Read Ebook: Worlds Within Worlds: The Story of Nuclear Energy Volume 2 (of 3) Mass and Energy; The Neutron; The Structure of the Nucleus by Asimov Isaac

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VOLUME 1 Introduction 5 Atomic Weights 6 Electricity 11 Units of Electricity 11 Cathode Rays 13 Radioactivity 17 The Structure of the Atom 25 Atomic Numbers 30 Isotopes 35 Energy 47 The Law of Conservation of Energy 47 Chemical Energy 50 Electrons and Energy 54 The Energy of the Sun 55 The Energy of Radioactivity 57

VOLUME 2 Mass and Energy 69 The Structure of the Nucleus 75 The Proton 75 The Proton-Electron Theory 76 Protons in Nuclei 80 Nuclear Bombardment 82 Particle Accelerators 86 The Neutron 92 Nuclear Spin 92 Discovery of the Neutron 95 The Proton-Neutron Theory 98 The Nuclear Interaction 101 Neutron Bombardment 107

VOLUME 3 Nuclear Fission 117 New Elements 117 The Discovery of Fission 122 The Nuclear Chain Reaction 127 The Nuclear Bomb 131 Nuclear Reactors 141 Nuclear Fusion 147 The Energy of the Sun 147 Thermonuclear Bombs 149 Controlled Fusion 151 Beyond Fusion 159 Antimatter 159 The Unknown 164 Reading List 166

MASS AND ENERGY

In 1900 it began to dawn on physicists that there was a vast store of energy within the atom; a store no one earlier had imagined existed. The sheer size of the energy store in the atom--millions of times that known to exist in the form of chemical energy--seemed unbelievable at first. Yet that size quickly came to make sense as a result of a line of research that seemed, at the beginning, to have nothing to do with energy.

Suppose a ball were thrown forward at a velocity of 20 kilometers per hour by a man on top of a flatcar that is moving forward at 20 kilometers an hour. To someone watching from the roadside the ball would appear to be travelling at 40 kilometers an hour. The velocity of the thrower is added to the velocity of the ball.

If the ball were thrown forward at 20 kilometers an hour by a man on top of a flatcar that is moving backward at 20 kilometers an hour, then the ball would seem to be not moving at all after it left the hand of the thrower. It would just drop to the ground.

There seemed no reason in the 19th century to suppose that light didn't behave in the same fashion. It was known to travel at the enormous speed of just a trifle under 300,000 kilometers per second, while earth moved in its orbit about the sun at a speed of about 30 kilometers per second. Surely if a beam of light beginning at some earth-bound source shone in the direction of earth's travel, it ought to move at a speed of 300,030 kilometers per second. If it shone in the opposite direction, against earth's motion, it ought to move at a speed of 299,970 kilometers per second.

Could such a small difference in an enormous speed be detected?

The German-American physicist Albert Abraham Michelson had invented a delicate instrument, the interferometer, that could compare the velocities of different beams of light with great precision. In 1887 he and a co-worker, the American chemist Edward Williams Morley , tried to measure the comparative speeds of light, using beams headed in different directions. Some of this work was performed at the U. S. Naval Academy and some at the Case Institute.

The results of the Michelson-Morley experiment were unexpected. It showed no difference in the measured speed of light. No matter what the direction of the beam--whether it went in the direction of the earth's movement, or against it, or at any angle to it--the speed of light always appeared to be exactly the same.

To explain this, the German-Swiss-American scientist Albert Einstein advanced his "special theory of relativity" in 1905. According to Einstein's view, speeds could not merely be added. A ball thrown forward at 20 kilometers an hour by a man moving at 20 kilometers an hour in the same direction would not seem to be going 40 kilometers an hour to an observer at the roadside. It would seem to be going very slightly less than 40 kilometers an hour; so slightly less that the difference couldn't be measured.

However, as speeds grew higher and higher, the discrepancy in the addition grew greater and greater until, at velocities of tens of thousands of kilometers per hour, that discrepancy could be easily measured. At the speed of light, which Einstein showed was a limiting velocity that an observer would never reach, the discrepancy became so great that the speed of the light source, however great, added or subtracted zero to or from the speed of light.

Accompanying this were all sorts of other effects. It could be shown by Einstein's reasoning that no object possessing mass could move faster than the speed of light. What's more, as an object moved faster and faster, its length in the direction of motion grew shorter and shorter, while its mass grew greater and greater. At 260,000 kilometers per second, its length in the direction of movement was only half what it was at rest, and its mass was twice what it was. As the speed of light was approached, its length would approach zero in the direction of motion, while its mass would approach the infinite.

Could this really be so? Ordinary objects never moved so fast as to make their lengths and masses show any measurable change. What about subatomic particles, however, which moved at tens of thousands of kilometers per second? The German physicist Alfred Heinrich Bucherer reported in 1908 that speeding electrons did gain in mass just the amount predicted by Einstein's theory. The increased mass with energy has been confirmed with great precision in recent years. Einstein's special theory of relativity has met many experimental tests exactly ever since and it is generally accepted by physicists today.

Einstein's theory gave rise to something else as well. Einstein deduced that mass was a form of energy. He worked out a relationship that is expressed as follows:

If mass is measured in grams and the speed of light is measured in centimeters per second, then the equation will yield the energy in a unit called "ergs". It turns out that 1 gram of mass is equal to 900,000,000,000,000,000,000 ergs of energy. The erg is a very small unit of energy, but 900 billion billion of them mount up.

The energy equivalent of 1 gram of mass would keep a 100-watt light bulb burning for 35,000 years.

Suppose, for instance, a gallon of gasoline is burned. The gallon of gasoline has a mass of 2800 grams and combines with about 10,000 grams of oxygen to form carbon dioxide and water, yielding 1.35 million billion ergs. That's a lot of energy and it will drive an automobile for some 25 to 30 kilometers. But by Einstein's equation all that energy is equivalent to only a little over a millionth of a gram. You start with 12,800 grams of reacting materials and you end with 12,800 grams minus a millionth of a gram or so that was given off as energy.

No instrument known to the chemists of the 19th century could have detected so tiny a loss of mass in such a large total. No wonder, then, that from Lavoisier on, scientists thought that the law of conservation of mass held exactly.

Radioactive changes gave off much more energy per atom than chemical changes did, and the percentage loss in mass was correspondingly greater. The loss of mass in radioactive changes was found to match the production of energy in just the way Einstein predicted.

It was no longer quite accurate to talk about the conservation of mass after 1905 . Instead, it is more proper to speak of the conservation of energy, and to remember that mass was one form of energy and a very concentrated form.

The mass-energy equivalence fully explained why the atom should contain so great a store of energy. Indeed, the surprise was that radioactive changes gave off as little energy as they did. When a uranium atom broke down through a series of steps to a lead atom, it produced a million times as much energy as that same atom would release if it were involved in even the most violent of chemical changes. Nevertheless, that enormous energy change in the radioactive breakdown represented only about one-half of 1% of the total energy to which the mass of the uranium atom was equivalent.

Once Rutherford worked out the nuclear theory of the atom, it became clear from the mass-energy equivalence that the source of the energy of radioactivity was likely to be in the atomic nucleus where almost all the mass of the atom was to be found.

The attention of physicists therefore turned to the nucleus.

THE STRUCTURE OF THE NUCLEUS

The Proton

As early as 1886 Eugen Goldstein, who was working with cathode rays, also studied rays that moved in the opposite direction. Since the cathode rays were negatively charged, rays moving in the opposite direction would have to be positively charged. In 1907 J. J. Thomson called them "positive rays".

Once Rutherford worked out the nuclear structure of the atom, it seemed clear that the positive rays were atomic nuclei from which a number of electrons had been knocked away. These nuclei came in different sizes.

Were the nuclei single particles--a different one for every isotope of every element? Or were they all built up out of numbers of still smaller particles of a very limited number of varieties? Might it be that the nuclei owed their positive electrical charge to the fact that they contained particles just like the electron, but ones that carried a positive charge rather than a negative one?

All attempts to discover this "positive electron" in the nuclei failed, however. The smallest nucleus found was that produced by knocking the single electron off a hydrogen atom in one way or another. This hydrogen nucleus had a single positive charge, one that was exactly equal in size to the negative charge on the electron. The hydrogen nucleus, however, was much more massive than an electron. The hydrogen nucleus with its single positive charge was approximately 1837 times as massive as the electron with its single negative charge.

Was it possible to knock the positive charge loose from the mass of the hydrogen nucleus? Nothing physicists did could manage to do that. In 1914 Rutherford decided the attempt should be given up. He suggested that the hydrogen nucleus, for all its high mass, should be considered the unit of positive electrical charge, just as the electron was the unit of negative electrical charge. He called the hydrogen nucleus a "proton" from the Greek word for "first" because it was the nucleus of the first element.

Why the proton should be so much more massive than the electron is still one of the unanswered mysteries of physics.

The Proton-Electron Theory

What about the nuclei of elements other than hydrogen?

All the other elements had nuclei more massive than that of hydrogen and the natural first guess was that these were made up of some appropriate number of protons closely packed together. The helium nucleus, which had a mass four times as great as that of hydrogen, might be made up of 4 protons; the oxygen nucleus with a mass number of 16 might be made up of 16 protons and so on.

This guess, however, ran into immediate difficulties. A helium nucleus might have a mass number of 4 but it had an electric charge of +2. If it were made up of 4 protons, it ought to have an electric charge of +4. In the same way, an oxygen nucleus made up of 16 protons ought to have a charge of +16, but in actual fact it had one of +8.

Could it be that something was cancelling part of the positive electric charge? The only thing that could do so would be a negative electric charge and these were to be found only on electrons as far as anyone knew in 1914. It seemed reasonable, then, to suppose that a nucleus would contain about half as many electrons in addition to the protons. The electrons were so light, they wouldn't affect the mass much, and they would succeed in cancelling some of the positive charge.

This "proton-electron theory" of nuclear structure accounted for isotopes very nicely. While oxygen-16 had a nucleus made up of 16 protons and 8 electrons, oxygen-17 had one of 17 protons and 9 electrons, and oxygen-18 had one of 18 protons and 10 electrons. The mass numbers were 16, 17, and 18, respectively, but the atomic number was 16 - 8, 17 - 9, and 18 - 10, or 8 in each case.

Again, uranium-238 has a nucleus built up, according to this theory, of 238 protons and 146 electrons, while uranium-235 has one built up of 235 protons and 143 electrons. In these cases the atomic number is, respectively, 238 - 146 and 235 - 143, or 92 in each case. The nucleus of the 2 isotopes is, however, of different structure and it is not surprising therefore that the radioactive properties of the two--properties that involve the nucleus--should be different and that the half-life of uranium-238 should be six times as long as that of uranium-235.

The presence of electrons in the nucleus not only explained the existence of isotopes, but seemed justified by two further considerations.

First, it is well known that similar charges repel each other and that the repulsion is stronger the closer together the similar charges are forced. Dozens of positively charged particles squeezed into the tiny volume of an atomic nucleus couldn't possibly remain together for more than a tiny fraction of a second. Electrical repulsion would send them flying apart at once.

On the other hand, opposite charges attract, and a proton and an electron would attract each other as strongly as 2 protons would repel each other. It was thought possible that the presence of electrons in a collection of protons might somehow limit the repulsive force and stabilize the nucleus.

Second, there are radioactive decays in which beta particles are sent flying out of the atom. From the energy involved they could come only out of the nucleus. Since beta particles are electrons and since they come from the nucleus, it seemed to follow that there must be electrons within the nucleus to begin with.

The proton-electron theory of nuclear structure also seemed to account neatly for many of the facts of radioactivity.

The manner of breakup fits the theory, too. Suppose a nucleus gives off an alpha particle. The alpha particle is a helium nucleus made up, by this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha particle, its mass number should decline by 4 and its atomic number by 4 - 2, or 2. And, indeed, when uranium-238 gives off an alpha particle, it becomes thorium-234 .

Suppose a beta particle is emitted. A beta particle is an electron and if a nucleus loses an electron, its mass number is almost unchanged. On the other hand, a unit negative charge is gone. One of the protons in the nucleus, which had previously been masked by an electron, is now unmasked. Its positive charge is added to the rest and the atomic number goes up by one. Thus, thorium-234 gives up a beta particle and becomes protactinium-234 .

If a gamma ray is given off, that gamma ray has no charge and the equivalent of very little mass. That means that neither the mass number nor the atomic number of the nucleus is changed, although its energy content is altered.

Even more elaborate changes can be taken into account. In the long run, uranium-238, having gone through many changes, becomes lead-206. Those changes include the emission of 8 alpha particles and 6 beta particles. The 8 alpha particles involve a loss of 8 x 4, or 32 in mass number, while the 6 beta particles contribute nothing in this respect. And, indeed, the mass number of uranium-238 declines by 32 in reaching lead-206. On the other hand the 8 alpha particles involve a decrease in atomic number of 8 x 2, or 16, while the 6 beta particles involve an increase in atomic number of 6 x 1, or 6. The total change is a decrease of 16 - 6, or 10. And indeed, uranium changes to lead .

It is useful to go into such detail concerning the proton-electron theory of nuclear structure and to describe how attractive it seemed. The theory appeared solid and unshakable and, indeed, physicists used it with considerable satisfaction for 15 years.

--And yet, as we shall see, it was wrong; and that should point a moral. Even the best seeming of theories may be wrong in some details and require an overhaul.

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