[Illustration: _Enrico Fermi 1901-1954_
Courtesy Chemical and Engineering News]
Physicists welcomed the neutron as a bullet that could strike any
nucleus, unopposed by electric repulsion. During the middle 1930s, a
number of investigators, chief among them the Italian physicist Enrico
Fermi, exposed many different isotopes of the chemical elements to beams
of neutrons to see what would happen.
What usually happened was that the bombarded nuclei would absorb
neutrons, emit alpha, beta, or gamma rays, and change into different
isotopes. The identification of the extremely small quantities of
isotopes produced required the development of a fantastic new branch of
chemistry known as radiochemistry, or, as one chemist put it, “phantom
chemistry.”
In some cases the absorption of a neutron by a nucleus was followed by
the emission of a negative electron (beta particle). This produced an
atom whose nuclear positive charge had been increased by one unit and
which therefore belonged at the next higher place on the periodic table.
Fermi and others then considered the fascinating possibility of doing
the same thing to uranium, the last-known element on the periodic table,
to create previously unknown chemical elements. The results of
bombarding uranium with neutrons turned out to be extremely complex, but
it eventually became clear that “transuranic” elements (those heavier
than uranium) could actually be made in this way.[2]
Some of the complex results of bombarding uranium with neutrons formed
an intriguing puzzle that kept various investigators busy for several
years. In 1939 the German chemists Otto Hahn and Fritz Strassmann and
the physicists Lise Meitner and Otto Frisch were able to announce a
solution. The absorption of a neutron by a certain uranium nucleus
(later shown to be that of the relatively rare isotope uranium-235) can
result in a splitting, or _fission_, of the nucleus into two parts with
separate weights that place them somewhere near the middle of the
periodic table.
[Illustration: _Lise Meitner and Otto Hahn in their laboratory in
the 1930s._
Courtesy Addison-Wesley Publishing Co.]
The announcement of this discovery created quite a stir among physicists
because a nuclear process of this nature must release a very large
amount of energy.
[Illustration: _Scale model of the CP-1 (Chicago Pile No. 1) used by
Enrico Fermi and his associates on December 2, 1942, to achieve the
first self-sustaining nuclear reaction. Alternate layers of
graphite, containing uranium metal and/or uranium oxide, were
separated by layers of solid graphite blocks. Graphite was used to
slow down neutrons to increase the likelihood of fissions._]
The excitement among physicists became even greater when it was realized
that this newly discovered process of fission was accompanied by the
release of several free neutrons from the splitting nucleus. Each new
neutron could, if properly slowed down by a moderating material, cause
another nucleus to split and release more energy and still more
neutrons, and so on, as illustrated in Figure 5. (A moderator is
necessary because fast, newly released neutrons are too readily absorbed
by uranium-238 nuclei, which rarely split.) Apparently all that was
needed to achieve this spectacular kind of a chain reaction was to
assemble enough uranium in one place so that the released neutrons would
have a good chance of finding another ²³⁵U nucleus before escaping from
the pile. The amount of fissionable material required to sustain a chain
reaction is termed the “critical mass.” A team of scientists led by
Fermi achieved the first self-sustaining nuclear reaction on December 2,
1942, under the grandstand at the University of Chicago’s athletic
field. This date is often referred to as the beginning of the Nuclear
Age.
[Illustration: Figure 5 _This diagram shows what happens in a chain
reaction resulting from fission of uranium-235 atoms._]
STRAY NEUTRON
²³⁵U
ORIGINAL FISSION
FISSION FRAGMENTS
One to three neutrons from fission process
A NEUTRON SOMETIMES LOST
²³⁸U
CHANGES TO PLUTONIUM
²³⁵U
ONE NEW FISSION
FISSION FRAGMENT
One to three neutrons again
²³⁵U
²³⁵U
TWO NEW FISSIONS
FISSION FRAGMENTS