Berlin, Germany (1938-1939)
The English word "atom" derives from the Greek word
"atomon" ("ατομον"),
which means "that which cannot be divided." In 1938, the
scientific community proved the Greek philosophers wrong by dividing
the atom.
Fission, the basis of the atomic bomb, was
discovered in Nazi Germany less
than a year before the beginning of the Second World War. It was
December 1938 when the radiochemists Otto Hahn (above, with Lise
Meitner) and Fritz Strassmann,
while bombarding elements with neutrons
in their Berlin laboratory, made their
unexpected discovery. They found that while the nuclei of most elements
changed somewhat during
neutron bombardment, uranium nuclei changed greatly and broke into two
roughly
equal pieces. They split and became not the new transuranic
elements that
some thought Enrico Fermi had
discovered but radioactive
barium isotopes (barium has the atomic number 56) and other fragments
of the uranium
itself.
The substances Fermi had created in his experiments, that
is, did
more than resemble lighter elements -- they were lighter
elements. The products of the Hahn-Strassmann experiment weighed
less than
that of the original uranium nucleus, and herein lay the primary
significance of
their findings. It folIowed from Albert Einstein's E=mc2 equation that the loss of
mass resulting from the splitting process must have been converted into
energy
in the form of kinetic energy that could in turn be converted into
heat.
Calculations
made by Hahn's former colleague, Lise
Meitner (above, with Otto Hahn), a refugee from Nazism then staying
in Sweden, and her nephew,
Otto Frisch, led to the conclusion that so much energy had been
released that a
previously undiscovered kind of process was at work. Frisch,
borrowing the
term for cell division in biology -- binary fission -- named the
process "fission." Fermi had produced fission in 1934; he had
just not recognized it.
It soon became clear that the
process of fission discovered by Hahn and
Strassmann had
another important characteristic besides the immediate
release of
enormous amounts of energy. This was the emission of
neutrons. The
energy released when fission occurred in uranium caused several
neutrons to
"boil off" the two main fragments as they flew apart. Given the
right set of circumstances, perhaps these secondary neutrons might
collide with
other atoms and release more neutrons, in turn
smashing into other
atoms and, at
the same time, continuously emitting energy. Beginning with a
single
uranium nucleus, fission could not only produce substantial amounts of
energy
but could also lead to a reaction creating ever-increasing amounts of
energy. The possibility of such a "chain
reaction" (left) completely
altered the prospects for releasing the energy stored in the
nucleus. A
controlled self-sustaining reaction could make it possible to generate
a large
amount of energy for heat and power, while an unchecked reaction could
create an explosión 
