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THE DISCOVERY OF FISSION Berlin, Germany (1938-1939)
Events >
Atomic Discoveries, 1890s-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 explosion of
huge force.
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Sources and notes for this page.
The text for this page was adapted from, and portions
were taken directly from, the
Office of History and Heritage Resources
publication:
F. G. Gosling,
The Manhattan Project: Making the Atomic Bomb
(DOE/MA-0001; Washington: History Division, Department
of Energy, January 1999), 2. The meaning of the word "atomon" is from the
entry on "Democritus" in
The Concise Oxford Companion to Classical
Literature, edited by M. C. Howatson and Ian Chilvers (New York:
Oxford University Press, 1993), 167-168. The
choice of the word "fission" is discussed in William R.
Shea, "Introduction: From Rutherford to Hahn," in
Otto Hahn and the Rise of Nuclear Physics,
edited by William R. Shea (Dordrecht, Holland: D. Reidel
Publishing Company, 1983), 15. The
fission chain reaction graphic is
adapted from a graphic originally produced by the
Washington State Department of Health; the modifications are original to the Department of
Energy's Office of History and Heritage Resources. The
photograph of Lise Meitner and Otto
Hahn is courtesy the
Department of Energy
(via the
National Archives;
the National Archives identifies the man as Ernest
Rutherford, but other sources agree in labeling this a
picture of Meitner and Hahn in their Kaiser Wilhelm
Institute Laboratory in Berlin). Click
here for more information on the comic book
images.
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