Volume 2: Nuclear Bombardment

Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume II, THE STRUCTURE OF THE NUCLEUS: Nuclear Bombardment

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 (“nuclear reactions”) 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.

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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 (like one billiard ball hitting another). The nucleus that shot ahead would strike a film of chemical that scintillated (sparkled) 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 (protons) 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 (atomic number 19).

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.

Man-made transmutation.

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 only way, apparently, in which nuclear reactions can be brought about), 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.

In fact, the most dramatic result of alpha particle bombardment had nothing to do with energy production, but rather the reverse. New nuclei were produced that had more energy than the starting nuclei, so that energy was absorbed by the nuclear reaction rather than given off.

This came about first in 1934, when a French husband-and-wife team of physicists, Frédéric Joliot-Curie (1900-1958) and Irène Joliot-Curie (1897-1956) were bombarding aluminum-27 (atomic number 13) 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 86that 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.

Frédéric and Irène Joliot-Curie

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 (atomic number 14). 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.

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49820/49820-h/49820-h.htm#c21

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