Friday, September 08, 2006

Atom Is Split

Atom Is Split

This article from the journal Scientific American was written in 1932, soon after the Cambridge physicists John Cockcroft and Ernest Walton succeeded in splitting the atom, a highly significant early step in harnessing nuclear energy and in understanding more about the origins of the universe. The “Mendeleeff table”, named after its deviser, the Russian scientist Dmitri Mendeleef or Mendeleyev, is usually known as the periodic table of elements.

Atomic Energy—Is It Nearer?

When some wise historian of the remote future undertakes to interpret our time he will dwell less on our wars and our political upheavals than on our scientific achievements. And the early decades of the 20th Century will call for special comment, because it was then that physicists began an attack on the atom which resulted in discoveries that changed the whole character of chemistry and engineering. To that historian the experiment conducted by Drs. J. D. Cockcroft and E. T. S. Walton of Cambridge University, England, which resulted in the splitting asunder of lithium atoms and the reuniting of their shattered nuclei in new combinations, will be singled out as an example of the method which finally led to the voluntary transmutation of the elements, to the controlled release of the energy that holds matter together and thus to a revelation of the whole plan and method of creation.

This being a machine age, we pay less attention to the understanding of the cosmos that will come out of our attacks upon the atom than to the unlimited energy that will be ours when we have mastered matter. Our civilization is based on coal and oil. If the invention and introduction of the steam engine could make coal and oil of such importance that nations were willing to fight for them and that the whole character of our living and thinking was transformed, what will happen to society when the energy in the atom is placed at the command of engineers?

If we are ever to utilize this energy we need something better than the hit-and-miss methods that must now be applied. When alpha particles from radium are directed on aluminum a nucleus is hit about once in a million times. At that rate an ounce of radium, costing several hundred thousand dollars, would not release enough energy from aluminum in a year to warm an ounce of water a degree or so. Cockcroft and Walton state that their protons hit the target only once in 10,000,000 times at 250,000 volts.

It is clear from this that we are only a little better off than the first savage who ever boiled water and saw steam rise. What did he know of the energy locked in the steam? How was he to divine that thousands of years after him engineers would be born who would devise cunning cylinders and pistons which would pump, haul, lift and do all that muscles would do? We are better than that savage in this: We know what mechanical energy can do. We know how much energy can be extracted from wood, coal, oil or a falling mass of water. We even realize the potentialities of the atom. This is the beginning of real progress.

We have still far to go before we can pretend to understand the atom and the secret of matter. But we have gone far enough to think of an engine which will harness the energy released in atom building. Not in our wildest speculations can we imagine what form that engine will assume. Perhaps some powerhouse engineer of the remote future will simply pour a few thimblefuls of sand into a disintegrating chamber. Perhaps he will actually change cheap metal into gold in the process of furnishing a city with light. Who knows?

What we have now in the form of engines, dynamos, and motors will seem quaint and amusing a few centuries hence. "To think that they actually burned coal to heat water and then used the steam to drive that funny engine and made the engine turn what they called a dynamo and in that way excited a current which they conducted to a lamp or a motor—how troublesome living and working must have been!" some boy will muse as he stands before a steam-engine of our day in a museum of the year 2500. Perhaps Lord Rutherford had something like this in mind when he said, years before Cockcroft and Walton came to his lectures as students in Cambridge, "the human race may trace its development from the discovery of a method of utilizing atomic energy."

Professor Millikan has often expressed the view that we are not likely to obtain enough energy for our industrial purposes by breaking down atoms through any electrical process. If we could collect all the radium thus far mined, the energy which it gives forth as it spontaneously disintegrates would not long suffice to run the peanut and popcorn roasters of the world, he has stated on more than one occasion. Too much work must be done on all the elements, except hydrogen, to make them give up their energy. "Man's only possible source of energy other than the sun is the up-building of the common elements out of hydrogen and helium or else the entire annihilation of positive and negative electrons," is the dictum he uttered at the Cleveland meeting of the American Association for the Advancement of Science. Probably this expresses the general view of physicists today.

The world is so obsessed by the preciousness of gold that whenever the subject of transmutation is broached it thinks only of increasing the monetary value of some base metal. To a physicist the change from lead to gold would be no more exciting than the change from nitrogen or aluminum to hydrogen which Rutherford effected long ago. True, if lead or mercury could be transmuted to gold the financial structure of the world might be threatened. But not for long. Some other standard of value would be adopted by international agreement.

A given weight of lead changing to gold would produce about a hundred million times as much heat as the same weight of burning coal. Hence a fraction of a grain of lead would do the work of a ton of coal. But the grain of lead turned into gold would bring only a fraction of a cent. The energy in the atom is worth far more than the price that the grain would bring in terms of gold. It is the owner of a coal mine or an oil field who has reason to worry if some day base metals could be cheaply converted into what we now regard as precious metals.

When they directed a stream of highspeed electric bullets—protons—on a layer of lithium in an exhausted tube, Drs. Cockcroft and Walton were not especially concerned with the transmutation of elements or the utilization of atomic energy. They had but one object—to penetrate deep into the atom's core and thus satisfy that natural curiosity on which all scientific investigation is based. They had reason to believe that if their bullets were fast enough, transmutation of some kind would occur, and that some energy would be released. But neither they nor any one else expected that out of the lithium nucleus would come helium particles. Their work is of the utmost scientific importance because of the new methods that they adopted, and because of their results. To understand what they have done, we must consider what the atom is, and realize how difficult it is to tear it apart in order to learn how nature put it together.

At this late day every one knows that an atom consists of a nucleus surrounded by electrons that vary in number with the weight of the atom. Hydrogen has one planetary electron. At the other end of the list is uranium with 92 planetary electrons. Between hydrogen and uranium lie the atoms of the other 90 elements, each atom having a number of planetary electrons that agrees with its numerical place in the Mendeleeff table of elements. Electrical forces are the cement that holds the atom together. Electrons are always negative. They are attracted by the positive nucleus. To smash the atom this bond must be broken; the mere stripping away of the outer rings of electrons is not enough. Most of the mass of an atom lies in the nucleus. It is the nucleus that determines just what kind of matter we deal with—whether it shall be a gas like hydrogen or a crystal like the diamond.

Originally, which means early in the present century, it was supposed that a nucleus consisted of positive electrons—positively charged particles which exactly counterbalanced the charges of the outer negative electrons. Later studies, notably those of Rutherford, showed that the nucleus is not so simple. It constitutes, in fact, a veritable atom within the atom. It is composed not only of protons, but also of electrons, in the case of elements higher in the scale than hydrogen. Physicists saw quickly enough that if they were ever to penetrate the secret of matter they must disrupt that nucleus.

A structure held together by force must be torn apart by force. And the binding force in this case is terrific. An atom is in itself invisible; the most powerful microscope that man can ever devise cannot magnify it so that it can be seen. The physicist who tries to disrupt the atom must smash indiscriminately. He cannot select a single atom and use it as a target. He must use millions of hammers on millions of atoms in the hope that an occasional telling blow will be struck. Even then he cannot know how successful he has been except by photographing the little tracks left by fragments of atoms that have been smashed.

The first man who succeeded in at least partly smashing an atom was Rutherford. He knew that he needed energy—that the blow struck must be violent. He cast about for the right kind of hammer. None that man could make would do. In radium he discovered the hammer that he needed. As it spontaneously disintegrates, radium shoots off alpha particles, which are the nuclei of helium atoms. They travel with a speed of about 12,000 miles a second, which is about 24,000 times faster than that of a rifle bullet—fast enough to go around the world in about two seconds.

Rutherford devised an apparatus in which different kinds of atoms could be hammered by alpha particles. They were both small and heavy, these particles. They crashed through the outer electrons easily. But at the nucleus they encountered the forces that hold the atom together. What physicists call a high-potential wall—an intangible wall of force—deffected them. Rutherford found it especially hard to make any impression on the nuclei of the heavy elements. With the lighter he was more successful. Nitrogen, boron, fluorine, aluminum, phosphorus were among those that yielded.

What was the result of this hammering? Always there came out of the nucleus protons—hydrogen nuclei. This was real transmutation. To change a score of different elements even partly into hydrogen was as startling as if lead had been changed into gold. But what was scientifically more important was the fact that always hydrogen nuclei or protons came out of widely different atoms. It was evident that protons, hydrogen nuclei, constituted the bases of all atoms—that the stuff out of which the universe was made must have been these protons and electrons.

The alpha particle method of attacking the nucleus of an atom has its decided limitations. If all the radium thus far mined and purified could be collected in one place it would give off only a known, fixed number of particles in a second. Furthermore, the speeds and energies of the rays from radium are not subject to control.

What the physicist wants is an electric gun which he can load to suit himself—a gun which will make it possible to attain projectile speeds higher than those of the particles given off by radium. With such a gun either protons or electrons can be fired at atoms. The propellant is high voltage. Since voltage can be raised or lowered it follows that the blows struck are subject to some control.

Drs. Cockcroft and Walton are not the only ones who have devised an electrical method of hurling particles at atoms. Drs. M. A. Tuve, L. R. Hafstad, and O. Dahl of the Carnegie Institution of Washington have been experimenting for many months with protons to which energies as high as 2,000,000 and 3,000,000 volts have been imparted. Their methods are much like those of the Cambridge scientists. In either case the protons are artificially produced.

The whole problem of wrecking atoms reduces itself to high voltages. In order to generate voltages higher than any thus far attained, Dr. R. J. Van de Graaff is now constructing for the Massachusetts Institute of Technology a giant apparatus with which protons can be shot against atoms at 10,000,000 to 15,000,000 volts. His apparatus consists of two large spheres within which the experimenters sit. Electricity accumulates like water on the outside of a sphere. When there is more of it than the surface of the sphere can hold, it spills over to the other sphere in a blinding flash of artificial lightning. Imprison the flash in a tube and protons are carried to the target—the atom.

This method, applied on a less magnificent scale by Cockcroft and Walton, has led to dramatically unexpected results. Protons fired at lithium actually reached the nucleus. One proton was occasionally captured. Thus a new combination of protons and electrons within the nucleus became possible. Out of the nucleus flew two alpha particles—in other words, helium nuclei. Such is the explanation of their results advanced by the Cambridge scientists. Rutherford fired alpha particles at atoms and drove out protons. Cockcroft and Walton fired protons and obtained alpha particles.

The arithmetic of the Cockcroft-Walton achievement is easy to understand. The lithium with which the experiment was conducted had a mass designated by 7. The proton had a mass equal to 1. Since a proton was caught and imprisoned lithium's mass was raised to 8. The capture of the proton was like pressing a hidden spring and disrupting the atom. Out of the atom flew two alpha particles, each of mass 4. An alpha particle consists of two protons and two electrons electrically cemented together. Never was a result more unexpectedly obtained. Moreover, this is the first time that an atom has been disrupted by purely electrical methods.

More startling than the formation of alpha particles (helium nuclei) is the amount of energy released from the atom. Ten million shots were fired at 250,000 volts and one hit scored. At 400,000 volts, the highest attainable with the apparatus, the marksmanship was better. But—and here we must hold tightly to our chairs—the energy of each of the two liberated alpha particles was 8,000,000 volts. A total of 16,000,000 volts obtained for an expenditure as little as 125,000 and as much as 400,000! In the history of laboratory experimentation with the atom nothing more startling has ever happened. Yet it is not thus that we shall obtain atomic energy for the practical purposes of the future.

The discovery of Cockcroft and Walton dovetails neatly with that of Bothe and Becker. Last year Professors Bothe and Becker, who are members of the faculty of the University of Giessen, Germany, bombarded beryllium with alpha particles, after the fashion of Rutherford, who had knocked out protons from beryllium. Their particles, however, came not from radium but from polonium. They obtained rays as penetrating as those which would be generated by a 14-million-volt X-ray tube if we could build one—rays which could pierce three inches of iron and still retain one third of their intensity.

The alpha particles entered the nucleus of the beryllium atom. A new type of carbon atom was created—a stepping-up of beryllium in the table of elements. And the building-up process was accompanied, quite as Einstein had predicted, by the release of energy which manifested itself in rays very much more penetrating or "harder" than the most powerful X-rays that can be produced in a laboratory or than the piercing gamma rays emitted by radium.

In the first erroneous accounts that came to us of the success achieved by Cockcroft and Walton, it was stated that hydrogen had been changed into helium with the liberation of energy. This is a building-up process of which physicists have long dreamed. Why energy should thus be obtained follows from Einstein. The atomic weight of hydrogen is 1.00778; that of helium, 4.00054, or a small fraction less than four times as much. When helium is created by the union of four hydrogen atoms something must become of matter equal to this small difference. This minute surplus becomes energy. The synthesis of a single gram of helium from one gram of hydrogen would produce as much heat as the combustion of 20 tons of coal.

Einstein has taught us that when an atom gives off energy it loses mass. The amount of mass which must be lost to yield energy is so small that it is scarcely perceptible.

Complete annihilation of a minute amount of matter—that is the engineering ideal of the Utopian who would dispense with coal, oil, and other fuels in the remote future. A single annihilated gram of matter, by which we mean the complete disappearance of its protons and electrons as atomic systems, would be more than enough, according to Haas, to lift a weight as heavy as all the buildings in New York combined to the height of the Empire State Building, or more than enough to raise the temperature of all the rooms in New York by several degrees.

Pessimists like Sir Oliver Lodge shudder when they speculate on the future. Man is not yet spiritually ripe for the possession of the secret of atomic energy, he reasons. Technically we are demi-gods, ethically still such barbarians that we would probably use the energy of the atom much as we used the less terrible forces that almost destroyed civilization during the last war.

Others are convinced that the new insight into nature which will be granted when the structure of the atom is at last known, and with it the method of controlling its energy, must be accompanied by a spiritual advance. Each new discovery about the atom makes man more consciously part of the world about him—links him with the stars, which are themselves composed of atoms, and with the dazzling light of the sun, which springs from atomic activity—and thus impresses him with the littleness of his greed and the puerility of his disputes.

Source: Reprinted with permission. Copyright © August 1932 by Scientific American, Inc. [ http://www.sciam.com/ ]. All rights reserved.
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

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