25.3. Nuclear Fission and Fusion http://www.ck12.org
It can be explained by likening the nucleus to a liquid drop, as shown inFigure25.6.
FIGURE 25.6
The liquid-drop model of atomic fission. (A) A neutron is about to be captured by a nucleus. (B) After capture, the
atomic mass is increased by 1 and it is in an excited state. Thermal energy causes the nucleus to expand. (C)
Expansion leads to the “fission” process of splitting. (D) The nucleus splits into two almost equal parts along with
the ejection of three neutrons.In addition to the daughter nuclei, three neutrons are also released in the process, as shown inFigure25.6. Physicists
can detect the tracks of subatomic particles on photographic plates, as well as observe them by a trail of tiny droplets
of water they produce when passing through a container filled with water vaporor alcohol (such containers are known
as “cloud chambers.”)
For more information on the tools physicists use to detect subatomic particles, follow the links below.
Cloud chambers: http://www.youtube.com/watch?v=zxHvqWcTfMk
Bubble chambers: http://www.youtube.com/watch?v=qcUwLH8L5AU
It wasn’t long before physicists realized the consequence of the liberation of additional neutrons within a reaction,
which depended upon neutrons in the first place! It was clear that the release of just a few neutrons could induce an
immense number of uranium nuclei to split in a very short period of time at the ever-increasing rate. The reaction
would be self-sustaining, also known as the chain reaction.
A more detailed explanation of the process is as follows:
- Slow neutrons cause uranium-235
( 235
92 U
)
to split into two nearly equal parts.- In the process, several neutrons are liberated from the parent nucleus, in addition to about 200 MeV of energy.
- The emitted neutrons, in turn, cause more uranium nuclei to split apart and more energy to be released.
- The continual emission of neutrons creates an exponential increase of nuclear fission events known as a chain
reaction. Using that principle, Fermi built the first nuclear reactor at the University of Chicago in December,
1942.
It was also understood that a nuclear chain reaction could be used to build an atomic bomb. During World War II, the