The fission products are of significance in the fission event for 
three reasons: (1) they are the recipients of most of the energy produced 
in fission by virtue of the kinetic energy imparted to them at the fission 
event (energy which is dissipated as heat); (2) fission products have 
probability for capture of neutrons so that they must ultimately be re- 
moved from the fuel, and (3) because many of them, unstable at birth, 
seek a stable nuclear configuration by the emission of beta particles 
and radiant energy called gamma rays. The distribution and yield of 
isotopes produced by fission is well knowns: isotopes of mass numbers 
72 to 162 are produced with individual yields varying from 6% to less 
than 0.0001% of the total. 
Moderators: The fission of un? , Ppu-??, or y°33 can be used by man 
in two ways: (1) as a weapon, (2) as a controlled, chain reacting "pile" 
for the production of useful power. The fission process yields neutrons 
with energy much greater than the "rest" or "thermal" energy of the 
fissionable or fertile nucleus. Fission can be produced by either "fast" 
neutrons or "slow" (thermal) neutrons, but to date most reactors, except 
the Experimental Breeder Reactor, have used slow neutrons. The energetic 
fission neutrons must be slowed down to match the thermal energy of the 
bombarded nucleus. Because a neutron has no charge, its energy is 
dissipated best by colliding, usually many times, with other particles or 
atoms approximately the same size as the neutron. Materials which have a 
low probability of parasitic neutron capture must be used for this purpose 
to preserve neutrons for fission or for useful capture. The light elements 
such as H, deuterium, carbon, aluminum, and beryllium possess these properties. 
Nuclear Reactor Principle: A nuclear reactor or "pile" is an assembly of 
moderator, fissionable materials, coolant, and in some cases, a fertile 
material arranged in such a geometry that will allow the production of a 
chain reaction. The fissionable material, or fuel, must be contained in 
some way to prevent the escape of fission products. 
DECLASSIFIED 
