems to fit the facts, but it has not yet been conclusively demonstrated. Nuclear structure is still a field of intense investigation, and will continue to be so for some time.
5.8 RADIOACTIVE ISOTOPES
In 1913, Theodore William Richards (1868-1928), of Harvard University, found two different atomic weights for lead obtained from two different sources. In the same year, FrancisWilliam Aston (1877-1945) separated neon atoms into two different atomic weight species. These were additional startling discoveries that led to the drastic alteration of the Daltonian concept of atomic structure.
The results were interpreted by Frederick Soddy in Great Britain (Sec. 3.10). He gave the name isotope to atoms or groups of atoms of the same element which have the same atomic number, but different atomic weights (different number of neutrons). They must have the same number of protons, or they are not atoms of the same element. Discovery and separation of isotopes of many other elements soon followed. Most of the first 83 elements have more than one stable isotopic species. The simplest example of an isotope is that of hydrogen, since its most common form, protium, is the simplest of atoms because it consists of one electron and one proton. However, another kind of hydrogen atom occurs which contains, in addition to the electron and proton, one neutron in its nucleus. This isotope of hydrogen is called deuterium. A third kind of hydrogen, called tritium, contains two neutrons in the nucleus and is radioactive. Tritium is one of the most expensive materials on earth.
There are many radioactive isotopes which occur naturally and many others which are artificially produced. Each radioactive isotope produces a characteristic type of radiation, either alpha or beta particles, usually in conjunction with some gamma radiation. Every radioactive isotope which emits alpha particles does so with a definite amount of energy, different from any other isotope. Beta emitters also give off particles with varying amounts of energy, but always below some maximum value which is characteristic of the particular atom. If the radiation is captured and measured, the isotope can be traced or identified. Radioactive isotopes are used in the laboratory and in industry to trace the path followed by various reactions and products. Studies using these radioactive tracers have led to important discoveries in science, and better products from industry.
It has been determined experimentally for large aggregates of radioactive atoms that the number of atoms which disintegrate in a unit of time is proportional to the number of atoms present. The percentage of atoms which disintegrate in any given period of time is constant for a given isotope. In order to compare the activity of various radioactive species, the length of time it takes for one-half of the atoms to disintegrate has been chosen as a standard, and is called the half-life.
|Astatine-218 - 2.0 sec.||Lead-210 - 19.4 yrs.||Strontium-90 - 28 yrs.|
|Barium-131 - 12.0 days||Lead-214 - 26.8 min.||Sulfur-35 - 87.1 days|
|Bismuth-210 - 5.0 days||Phosphorus-32 - 14.3 days||Thallium-206 - 4.20 days|
|Bismuth-212 - 60.5 min.||Polonium-210 - 138.4 days||Thallium-210 - 1.32 min.|
|Bismuth-214 - 19.7 min.||Polonium-214 - 1.64 x 10-4 days||Thorium-230 - 8.0 x 104 yrs.|
|Calcium-45 - 152.0 days||Polonium-215 - 0.0018 sec.||Thorium-234 - 24.1 days|
|Carbon-14 - 5,760 yrs.||Polonium-216 - 0.16 sec.||Uranium-234 - 2.48 x 106 yrs.|
|Chlorine-36 - 4 x 105 yrs.||Polonium-218 - 3.05 min.||Uranium-235 - 7.1 x 108 yrs.|
|Cobalt-60 - 5.26 yrs.||Protactinium-234 - 1.18 min.||Uranium-238 - 4.51 x 109 yrs.|
|Gold-198 - 2.7 days||Radium-226 - 1,620 yrs.||Uranium-239 - 23.5 min.|
|Iodine-131 - 8.14 days||Radon-222 - 3.82 days|
|Iron-59 - 46.3 days||Sodium-24 - 15.0 hrs.|
Knowledge of the half-life of an isotope is useful in almost all calculations involving tracer isotopes. It also leads to an interesting use for naturally occurring isotopes: dating of ancient objects. In the upper atmosphere, radioactive CO2 (C14) is formed as a result of bombardment of the upper atmosphere by cosmic rays. Through mixing by winds, the distribution of this form of carbon dioxide in the atmosphere remains virtually uniform. Since carbon dioxide is constantly being removed from the air by plants and created by cosmic rays, we can assume that the percentage of carbon dioxide in the air has been approximately the same for several hundred million years. All plants have a constant concentration of C14 in their composition, because they use the radioactive C-14 in photosynthesis. When the plant is alive, C14 is continually disintegrating, but it is continually being replaced by photosynthesis. However, when the plant dies, no more CO2 containing C14 is replaced by photosynthesis. Now, only disintegration is occurring and the C14 concentration in the plant begins to decrease. By measuring the C14 level in the plant, it is possible to tell how long the plant has been dead. If an archaeologist unearths logs in an excavation of an ancient city, radiocarbon dating of the timbers will indicate approximately when the trees were cut down.
By employing various isotopes in a somewhat similar fashion, geologists have been able to date the formation of many ancient rocks and even to roughly date the formation of the earth. In uranium-bearing rocks, the radioactive uranium decomposes through a series of steps. The final product of this decay is lead. In the process, eight alpha particles are given off for each atom of uranium converted to lead. By measuring entrapped He to U and Pb to U ratios, it is possible to date the rocks. The half-life of uranium must, of course, be known for the calculation.
|Calcium-45||Studying plant nutrition.|
|Carbon-14||Treating brain tumors, measuring age of ancient objects.|
|Cobalt-60||Treating cancer, irradiating food, inducing mutations.|
|Iodine-131||Studying and treating the thyroid gland, finding leaks in water pipes.|
|Iron-59||Studying the blood.|
|Phosphorus-32||Studying plants' use of fertilizer.|
|Sodium-24||Diagnosing circulatory disease.|
|Strontium-90||Treating small lesions.|
|Sulfur-35||Studying the body's use of certain amino acids.|
5.10 STABILITY OF RADIOACTIVE ISOTOPES
Not all isotopes of an element are equally stable. It is possible to estimate which isotopes will be most stable by applying the three rules which follow.
If a very heavy nucleus becomes too unstable, it breaks into two approximately equal parts, with the release of a large amount of energy, by a process called FISSION. A nucleus can be made unstable by bombardment with a number of particles, including neutrons. The heaviest elements are the only ones which exhibit the phenomenon of fission. When a heavy nucleus breaks up, usually one or more neutrons are emitted, in addition to the production of the two heavy fragments. It is possible for the neutrons emitted to produce a chain reaction. One atom splits, due to bombardment by a neutron. In the process of splitting, the atom gives off a neutron, which in turn can cause a second atom to split. This continues until a very large number of the atoms present have reacted. Since a large energy change is involved in the fission process, a chain reaction can serve as an energy source for various purposes. A nuclear reactor is a device for controlling nuclear fission.
If a nuclear reaction takes place almost instantaneously, as in an atomic bomb, a tremendous amount of energy is released in a very short period of time. The result is an explosion. However, fission explosions are small when compared with the fusion explosions of hydrogen bombs.
In this process, which can take place only in certain very heavy nuclei such as 92U235, the absorption of an incoming neutron causes the target nucleus to split into two smaller nuclei called fission fragments. Because stable light nuclei have proportionately fewer neutrons than do heavy nuclei the fragments are unbalanced when they are formed and at once release one or two neutrons each. Usually the fragments are still somewhat unstable, and may undergo beta decays (accompanied by gamma decays) to achieve appropriate neutron:proton ratios. The products of fission, such as the fallout from a nuclear bomb burst, are accordingly highly radioactive. Although a variety of nuclear species may appear as fission fragments, we might cite as a typical reaction
92U235 + 0n1 92U236 54Xe140 + 38Sr94 + 0n1 + 0n1 + v + 200 Mev.
The 92U236 which is first formed is a compound nucleus, and it is this nucleus that splits in two. The fission fragments 54Xe140 and 38Sr94 are both beta radioactive; the former decays four successive times until it becomes the stable isotope 58Ce140, and the latter decays twice in becoming the stable isotope 40Zr94. About 84% of the total energy liberated during fission appears as kinetic energy of the fission fragments, about 2.5% as kinetic energy of the neutrons, and about 2.5% in the form of instantaneously emitted gamma rays, with the remaining 11% being given off in the decay of the fission fragments.
The liquid drop model of the nucleus provides a plausible mechanism for the fission process. We are all familiar with the characteristic oscillations of a stretched string, and of a taut membrane. Less familiar perhaps are the characteristic oscillations of a liquid drop. If we accept the picture of a nucleus as a liquid drop, we can suppose that the absorption of a neutron by a heavy nucleus is enough to set it vibrating. The difference between an ordinary liquid drop and a nucleus is that, when the latter is distorted from a spherical shape, the short-range nuclear forces holding it together lose much of their effectiveness owing to the larger nuclear surface where nucleons have fewer bonds. Though the distortion may seriously weaken the attractive forces in the nucleus, the repulsive electrostatic forces still predominate, the excitation energy of the nucleus eventually is lost through gamma decay: a neutron is captured and a gamma ray is emitted. If the repulsive forces predominate, however, the distortion grows larger and larger until the nucleus splits in two, which is observed as fission.
Because each fission event liberates two or three neutrons while only one fission is required to initiate it, a rapidly multiplying sequence of fissions can occur in a lump of suitable material. When uncontrolled, such a chain reaction evolves an immense amount of energy in a short time. If we assume that two neutrons emitted in each fission are able to induce further fissions (the average figure is lower in practice) and that 10-8 sec elapses between the emission of a neutron and its subsequent absorption, a chain reaction starting with a single fission will release 2 x 1013 joules of energy in less than 10-6 sec! An uncontrolled chain reaction evidently can cause an explosion of exceptional magnitude. When properly controlled so as to assure that exactly one neutron per fission causes another fission, a chain reaction occurs at a constant level of power output. A reaction of this kind makes a very efficient source of power: an output of about 1000 kilowatts is produced by the fission of 1 gram of a suitable isotope per day, as compared with the consumption of over 3 tons of coal per day per 1000 kilowatts in a conventional power plant.
FUSION is another kind of nuclear reaction, and is the reverse of the fission process. The energy of the sun comes from a continual fusion reaction. A great deal of energy is released when several small nuclei are fused into one larger nucleus. In the sun, the four hydrogen atoms fuse into one helium atom (Each second the sun produces 400 million tons of Helium). This is the same type of reaction employed in the hydrogen bomb, in which various isotopes of hydrogen react to produce helium. If such a reaction takes place among a very large number of hydrogen atoms in a very short time, the energy released is tremendous. For such a reaction to take place, it is necessary for the reactants to reach a very high temperature and pressure. These conditions are usually created by a fission reaction. A hydrogen bomb may use a fission bomb as a trigger.
Virtually all of the energy in the universe originates in the fusion of hydrogen nuclei into helium nuclei in stellar interiors, where hydrogen is the most abundant element. Two different reaction sequences are possible, with the likelihood of each depending upon the properties of the star involved. The proton-proton cycle is the predominant energy source of stars whose interiors are cooler than that of the sun, perhaps 2 x 106 oK. The proton-proton cycle proceeds by means of the following reactions:
1H1 + 1H1 1H2 + e+ + 0.4 Mev,
1H1 = 1H2 2He3 + 5.5 Mev,
2He3 + 2He3 2He4 + 2 1H1 + 12.9 Mev.
The first two of these reactions must each occur twice for every synthesis of 2He4, so that the total energy produced per cycle is 24.7 Mev.
Stars hotter than the sun obtain their energy from the carbon cycle. This cycle requires a 6C12 nucleus for its first step and in its last step regenerates a 6C12 nucleus, so that this isotope may be thought of as a catalyst for the process. The carbon cycle proceeds as follows:
1H1 + 6C12 7N13 + 2.0 Mev,
7N13 6C13 + e+ + 1.2 Mev,
6C13 + 1H1 7N14 + 7.6 Mev,
7N14 + 1H1 8O15 + 7.3 Mev,
8O15 7N15 + e+ + 1.7 Mev,
7N15 + 1H1 6C12 + 2He4 + 4.9 Mev.
Here again the net result is the formation of an alpha particle and two positrons from four protons with a total of 24.7 Mev of energy evolved. In the sun both the proton-proton and carbon cycles take place with comparable probabilities.
The energy liberated by nuclear fusion is often called thermonuclear energy. High temperatures and densities are necessary for fusion reactions to occur in such quantity that a substantial amount of thermonuclear energy is produced: the high temperature assures that the initial light nuclei have enough thermal energy to overcome their mutual electrostatic repulsion and come close enough together to react, while the high density assures that such collisions are frequent. A further condition for the proton-proton and carbon cycles is a large reacting mass, such as that of a star, since a number of separate steps is involved in each cycle and much time may elapse between the initial fusion of a particular proton and its ultimate incorporation in an alpha particle. On earth, where any reacting mass must be very limited in size, an efficient thermonuclear process cannot involve more than a single step. The reactions that appear most promising as sources of commercial power involve the combination of two deuterons to form a triton and a proton,
1H2 + 1H2 1H3 + 1H1 + 4.0 Mev,
or their combinations to form a 2He3 nucleus and a neutron,
1H2 + 1H2 2He3 + 0n1 + 3.3 Mev.
Both reactions have about equal probabilities. A major advantage of these reactions is that deuterium (the variety of hydrogen whose atomic nuclei are deuterons, sometimes called "heavy hydrogen") is relatively abundant on the earth, so that there should be no fuel problems in power plants operating on deuteron fusion. Wile there are many difficulties to surmount in achieving practical thermonuclear power, it will almost certainly become an eventual reality.
Nuclear reactions can result in the change of one element into another, because these reactions can change the number of protons in the nucleus. This is called TRANSMUTATION. In natural uranium, the most common isotope is U238, which decays by emitting an alpha particle. The resulting atom contains two less protons and two less neutrons than uranium, and has an atomic mass of 234. The new atom is an atom of thorium, Th234. By a natural process, uranium will transmute to thorium. Th234 decays by emitting a beta particle. Since the mass of an electron is negligible, the new atom has the same mass number, 234, but has one more positive charge than before, or 91. The new atom is protactinium, Pa234. The uranium series of disintegration's, ends with lead, Pb206, which is a stable isotope.
The earliest artificial transmutation was performed by Lord Rutherford in 1911 by bombarding nitrogen-14 with alpha particles from radioactive bismuth-214, to produce oxygen-17 and protons as products. To enter and be captured by nitrogen nuclei, the alpha particles had to be traveling at high speed with great energy. They attained this speed from the energy "created" by the "destruction" of matter when the atoms of bismuth-214 decayed.
All of the twelve transuranium elements thus far synthesized have been produced by converting a lighter element into a heavier one by increasing the number of protons in the nucleus. One of the processes of synthetic transmutation requires that a nuclear reactor produce a high concentration of neutrons which are "packed" into the nucleus of an element, such as plutonium-239. As the mass number builds up, a beta particle is emitted. When this happens, a neutron is converted into a proton with no significant loss of mass. This process produces an element with an atomic number greater than the original element.
This element in turn can be used as a target to produce another with a higher atomic number. Fermiun-256 is the element with the highest atomic number reached in this manner.
A second method of creating transuranium elements makes use of nuclear explosions which produce vast amounts of neutrons. Some neutrons are captured by uranium atoms. Successive electron emissions produce new elements in the same manner as above. Again, fermium-256 is the element with the highest atomic number produced in this manner.
Elements with atomic numbers greater than 100 have been produced by using other elements to bombard target elements. To create mendelevium, number 101, einsteinium was bombarded by alpha particles. Element 102, nobelium, was created by using carbon ions and curium. The production of lawrencium, number 103, made use of boron and californium.
Scientists have designed accelerators capable of making bombardments with even larger particles than those required in producing the preceding examples. Accelerators are being prepared which make use of nuclei as heavy as argon.
It is highly possible that elements with even greater atomic numbers can be produced. Difficulties are caused by the present low yield of transuranium elements and by their extremely short half-lives. Only a few atoms of element 103 were first prepared, for example, and these had a half-life of 8 seconds. However, most nuclear scientists believe that prospects of producing elements with atomic numbers up to 110 are reasonably good and even higher numbers are in the realm of possibility.
5.14 PARTICLE ACCELERATORS
Before the days of the nuclear reactor, scientists had produced isotopes by means of the cyclotron, a device that accelerates positively charged nuclear particles to tremendously high speed. Today the cyclotron is just one of a group of devices called particle accelerators which cause particles to move at speeds approaching that of light. When traveling at such speeds, particles have enormous energy.
It was a particle accelerator that first enabled scientists to achieve the dream of the alchemists: to turn a base metal into gold. This is done by bombarding mercury with deuterons, the nuclei of heavy hydrogen:
Hg200 + H2 Au198 + He4
Unfortunately the gold resulting from this transmutation is radioactive and has a short half life of 2.7 days. Moreover it costs far more to turn mercury into gold by this means than it would to buy the same quantity.
In 1928 George Gamow (1904-1968) suggested that other charged particles, traveling at high speeds, might be even more effective than alpha particles in causing transmutations. Gamow's prediction was realized in 1932 by John Douglas Cockcroft (1897-1967) and Ernest Thomas Sinton Walton (1903-1995), two workers in Rutherford's laboratory. They first produced protons (hydrogen nuclei) by passing high voltage electricity through hydrogen gas. Then they accelerated the speed of these protons. With the accelerated protons they bombarded a thin film of lithium oxide which contained stable atoms of lithium-7. Cloud chamber tracks showed that some of the lithium-7 atoms captured protons and disintegrated to form two atoms of helium:
Li7 + H1 He4 + He4
This was the first time that artificially accelerated particles had been used to cause disintegration.
THE TRANSURANIC ELEMENTS
Neptunium 93 E.M. McMillan and P.H. Abelson. 1940. Bombardment of uranium with deuterons.
Plutonium 94 G.T. Seaborg, E.M. McMillan, J.W. Kennedy, and A.C.Wahl. 1941. Bombardment of uranium with deuterons.
Americium 95 G.T. Seaborg, R.A. James, L.O. Morgan, and A. Ghiorso. 1944-45. Bombardment of uranium with helium ions.
Curium 96 G.T. Seaborg, R.A. James, and A. Ghiorso. 1945. Bombardment of plutonium with helium ions.
Berkelium 97 S.G. Thompson, A. Ghiorso, and G.T. Seaborg. 1949. Bombardment of americium with helium ions.
Californium 98 S.G. Thompson, K. Street, Jr., A. Ghiorso, and G.T. Seaborg. 1950. Bombardment of curium with helium ions.
Einsteinium 99 A. Ghiorso et al. 1952-53. Bombardment of uranium with nitrogen ions.
Fermium 100 A. Ghiorso et al. 1952-53. Bombardment of uranium with oxygen ions.
Mendelevium 101 A. Ghiorso, B.G. Harvey, G.R. Choppin, S.G. Thompson, and G.T. Seaborg. 1955. Bombardment of einsteinium with helium ions.
Nobelium 102 A. Ghiorso, T. Sikkeland, J.R. Walton, and G.T. Seaborg. 1958. Bombardment of curium with carbon ions.
Lawrencium 103 A. Ghiorso, T. Sikkeland, A.E. Larsh, and R.M. Latimer. 1961. Bombardment of californium with boron ions.
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