Part 8

For the next three years, it can thus be seen, and possibly for a considerably longer period, the initiative, as far as atomic weapons are concerned, will remain with us. Let us therefore be done with all visionary plans for destroying the shield that now protects civilization as we know it, and proceed to build bigger and better shields, hoping that by our very act of doing so we can prevent the ultimate cataclysm. Right now the outlook is not bright, but our strength, physical and spiritual, should give us faith that the forces of good will prevail in the end over the forces of evil, as they have always done throughout history; that the four freedoms will triumph over the Four Horsemen of the Apocalypse.

V A PRIMER OF ATOMIC ENERGY

The material universe, the earth and everything in it, all things living and non-living, the sun and its planets, the stars and the constellations, the galaxies and the supergalaxies, the infinitely large and the infinitesimally small, manifests itself to our senses in two forms, matter and energy. We do not know, and probably never can know, how the material universe began, and whether, indeed, it ever had a beginning, but we do know that it is constantly changing and that it did not always exist in its present form. We also know that in whatever form the universe may have existed, matter and energy have always been inseparable, no energy being possible without matter, and no matter without energy, each being a form of the other.

While we do not know how and when matter and energy came into being, or whether they ever had a beginning in time as we perceive it, we do know that while the relative amounts of matter and energy are constantly changing, the total amount of both, in one form or the other, always remains the same. When a plant grows, energy from the sun, in the form of heat and light, is converted into matter, so that the total weight of the plant is greater than that of the elementary material constituents, water and carbon-dioxide gas, out of which its substance is built up. When the substance of the plant is again broken up into its original constituents by burning, the residual ashes and gases weigh less than the total weight of the intact plant, the difference corresponding to the amount of matter that had been converted into energy, liberated once again in the form of heat and light.

All energy as we know it manifests itself through motion or change in the physical or chemical state of matter, or both, though these changes and motions may be so slow as to be imperceptible. As the ancient Greek philosopher Heraclitus perceived more than two thousand years ago, all things are in a constant state of flux, this flux being due to an everlasting conversion of matter into energy and energy into matter, everywhere over the vast stretches of the material universe, to its outermost and innermost limits, if any limits there be.

Each manifestation of energy involves either matter in motion or a change in its physical state, which we designate as physical energy; a change in the chemical constitution of matter, which we know as chemical energy; or a combination of the two. Physical energy can be converted into chemical energy and vice versa. For example, heat and light are forms of physical energy, each consisting of a definite band of waves of definite wave lengths in violent, regular, rhythmic oscillations. A mysterious mechanism in the plant, known as photosynthesis, uses the heat and light energy from the sun to create complex substances, such as sugars, starches, and cellulose, out of simpler substances, such as carbon dioxide and water, converting physical energy, heat, and light into the chemical energy required to hold together the complex substances the plant produces. When we burn the cellulose in the form of wood or coal (coal is petrified wood), the chemical energy is once again converted into physical energy in the form of the original heat and light. As we have seen, the chemical energy stored in the plant manifested itself by an increase in the plant’s weight as compared with that of its original constituents. Similarly, the release of the energy manifests itself through a loss in the total weight of the plant’s substance.

It can thus be seen that neither matter nor energy can be created. All we can do is to manipulate certain types of matter in a way that liberates whatever energy had been in existence, in one form or another, since the beginning of time. All the energy that we had been using on earth until the advent of the atomic age had originally come from the sun. Coal, as already said, is a petrified plant that had stored up the energy of the sun in the form of chemical energy millions of years ago, before man made his appearance on the earth. Oil comes from organic matter that also had stored up light and heat from the sun in the form of chemical energy. Water power and wind power are also made possible by the sun’s heat, since all water would freeze and no winds would blow were it not for the sun’s heat energy keeping the waters flowing and the air moving, the latter by creating differences in the temperature of air masses.

There are two forms of energy that we take advantage of which are not due directly to the sun’s radiations—gravitation and magnetism—but the only way we can utilize these is by employing energy derived from the sun’s heat. In harnessing Niagara, or in the building of great dams, we utilize the fall of the water because of gravitation. But as I have already pointed out, without the sun’s heat water could not flow. To produce electricity we begin with the chemical energy in coal or oil, which is first converted into heat energy, then to mechanical energy, and finally, through the agency of magnetism, into electrical energy.

The radiations of the sun, of the giant stars millions of times larger than the sun, come from an entirely different source, the greatest source of energy in the universe, known as atomic or, more correctly, nuclear energy. But even here the energy comes as the result of the transformation of matter. The difference between nuclear energy and chemical energy is twofold. In chemical energy, such as the burning of coal, the matter lost in the process comes from the outer shell of the atoms, and the amount of matter lost is so small that it cannot be weighed directly by any human scale or other device. In nuclear energy, on the other hand, the matter lost by being transformed into energy comes from the nucleus, the heavy inner core, of the atom, and the amount of matter lost is millions of times greater than in coal, great enough to be weighed.

An atom is the smallest unit of any of the elements of which the physical universe is constituted. Atoms are so small that if a drop of water were magnified to the size of the earth the atoms in the drop would be smaller than oranges.

The structure of atoms is like that of a minuscule solar system, with a heavy nucleus in the center as the sun, and much smaller bodies revolving around it as the planets. The nucleus is made up of two types of particles: protons, carrying a positive charge of electricity, and neutrons, electrically neutral. The planets revolving about the nucleus are electrons, units of negative electricity, which have a mass about one two-thousandth the mass of the proton or the neutron. The number of protons in the nucleus determines the chemical nature of the element, and also the number of planetary electrons, each proton being electrically balanced by an electron in the atom’s outer shells. The total number of protons and neutrons in the nucleus is known as the mass number, which is very close to the atomic weight of the element but not quite equal. Protons and neutrons are known under the common name “nucleons.”

There are two important facts to keep constantly in mind about protons and neutrons. The first is that the two are interchangeable. A proton, under certain conditions, loses its positive charge by emitting a positive electron (positron) and thus becomes a neutron. Similarly, a neutron, when agitated, emits a negative electron and becomes a proton. As we shall see, the latter process is taken advantage of in the transmutation of nonfissionable uranium into plutonium, and of thorium into fissionable uranium 233. The transmutation of all other elements, age-old dream of the alchemists, is made possible by the interchangeability of protons into neutrons, and vice versa.

The second all-important fact about protons and neutrons, basic to the understanding of atomic energy, is that each proton and neutron in the nuclei of the elements weighs less than it does in the free state, the loss of weight being equal to the energy binding the nucleons. This loss becomes progressively greater for the elements in the first half of the periodic table, reaching its maximum in the nucleus of silver, element 47. After that the loss gets progressively smaller. Hence, if we were to combine (fuse) two elements in the first half of the periodic table, the protons and the neutrons would lose weight if the newly formed nucleus is not heavier than that of silver, but would gain weight if the new nucleus thus formed is heavier than silver. The opposite is true with the elements in the second half of the periodic table, the protons and neutrons losing weight when a heavy element is split into two lighter ones, and gaining weight if two elements are fused into one.

Since each loss of mass manifests itself by the release of energy, it can be seen that to obtain energy from the atom’s nucleus requires either the fusion of two elements in the first half of the periodic table or the fission of an element in the second half. From a practical point of view, however, fusion is possible only with two isotopes (twins) of hydrogen, at the beginning of the periodic table, while fission is possible only with twins of uranium, U-233 and U-235, and with plutonium, at the lower end of the table.

The diameter of the atom is 100,000 times greater than the diameter of the nucleus. This means that the atom is mostly empty space, the volume of the atom being 500,000 billion times the volume of the nucleus. It can thus be seen that most of the matter in the universe is concentrated in the nuclei of the atoms. The density of matter in the nucleus is such that a dime would weigh 600 million tons if its atoms were as tightly packed as are the protons and neutrons in the nucleus.

The atoms of the elements (of which there are ninety-two in nature, and six more man-made elements) have twins, triplets, quadruplets, etc., known as isotopes. The nuclei of these twins all contain the same number of protons and hence all have the same chemical properties. They differ, however, in the number of neutrons in their nuclei and hence have different atomic weights. For example, an ordinary hydrogen atom has a nucleus of one proton. The isotope of hydrogen, deuterium, has one proton plus one neutron in its nucleus. It is thus twice as heavy as ordinary hydrogen. The second hydrogen isotope, tritium, has one proton and two neutrons in its nucleus and hence an atomic mass of three. On the other hand, a nucleus containing two protons and one neutron is no longer hydrogen but helium, also of atomic mass three.

There are hundreds of isotopes, some occurring in nature, others produced artificially by shooting atomic bullets, such as neutrons, into the nuclei of the atoms of various elements. A natural isotope of uranium, the ninety-second and last of the natural elements, contains 92 protons and 143 neutrons in its nucleus, hence its name U-235, one of the two atomic-bomb elements. The most common isotope of uranium has 92 protons and 146 neutrons in its nucleus and hence is known as U-238. It is 140 times more plentiful than U-235, but cannot be used for the release of atomic energy.

Atomic, or rather nuclear, energy is the cosmic force that binds together the protons and the neutrons in the nucleus. It is a force millions of times greater than the electrical repulsion force existing in the nucleus because of the fact that the protons all have like charges. This force, known as the coulomb force, is tremendous, varying inversely as the square of the distance separating the positively charged particles. Professor Frederick Soddy, the noted English physicist, has figured out that two grams (less than the weight of a dime) of protons placed at the opposite poles of the earth would repel each other with a force of twenty-six tons. Yet the nuclear force is millions of times greater than the coulomb force. This force acts as the cosmic cement that holds the material universe together and is responsible for the great density of matter in the nucleus.

We as yet know very little about the basic nature of this force, but we can measure its magnitude by a famous mathematical equation originally presented by Dr. Einstein in his special theory of relativity in 1905. This formula, one of the great intellectual achievements of man, together with the discovery of the radioactive elements by Henri Becquerel and Pierre and Marie Curie, provided the original clues as well as the key to the discovery and the harnessing of nuclear energy.

Einstein’s formula, E = mc², revealed that matter and energy are two different manifestations of one and the same cosmic entity, instead of being two different entities, as had been generally believed. It led to the revolutionary concept that matter, instead of being immutable, was energy in a frozen state, while, conversely, energy was matter in a fluid state. The equation revealed that any one gram of matter was the equivalent in ergs (small units of energy) to the square of the velocity of light in centimeters per second—namely, 900 billion billion ergs. In more familiar terms, this means that one gram of matter represents 25,000,000 kilowatt-hours of energy in the frozen state. This equals the energy liberated in the burning of three billion grams (three thousand tons) of coal.

The liberation of energy in any form, chemical, electrical, or nuclear, involves the loss of an equivalent amount of mass, in accordance with the Einstein formula. When 3,000 metric tons of coal are burned to ashes, the residual ashes and the gaseous products weigh one gram less than 3,000 tons; that is, one three-billionth part of the original mass will have been converted into energy. The same is true with the liberation of nuclear energy by the splitting or fusing (as will be explained later) of the nuclei of certain elements. The difference is merely that of magnitude. In the liberation of chemical energy by the burning of coal, the energy comes from a very small loss of mass resulting from the rearrangement of electrons on the surface of the atoms. The nucleus of the coal atoms is not involved in any way, remaining exactly the same as before. The amount of mass lost by the surface electrons is one thirtieth of one millionth of one per cent.

On the other hand, nuclear energy involves vital changes in the atomic nucleus itself, with a consequent loss of as high as one tenth to nearly eight tenths of one per cent in the original mass of the nuclei. This means that from one to nearly eight grams per thousand grams are liberated in the form of energy, as compared with only one gram in three billion grams liberated in the burning of coal. In other words, the amount of nuclear energy liberated in the transmutation of atomic nuclei is from 3,000,000 to 24,000,000 times as great as the chemical energy released by the burning of an equal amount of coal. In terms of TNT the figure is seven times greater than for coal, as the energy from TNT, while liberated at an explosive rate, is about one seventh the total energy content for an equivalent amount of coal. This means that the nuclear energy from one kilogram of uranium 235, or plutonium, when released at an explosive rate, is equal to the explosion of twenty thousand tons of TNT.

Nuclear energy can be utilized by two diametrically opposed methods. One is fission—the splitting of the nuclei of the heaviest chemical elements into two uneven fragments consisting of nuclei of two lighter elements. The other is fusion—combining, or fusing, two nuclei of the lightest elements into one nucleus of a heavier element. In both methods the resulting elements are lighter than the original nuclei. The loss of mass in each case manifests itself in the release of enormous amounts of nuclear energy.

When two light atoms are combined to form a heavier atom, the weight of the heavier is less than the total weight of the two light atoms. If the heavier atom could again be split into the two lighter ones, the latter would resume their original weight. As explained before, however, this is true only with the light elements, such as hydrogen, deuterium, and tritium, in the first half of the periodic table of the elements. The opposite is true with the heavier elements of the second half of the periodic table. For example, if krypton and barium, elements 36 and 56, were to be combined to form uranium, element 92, the protons and the neutrons in the uranium nucleus would each weigh about 0.1 per cent more than they weighed in the krypton and barium nuclei. It can thus be seen that energy could be gained either through the loss of mass resulting from the fusion of two light elements, or from the similar loss of mass resulting from the fission of one heavy atom into two lighter ones.

In the fusion of two lighter atoms, the addition of one and one yields less than two, and yet half of two will be more than one. In the case of the heavy elements the addition of one and one yields more than two, yet half of two makes less than one. This is the seeming paradox of atomic energy.

Three elements are known to be fissionable. Only one of these is found in nature: the uranium isotope 235 (U-235). The other two are man-made. One is plutonium, transmuted by means of neutrons from the nonfissionable U-238, by the addition of one neutron to the 146 present in the nucleus, which leads to the conversion of two of the 147 neutrons into protons, thus creating an element with a nucleus of 94 protons and 145 neutrons. The second man-made element (not yet in wide use, as far as is known) is uranium isotope 233 (92 protons and 141 neutrons), created out of the element thorium (90 protons, 142 neutrons) by the same method used in the production of plutonium.

When the nucleus of any one of these elements is fissioned, each proton and neutron in the two resulting fragments weighs one tenth of one per cent less than it weighed in the original nucleus. For example, if U-235 atoms totaling 1,000 grams in weight are split, the total weight of the fragments will be 999 grams. The one missing gram is liberated in the form of 25,000,000 kilowatt-hours of energy, equivalent in explosive terms to 20,000 tons of TNT. But the original number of protons and neutrons in the 1,000 grams does not change.

The fission process, the equivalent of the “burning” of nuclear fuels, is maintained by what is known as a chain reaction. The bullets used for splitting are neutrons, which, because they do not have an electric charge, can penetrate the heavily fortified electrical wall surrounding the positively charged nuclei. Just as a coal fire needs oxygen to keep it going, a nuclear fire needs the neutrons to maintain it.

Neutrons do not exist free in nature, all being tightly locked up within the nuclei of atoms. They are liberated, however, from the nuclei of the three fissionable elements by a self-multiplication process in the chain reaction. The process begins when a cosmic ray from outer space, or a stray neutron, strikes one nucleus and splits it. The first atom thus split releases an average of two neutrons, which split two more nuclei, which in turn liberate four more neutrons, and so on. The reaction is so fast that in a short time trillions of neutrons are thus liberated to split trillions of nuclei. As each nucleus is split, it loses mass, which is converted into great energy.

There are two types of chain reactions: controlled and uncontrolled. The controlled reaction is analogous to the burning of gasoline in an automobile engine. The atom-splitting bullets—the neutrons—are first slowed down from speeds of more than ten thousand miles per second to less than one mile per second by being made to pass through a moderator before they reach the atoms at which they are aimed. Neutron-“killers”—materials absorbing neutrons in great numbers—keep the neutrons liberated at any given time under complete control in a slow but steady nuclear fire.

The uncontrolled chain reaction is one in which there is no moderator—and no neutron-absorbers. It is analogous to the dropping of a match in a gasoline tank. In the uncontrolled chain reaction the fast neutrons, with nothing to slow them down or to devour them, build up by the trillion and quadrillion in a fraction of a millionth of a second. This leads to the splitting of a corresponding number of atoms, resulting in the release of unbelievable quantities of nuclear energy at a tremendously explosive rate. One kilogram of atoms split releases energy equivalent to that of 20,000,000 kilograms (20,000 metric tons) of TNT.

It is the uncontrolled reaction that is employed in the explosion of the atomic bomb. The controlled reaction is expected to be used in the production of vast quantities of industrial power. It is now being employed in the creation of radioactive isotopes, for use in medicine and as the most powerful research tool since the invention of the microscope for probing into the mysteries of nature, living and non-living.

In the controlled reaction the material used is natural uranium, which consists of a mixture of 99.3 per cent U-238 and 0.7 of the fissionable U-235. The neutrons from the U-235 are made to enter the nuclei of U-238 and convert them to the fissionable element plutonium, for use in atomic bombs. The large quantities of energy liberated by the split U-235 nuclei in the form of heat is at too low a temperature for efficient utilization as power, and is at present wasted. To be used for power, nuclear reactors capable of operating at high temperatures are now being designed.

In the atomic bomb only pure U-235, or plutonium, is used.

In both the controlled and the uncontrolled reactions a minimum amount of material, known as the “critical mass,” must be used, as otherwise too many neutrons would escape and the nuclear fire would thus be extinguished, as would an ordinary fire for lack of oxygen. In the atomic bomb two masses, each less than a critical mass, which together equal or exceed it, are brought in contact at a predetermined instant. The uncontrolled reaction then comes automatically, since, in the absence of any control, the neutrons, which cannot escape to the outside, build up at an unbelievable rate.