MadSci Network: Physics

Re: Did Enrico Fermi ran into any problems discovering the chain reaction?

Date: Sat Dec 5 14:00:54 1998
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 911770971.Ph

Hi Bruce,

That is an interesting question.  A historian might provide you with a 
better answer, but here goes.  Sir James Chadwick discovered the neutron 
in 1932.  In the following years Fermi studied the effects of neutrons on 
elements.  The neutrons emitted by radioactive decay tend to have a lot of 
energy, which means they are moving at high velocity.  In 1934 Fermi 
discovered that neutrons could be slowed down or moderated by passing them 
through light elements (low atomic number) such as carbon.  Fermi found 
that the slow or thermal neutrons produced reactions more frequently than 
fast neutrons.  Slow neutrons were essential to producing a chain 
reaction.  The chain reaction occurs because each atom of uranium that 
splits produces two or more neutrons.  Each of the neutrons produced by 
the first reaction can then start another reaction.  Physicist later found 
ways to use fast neutrons in reactions, which enabled the development of 
the fast breeder reactor.

Enrico Fermi received the Nobel Prize in physics in 1938 for his work on 
neutron reactions.  The World Book encyclopedia states that Fermi believed 
the neutrons were absorbed by the uranium atoms to produce elements 
heavier than uranium.  In 1938 Lise Meitner and Otto Frisch showed that 
the uranium atom had been split, and named the process nuclear fission. 
The discovery that the reaction was fission (splitting) accompanied with 
the production of excess neutrons led to the realization that fission 
could be accelerated, and controlled, with a chain reaction by using slow 
neutrons.  Had Enrico Fermi recognized that the reactions he produced were 
fission reactions, the development of the chain reaction might have 
occurred several years sooner.

Fermi was correct that the truansuranic elements (atomic number greater 
than 92) can be produced by the capture of slow neutrons.  Bombarding 
uranium-238 with slow neutrons produces plutonium-239.  The capture of the 
neutron by the uranium-238 nucleus forms neptunium-239, which decays by 
beta particle emission to form plutonium-239.

Both the fissioning of uranium-235 and the capture of a neutron to produce 
Plutonuim-239 occur in modern fission reactors which use uranium enriched 
in the 235 isotope for fuel.  

For more information you might try using the keywords Physics and history 
with your web browser.  There a many good web sites devoted to the history 
of science and physics.

Bob Novak
Sr. Process Engineer
Carpenter Technology

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