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.
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.
[Illustration: _Man-made transmutation._]
Nitrogen-14 (7N,7P) + Helium-4 (2N,2P) (Alpha particle)
→ Oxygen-17 (9N,8P) + Hydrogen-1 (1P)
Neutron=N, Proton=P
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 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.
[Illustration: _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.