Part 2

Patrick Blackett in England and W. D. Harkins in the United States soon proved independently that, during the nuclear event reported by Rutherford in his 1919 paper, an alpha particle combines with a nitrogen nucleus and that the resulting unstable combination immediately emits a proton and ends up as one of the isotopes of oxygen. This was the first instance of deliberate transmutation of one stable chemical element into another. Since that time practically every known element has been transmuted by bombardment. The dream of the alchemists has been partially fulfilled in that mercury has been changed into gold. We say “partially fulfilled” because the process is much too expensive to be economically profitable.

Some Particles Have No Electric Charge

During the early 1920s a number of investigators, including Harkins in the United States, Orme Masson in Australia, and Rutherford and his assistant James Chadwick in England, seriously considered the possibility that a neutral particle might exist in nature, possibly formed by the very close association of a proton and an electron. However, strenuous efforts to produce such particles by combining protons and electrons were unsuccessful.

During these years the new technique of bombarding all kinds of matter with alpha particles to see what would happen was widely exploited, and it gradually became clear that in a few instances a peculiar and highly penetrating kind of radiation was produced. In 1932, Chadwick succeeded in showing that the peculiar radiation must consist of a stream of particles, each weighing about the same as a proton but having no electrical charge.

The name “neutron” for a possible neutral particle of this type was suggested by Harkins in the United States in 1921. Much evidence now exists that the neutron is a fundamental particle in its own right and that it should not be thought of merely as a particle formed by a very close association between a proton and an electron.

The new particle discovered by Chadwick was destined to play a totally unexpected role, not only in the history of atomic science but also in the fate of nations. It immediately outmoded a previous concept of the nucleus that pictured it as a cluster of protons approximately half of which were neutralized by electrons crowded into the nucleus. A nucleus is now thought of as containing just protons and neutrons.

The neutron was also greeted by nuclear workers as a practically perfect kind of bullet. Unlike charged alpha particles, uncharged neutrons can approach a charged nucleus completely unopposed. It is physically impossible for any kind of container to hold a swarm of free neutrons; they seep right through its walls.

Matter Is Energy; Energy Is Matter

So far, in the story about man’s curiosity concerning the fundamental nature and structure of matter, the development of ideas about _structure_ has been emphasized. We will now take a brief look at a development which strongly influenced our ideas about the fundamental _nature_ of matter.

In 1887 reports appeared on a famous study, often referred to as the Michelson-Morley experiment, which was aimed at determining the earth’s speed through absolute space. The entirely unexpected results of the experiment had a great impact on the concepts of space and time. We will here concern ourselves with just one outcome of the experiment.

In 1905, a young German-born physics student named Albert Einstein, who was working as a patent examiner in Switzerland, published three papers, each of which had a profound effect on a different field of physics.

One of the papers dealt with some peculiar speculations about space and time which began to interest him when he was studying the Michelson-Morley experiment. The contents of the paper are now referred to as the Special Theory of Relativity. This paper contains several predictions that seemed incredible to the average physicist of that day. These predictions have, however, long since been proved valid.

One of Einstein’s predictions had to do with the equivalence of matter and energy. Until 1905 _matter_ had been considered as something that has mass or inertia; _energy_, on the other hand, had been regarded as the ability to do work. It was believed that the two were as different from each other as, say, a square yard is different from an hour. Einstein’s theory, however, implies that matter and energy are merely two different manifestations of the same fundamental physical reality, and that each may be converted into the other according to the famous equation:

E = MC²

where E = quantity of energy, M = quantity of matter, and C = speed of light in a vacuum.

Nuclei Contain Energy

One more piece of information must be fitted into the story of the atom before it becomes clear why some people began to realize during the 1920s that atomic nuclei contain vast stores of energy that might some day revolutionize civilization. This last item has to do with a nuclear phenomenon known as the packing fraction.

Since any nucleus consists of a certain number of protons and neutrons, it seems logical that the total weight of the nucleus could be determined by adding together the individual weights of the particles in it. When mass spectrographs of sufficiently high accuracy became available, however, it was found that in the case of nuclear weights, the whole was not equal to the sum of its parts! All nuclei (except hydrogen) weigh less than the sum of the weights of the particles in them.

For example, the atomic weight of a proton is 1.00812 and that of a neutron is 1.00893. (These are relative weights based on an internationally accepted scale.) It would seem then that a nucleus of helium containing two protons and two neutrons should have an atomic weight of 2 × 1.00812 plus 2 × 1.00893 or 4.0341. Actually the atomic weight of helium as measured by the mass spectrograph is only 4.0039. (See Figure 4.)

HELIUM NUCLEUS TWO PROTONS AND TWO NEUTRONS

What happens to the missing atomic weight of 0.0302? Physicists now realize that, as postulated in Einstein’s formula, it must be converted into energy! The conversion occurs when the protons and neutrons are drawn together into a helium nucleus by the powerful nuclear forces between them.

When the missing atomic weight 0.0302 is multiplied by the square of the velocity of light according to Einstein’s theory, it is found to represent a tremendous amount of energy. Indeed, the energy released in forming a helium nucleus from two protons and two neutrons turns out to be seven million times that released when a carbon atom combines with an oxygen molecule to produce a molecule of carbon dioxide in the familiar process of combustion.

The general behavior of such losses in atomic weight for atoms throughout the periodic table had been determined as early as 1927, largely through the work of Aston, the English scientist who developed the first mass spectrograph. His results show that, in general, if two light nuclei combine to form a heavier one, the new nucleus does not weigh as much as the sum of the original ones. This behavior continues up to the level of the so-called “transition metals”—iron, nickel, and cobalt—in the periodic table. But if two nuclei heavier than iron are coalesced into a single very heavy nucleus found near the end of the periodic table (such as uranium), the new nucleus weighs more than the sum of the two nuclei that formed it.

Thus, if a very heavy nucleus could be divided into parts, energy would be released, and the sum of the weights of the fragments would be less than that of the original nucleus.

In these two types of nuclear reactions, a small amount of matter would actually vanish! Einstein’s Special Theory of Relativity states that the vanished matter would reappear as an enormous quantity of energy.

During the late 1920s scientists began saying that a small amount of matter could supply enough energy to drive a large ship across the ocean. As we know, this prediction has since been borne out by the performance of nuclear submarines and surface vessels.

CHRONOLOGY

1800 Dalton firmly establishes atomic theory of matter. 1890-1900 Thomson’s experiments with cathode rays prove the existence of electrons. Atoms are found to contain negative electrons and positive electric charge. Becquerel discovers unstable (radioactive) atoms. 1905 Einstein postulates the equivalence of mass and energy. 1911 Rutherford recognizes nucleus. 1919 Rutherford achieves transmutation of one stable chemical element (nitrogen) into another (oxygen). 1920-1925 Improved mass spectrographs show that changes in mass per nuclear particle accompanying transmutation account for energy released by nucleus. 1932 Chadwick identifies neutrons. 1939 Discovery of uranium fission by German scientists. 1940 Discovery of neptunium by Edwin M. McMillan and Philip H. Abelson and of plutonium by Glenn T. Seaborg and associates at the University of California. 1942 Achievement of first self-sustaining nuclear reaction, University of Chicago. 1945 First successful test of an atomic device, near Alamagordo, New Mexico, followed by the dropping of atomic bombs on Hiroshima and Nagasaki, Japan. 1946 U. S. Atomic Energy Commission established by Act of Congress. First shipment of radioisotopes from Oak Ridge goes to hospital in St. Louis, Missouri. 1951 First significant amount of electricity (100 kilowatts) produced from atomic energy at testing station in Idaho. 1952 First detonation of a thermonuclear bomb, Eniwetok Atoll, Pacific Ocean. 1953 President Eisenhower announces U. S. Atoms-for-Peace program and proposes establishment of an international atomic energy agency. 1954 First nuclear-powered submarine, _Nautilus_, commissioned. 1955 First United Nations International Conference on Peaceful Uses of Atomic Energy held in Geneva, Switzerland. 1957 First commercial use of power from a civilian reactor takes place in California. Shippingport Atomic Power Plant in Pennsylvania reaches full power of 60,000 kilowatts. International Atomic Energy Agency formally established. 1959 First nuclear-powered merchant ship, the _Savannah_, launched at Camden, New Jersey. Commissioning of first nuclear-powered Polaris missile-launching submarine _George Washington_. 1961 A radioisotope-powered electric power generator placed in orbit, the first use of nuclear power in space. 1962 Nuclear power plant in the Antarctic becomes operational. 1963 President Kennedy ratified the Limited Test Ban Treaty for the United States on October 7. 1964 President Johnson signed law permitting private ownership of certain nuclear materials.

Fission is Explained

Physicists welcomed the neutron as a bullet that could strike any nucleus, unopposed by electric repulsion. During the middle 1930s, a number of investigators, chief among them the Italian physicist Enrico Fermi, exposed many different isotopes of the chemical elements to beams of neutrons to see what would happen.

What usually happened was that the bombarded nuclei would absorb neutrons, emit alpha, beta, or gamma rays, and change into different isotopes. The identification of the extremely small quantities of isotopes produced required the development of a fantastic new branch of chemistry known as radiochemistry, or, as one chemist put it, “phantom chemistry.”

In some cases the absorption of a neutron by a nucleus was followed by the emission of a negative electron (beta particle). This produced an atom whose nuclear positive charge had been increased by one unit and which therefore belonged at the next higher place on the periodic table. Fermi and others then considered the fascinating possibility of doing the same thing to uranium, the last-known element on the periodic table, to create previously unknown chemical elements. The results of bombarding uranium with neutrons turned out to be extremely complex, but it eventually became clear that “transuranic” elements (those heavier than uranium) could actually be made in this way.[2]

Some of the complex results of bombarding uranium with neutrons formed an intriguing puzzle that kept various investigators busy for several years. In 1939 the German chemists Otto Hahn and Fritz Strassmann and the physicists Lise Meitner and Otto Frisch were able to announce a solution. The absorption of a neutron by a certain uranium nucleus (later shown to be that of the relatively rare isotope uranium-235) can result in a splitting, or _fission_, of the nucleus into two parts with separate weights that place them somewhere near the middle of the periodic table.

The announcement of this discovery created quite a stir among physicists because a nuclear process of this nature must release a very large amount of energy.

The excitement among physicists became even greater when it was realized that this newly discovered process of fission was accompanied by the release of several free neutrons from the splitting nucleus. Each new neutron could, if properly slowed down by a moderating material, cause another nucleus to split and release more energy and still more neutrons, and so on, as illustrated in Figure 5. (A moderator is necessary because fast, newly released neutrons are too readily absorbed by uranium-238 nuclei, which rarely split.) Apparently all that was needed to achieve this spectacular kind of a chain reaction was to assemble enough uranium in one place so that the released neutrons would have a good chance of finding another ²³⁵U nucleus before escaping from the pile. The amount of fissionable material required to sustain a chain reaction is termed the “critical mass.” A team of scientists led by Fermi achieved the first self-sustaining nuclear reaction on December 2, 1942, under the grandstand at the University of Chicago’s athletic field. This date is often referred to as the beginning of the Nuclear Age.

STRAY NEUTRON ²³⁵U ORIGINAL FISSION FISSION FRAGMENTS One to three neutrons from fission process A NEUTRON SOMETIMES LOST ²³⁸U CHANGES TO PLUTONIUM ²³⁵U ONE NEW FISSION FISSION FRAGMENT One to three neutrons again ²³⁵U ²³⁵U TWO NEW FISSIONS FISSION FRAGMENTS

The Fission Bomb Is Exploded

The American scientists present on that historic December day were part of the tremendous super-secret scientific and industrial complex that bore the unrevealing title Manhattan District. The United States had been at war almost a year. An uncontrolled fission reaction gave promise of producing an explosion of untold proportions. This promise, coupled with the possibility that enemy scientists might be nearing such a goal, had launched a vast Allied effort.

The Manhattan Project, as it was commonly known, included a variety of “hush-hush” facilities. Each of these installations, in New York, Illinois, Tennessee, New Mexico, California, and Washington, had its own experts working night and day to solve the baffling problems surrounding development of a fission weapon.

Ordinary uranium as found in nature was not suitable for an atomic bomb because less than one percent of the atoms in it are fissionable isotope ²³⁵U.[3] It therefore became necessary to find some means for separating the rare ²³⁵U from the large quantity of ²³⁸U. Chemistry could not do it since the two isotopes are identical chemically.

Several methods of achieving large-scale separation were tried. The most successful and economical, known as “gaseous diffusion,” involves compressing normal uranium, in the form of uranium hexafluoride gas, against a porous barrier containing millions of holes, each smaller than two-millionths of an inch. Since the ²³⁵U molecules are slightly lighter than the ²³⁸U, they bounce against the barrier more frequently and have a greater chance of penetrating. Thus, although the gas at first contains only 0.7% ²³⁵U, the process of compression is repeated several thousand times, and the proportion gradually increases until the necessary concentration is reached.

For this operation an enormous plant containing a very large barrier area, miles of piping, and countless pumps was built at Oak Ridge, Tennessee.

At the same time that vast efforts were being made to produce a ²³⁵U bomb, another project of equal importance was being pursued to develop a different kind of fission bomb. Uncertainty as to whether it would be possible to separate usable amounts of ²³⁵U led to a decision to exploit a highly significant discovery about one of the transuranic elements.

By 1941 Glenn T. Seaborg, Edwin M. McMillan, Philip H. Abelson, and others at the Radiation Laboratory, Berkeley, California, had identified isotopes of two new transuranic elements developed when they bombarded ²³⁸U nuclei with neutrons. The new elements were named neptunium and plutonium after the planets Neptune and Pluto, which lie beyond Uranus in the solar system.[4] One isotope of plutonium, plutonium-239, which resulted from the absorption of a neutron by a ²³⁸U nucleus and the emission of two beta particles, was discovered to be as fissionable as ²³⁵U and hence theoretically just as feasible for a bomb. Since plutonium is chemically different from uranium, it offered the tremendous advantage that it could readily be concentrated by conventional chemical techniques.

The way to manufacture usable amounts of plutonium, an element that had never before been detected on earth, is to expose uranium to a very intense neutron bombardment. The best-known place to find a rich supply of neutrons was the heart of a self-sustaining chain-reacting pile of uranium. Accordingly, very large piles, or _reactors_, were rushed to completion near the Columbia River at Hanford, Washington, to make plutonium.

On July 16, 1945, a plutonium bomb, carefully assembled by another group of scientists at “Project Y,” Los Alamos, New Mexico, was successfully tested in the New Mexico desert. The heat from that first man-made nuclear explosion completely vaporized a tall steel tower and melted several acres of surrounding surface sand. The flash of light was the brightest the earth had ever witnessed.

A ²³⁵U bomb was dropped on Hiroshima, Japan, on August 6, 1945. Three days later a plutonium bomb was dropped on Nagasaki, Japan. Hostilities ended on August 14, 1945.

Nuclear Energy Is Needed for the Future

The chief source of the enormous quantities of energy used daily by modern civilization is fossil fuels in the form of coal, petroleum, and natural gas. Concentrated sources of these fuels, though large, are far from inexhaustible, and it has been said that future historians may refer to the brief time when they were used as “the fossil-fuel incident.”

The next great source of energy will probably be nuclear reactors, in which controlled chain reactions release energy from the large store of fissionable materials in the world.[5]

The accomplishments of nuclear power in the propulsion of ships have already been noted. In addition, there is now going on in industrialized countries in different parts of the world a large-scale development of nuclear power plants for production of electricity. Nuclear electric power is approaching the point where it will be economically competitive with power from hydroelectric plants or those burning coal, oil, or gas as fuels. Improvements in nuclear power technology are rapidly being made, and it is now widely predicted that before the end of this century most new electric power plants will be nuclear.

Fusion Has Potential

One of the greatest puzzles to be solved by physicists arose from the work of geologists. When it became clear that coal and other fossil remains of living things date from many hundreds of millions of years ago, it was obvious that the earth’s sun had been shining at a quite steady rate for an extremely long time.

How does it manage to do it? What is its source of energy? Chemical energy supplied by combustion and gravitational potential energy supplied by contraction are thousands of times too small to have kept the sun going for such a long time.

The principle illustrated by Figure 4 suggests the most probable source of energy for the sun and all the other stars as well. It is known that the sun consists chiefly of hydrogen and that it has a temperature of about 40,000,000 degrees Fahrenheit near its center. Several kinds of nuclear reactions produced in atom smashers have demonstrated that hydrogen nuclei, if energized by being heated to a very high temperature, can actually combine, or fuse, to form helium nuclei.

The accompanying loss of weight per particle indicated by Figure 4 must result in the appearance of sufficient energy to balance Einstein’s famous equation. In fact, calculations by the German-born American physicist Hans A. Bethe and others show that, based on reasonable estimates of the conditions within the sun, familiar nuclear reactions account for its energy. The calculations predict, furthermore, that the sun can continue to operate at its present level for many billions of years.