MadSci Network: Physics
Query:

Re: Why do neutrons have to be slow to cause fission?

Date: Mon May 10 13:32:28 1999
Posted By: Michael Kay, Staff, Chem, Haz. Mat Mgmt, Health Physics, Nuclear Science, AMBRY, Inc
Area of science: Physics
ID: 925942436.Ph
Message:

I will break my answer into two parts--the formation of the unstable 
nucleus that will undergo fission, and then why it fissions.

Thermal (slow) neutrons are required for fission because the neutron must 
be captured to form the unstable nucleus (U-235 + n --> U-236* or 
Th-228 + n --> Th-229* are examples of neutron capture leaving the product 
nucleus in a high energy state). Fast neutrons will cause nuclear reactions 
such as (n,p) where a fast neutron collides with a nucleus and then a 
proton is emitted. You can also have an (n, 2n) reaction where a fast 
neutron collides with a nucleus and 2 neutrons are emitted. It is also 
possible to cause fission in some nuclides with fast neutrons (Ra-226, 
Th-232, Pa-231, U-238, and Pu-242 are examples) where the neutron energy is 
in the range 0.2 to 1.7 MeV. The reference by Friedlander, et.al. gives 
graphs of the probability of fission as a function of neutron energy. The 
probability of fission for a thermal neutron hitting a nucleus that will 
fission with capture of a thermal neutron is hundreds of times greater than 
that for fission induced by a fast neutron.

The neutron to be captured must be of low enough energy that it can "fall" 
into the potential energy well of the target nucleus and form a compound 
nucleus in an excited state. In Friedlander et.al. there is a discussion of 
the formation of the excited compound nucleus (in the above examples U-236* 
and Th-229*). This is part of the general theory of Formation and Decay of 
the Compound Nucleus as a part of the theory of particle-induced nuclear 
reactions.

The decay of the compound nucleus usually has many possible pathways, in 
the case of U-236*, it can decay by the emission of gamma rays, beta 
particles, alpha particles, and nuclear fission. In this case, fission is 
the most probable method of decay because of the greater stability of the 
product nuclei. The Binding Energy of a nucleus (the difference between the 
mass of A hydrogen atoms and N neutrons and the mass of the nuclide peaks 
at iron (Fe-56 is the nuclide with the highest binding 
energy--equivalently, the greatest difference in mass between 28 Hydrogen 
Atoms plus 28 Neutrons minus the mass of Fe-56. This resultant difference 
is converted to energy (about 941 MeV per atomic mass unit). 

There are two other major effects that enter into why fission occurs: the 
first is the nuclear shell theory which shows that certain nuclei with 
"magic numbers" of protons or neutrons are especially stable (nuclear 
equivalents of inert gases; Z=50 and N=82 are important in fission). The 
second is the liquid-drop model of fission. Large nuclei are not spherical, 
but usually oblate spheriods. The addition of a neutron sets up 
oscillations that can lead to fission. The large amount of energy emitted 
from fission (appx 200 MeV per fission) yielding more stable products than 
alpha or beta decay also drives the reaction. The fission products are also 
excited, but they do not have the energy for fission, so they decay by beta 
and gamma emission.


Ref: Nuclear and Radiochemistry, 3rd Edition by Friedlander, Kennedy, 
Macias, and Miller (Wiley Interscience 1981 ISBN 0-471-86255-X pbk)

The Atomic Nucleus, R.D. Evans, McGraw Hill 1955, 14th printing May 26, 
1972 Library of Congress Catalog Card Number 55-7275. This is considered to 
be the bible of low energy nuclear physics.

This is a long answer, possibly quite complicated, but using the 
references, or other books on Nuclear Chemistry will make it clearer.


Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-1999. All rights reserved.