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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--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.

Protons in Nuclei

Let us, nevertheless, go on to describe some of the progress made in the 1920s in terms of the proton-electron theory that was then accepted.

Since a nucleus is made up of a whole number of protons, its mass ought to be a whole number if the mass of a single proton is considered 1.

When isotopes were first discovered this indeed seemed to be so. However, Aston and his mass spectrometer kept measuring the mass of different nuclei more and more closely during the 1920s and found that they differed very slightly from whole numbers. Yet a fixed number of protons turned out to have different masses if they were first considered separately and then as part of a nucleus.

Using modern standards, the mass of a proton is 1.007825. Twelve separate protons would have a total mass of twelve times that, or 12.0939. On the other hand, if the 12 protons are packed together into a carbon-12 nucleus, the mass is 12 so that the mass of the individual protons is 1.000000 apiece. What happens to this difference of 0.007825 between the proton in isolation and the proton as part of a carbon-12 nucleus?

According to Einstein's special theory of relativity, the missing mass would have to appear in the form of energy. If 12 hydrogen nuclei plus 6 electrons are packed together to form a carbon nucleus, a considerable quantity of energy would have to be given off.

In general, Aston found that as one went on to more and more complicated nuclei, a larger fraction of the mass would have to appear as energy until it reached a maximum in the neighborhood of iron.

Iron-56, the most common of the iron isotopes, has a mass number of 55.9349. Each of its 56 protons, therefore, has a mass of 0.9988.

For nuclei more complicated than those of iron, the protons in the nucleus begin to grow more massive again. Uranium-238 nuclei, for instance, have a mass of 238.0506, so that each of the 238 protons they contain has a mass of 1.0002.

This demonstration that energy was released in any shift away from either extreme of the list of atoms according to atomic number fits the case of radioactivity, where very massive nuclei break down to somewhat less massive ones.

Consider that uranium-238 gives up 8 alpha particles and 6 beta particles to become lead-206. The uranium-238 nucleus has a mass of 238.0506; each alpha particle has one of 4.0026 for a total of 32.0208; each beta particle has a mass of 0.00154 for a total of 0.00924; and the lead-206 nucleus has one of 205.9745.

This means that the uranium-238 nucleus changes into 8 alpha particles, 6 beta particles, and a lead-206 nucleus . The starting mass is 0.0461 greater than the final mass and it is this missing mass that has been converted into energy and is responsible for the gamma rays and for the velocity with which alpha particles and beta particles are discharged.

Nuclear Bombardment

Once scientists realized that there was energy which became available when one kind of nucleus was changed into another, an important question arose as to whether such a change could be brought about and regulated by man and whether this might not be made the source of useful power of a kind and amount undreamed of earlier.

Chemical energy was easy to initiate and control, since that involved the shifts of electrons on the outskirts of the atoms. Raising the temperature of a system, for instance, caused atoms to move more quickly and smash against each other harder, and that in itself was sufficient to force electrons to shift and to initiate a chemical reaction that would not take place at lower temperatures.

To shift the protons within the nucleus and make nuclear energy available was a harder problem by far. The particles involved were much more massive than electrons and correspondingly harder to move. What's more, they were buried deep within the atom. No temperatures available to the physicists of the 1920s could force atoms to smash together hard enough to reach and shake the nucleus.

In fact, the only objects that were known to reach the nucleus were speeding subatomic particles. As early as 1906, for instance, Rutherford had used the speeding alpha particles given off by a radioactive substance to bombard matter and to show that sometimes these alpha particles were deflected by atomic nuclei. It was, in fact, by such an experiment that he first demonstrated the existence of such nuclei.

Rutherford had continued his experiments with bombardment. An alpha particle striking a nucleus would knock it free of the atom to which it belonged and send it shooting forward . The nucleus that shot ahead would strike a film of chemical that scintillated under the impact. In a rough way, one could tell the kind of nucleus that struck from the nature of the sparkling.

In 1919 Rutherford bombarded nitrogen gas with alpha particles and found that he obtained the kind of sparkling he associated with the bombardment of hydrogen gas. When he bombarded hydrogen, the alpha particles struck hydrogen nuclei and shot them forward. To get hydrogen-sparkling out of the bombardment of nitrogen, Rutherford felt, he must have knocked protons out of the nitrogen nuclei. Indeed, as was later found, he had converted nitrogen nuclei into oxygen nuclei.

This was the first time in history that the atomic nucleus was altered by deliberate human act.

Rutherford continued his experiments and by 1924 had shown that alpha particles could be used to knock protons out of the nuclei of almost all elements up to potassium .

There were, however, limitations to the use of natural alpha particles as the bombarding agent.

First, the alpha particles used in bombardment were positively charged and so were the atomic nuclei. This meant that the alpha particles and the atomic nuclei repelled each other and much of the energy of the alpha particles was used in overcoming the repulsion. For more and more massive nuclei, the positive charge grew higher and the repulsion stronger until for elements beyond potassium, no collision could be forced, even with the most energetic naturally occurring alpha particles.

Second, the alpha particles that are sprayed toward the target cannot be aimed directly at the nuclei. An alpha particle strikes a nucleus only if, by chance, they come together. The nuclei that serve as their targets are so unimaginably small that most of the bombarding particles are sure to miss. In Rutherford's first bombardment of nitrogen, it was calculated that only 1 alpha particle out of 300,000 managed to strike a nitrogen nucleus.

The result of these considerations is clear. There is energy to be gained out of nuclear reactions, but there is also energy that must be expended to cause these nuclear reactions. In the case of nuclear bombardment by subatomic particles , the energy expended seems to be many times the energy to be extracted. This is because so many subatomic particles use up their energy in ionizing atoms, knocking electrons away, and never initiate nuclear reactions at all.

It was as though the only way you could light a candle would be to strike 300,000 matches, one after the other. If that were so, candles would be impractical.

This came about first in 1934, when a French husband-and-wife team of physicists, Fr?d?ric Joliot-Curie and Ir?ne Joliot-Curie were bombarding aluminum-27 with alpha particles. The result was to combine part of the alpha particle with the aluminum-27 nucleus to form a new nucleus with an atomic number two units higher--15--and a mass number three units higher--30.

The element with atomic number 15 is phosphorus so that phosphorus-30 was formed. The only isotope of phosphorus that occurs in nature, however, is phosphorus-31. Phosphorus-30 was the first man-made nucleus--the first to be manufactured by nuclear reactions in the laboratory.

The reason phosphorus-30 did not occur in nature was that its energy content was too high to allow it to be stable. Its energy content drained away through the emission of particles that allowed the nucleus to change over into a stable one, silicon-30 . This was an example of "artificial radioactivity".

Since 1934, over a thousand kinds of nuclei that do not occur in nature have been formed in the laboratory through various kinds of bombardment-induced nuclear reactions. Every single one of them proved to be radioactive.

Particle Accelerators

Was there nothing that could be done to make nuclear bombardment more efficient and increase the chance of obtaining useful energy out of nuclear reactions?

In 1928 the Russian-American physicist George Gamow suggested that protons might be used as bombarding agents in place of alpha particles. Protons were only one-fourth as massive as alpha particles and the collision might be correspondingly less effective; on the other hand, protons had only half the positive charge of alpha particles and would not be as strongly repelled by the nuclei. Then, too, protons were much more easily available than alpha particles. To get a supply of protons one only had to ionize the very common hydrogen atoms, i.e., get rid of the single electron of the hydrogen atom, and a single proton is left.

Of course, protons obtained by the ionization of hydrogen atoms have very little energy, but could energy be imparted to them? Protons carry a positive charge and a force can therefore be exerted upon them by an electric or magnetic field. In a device that makes use of such fields, protons can be accelerated , and thus gain more and more energy. In the end, if enough energy is gained, the proton could do more damage than the alpha particle, despite the former's smaller mass. Combine that with the smaller repulsion involved and the greater ease of obtaining protons--and the weight of convenience and usefulness would swing far in the direction of the proton.

Physicists began to try to design "particle accelerators" and the first practical device of this sort was produced in 1929 by the two British physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton . Their device, called an "electrostatic accelerator", produced protons that were sufficiently energetic to initiate nuclear reactions. In 1931 they used their accelerated protons to disrupt the nucleus of lithium-7. It was the first nuclear reaction to be brought about by man-made bombarding particles.

Other types of particle accelerators were also being developed at this time. The most famous was the one built in 1930 by the American physicist Ernest Orlando Lawrence . In this device a magnet was used to make the protons move in gradually expanding circles, gaining energy with each lap until they finally moved out beyond the influence of the magnet and then hurtled out of the instrument in a straight line at maximum energy. This instrument was called a "cyclotron".

The cyclotron was rapidly improved, using larger magnets and increasingly sophisticated design. There are now, at this time of writing, "proton synchrotrons" that produce particles with over a million times the energy of those produced by Lawrence's first cyclotron. Of course, the first cyclotron was only a quarter of a meter wide, while the largest today has a diameter of some 2000 meters.

As particle accelerators grew larger, more efficient, and more powerful, they became ever more useful in studying the structure of the nucleus and the nature of the subatomic particles themselves. They did not serve, however, to bring the dream of useful nuclear energy any closer. Though they brought about the liberation of vastly more nuclear energy than Rutherford's initial bombardments could, they also consumed a great deal more energy in the process.

It is not surprising that Rutherford, the pioneer in nuclear bombardment, was pessimistic. To the end of his days he maintained that it would be forever impossible to tap the energy of the nucleus for use by man. Hopes that "nuclear power" might some day run the world's industries were, in his view, an idle dream.

THE NEUTRON

Nuclear Spin

What Rutherford did not take into account were the consequences of a completely new type of nuclear bombardment involving a type of particle unknown in the 1920s .

The beginnings of the new path came about through the reluctant realization that there was a flaw in the apparently firmly grounded proton-electron picture of nuclear structure.

The flaw involved the "nuclear spin". In 1924 the Austrian physicist Wolfgang Pauli worked out a theory that treated protons and electrons as though they were spinning on their axes. This spin could be in either direction . Quantum theory has shown that a natural unit exists for what is called the angular momentum of this spin. Measured in terms of this natural unit of spin, the proton and the electron have spin 1/2 . If the particle spun in one direction it was + 1/2 , if in the other it was - 1/2 .

When subatomic particles came together to form an atomic nucleus, each kept its original spin, and the nuclear spin was then equal to the total angular momentum of the individual particles that made it up.

For instance, suppose the helium nucleus is made up of 4 protons and 2 electrons, as was thought in the 1920s. Of the 4 protons, suppose that two had a spin of + 1/2 and two of - 1/2 . Suppose also that of the 2 electrons, one had a spin of + 1/2 and one of - 1/2 . All the spins would cancel each other. The total angular momentum would be zero.

Of course, it is also possible that all 6 particles were spinning in the same direction; all + 1/2 or all - 1/2 . In that case the nuclear spin would be 3, either in one direction or the other. If 5 particles were spinning in one direction and 1 in the other, then the total spin would be 2, in one direction or the other.

In short if you have an even number of particles in a nucleus, each with a spin of + 1/2 or - 1/2 , then the total spin is either zero or a whole number, no matter what combination of positive and negative spins you choose.

Consequently, if one measures the spin of a particular atomic nucleus one can tell at once whether that nucleus contains an even number of particles or an odd number.

This quickly raised a problem. The nuclear spin of the common isotope, nitrogen-14, was measured accurately over and over again and turned out to be 1. There seemed no doubt about that and it could therefore be concluded that there were an even number of particles in the nitrogen-14 nucleus.

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