Why Are Certain Nuclei Fissionable with Slow Neutrons?

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Discussion Overview

The discussion centers around the fissionability of certain nuclei with slow neutrons, specifically focusing on U233, U235, and Pu239, and the underlying reasons for their fission characteristics. Participants explore theoretical aspects, including the pairing force and binding energy, while also referencing specific isotopes and their properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the pairing force explains why certain nuclei, such as U235, can fission with slow neutrons, as it lowers the ground state energy of the nucleus after neutron absorption.
  • Others mention that U238 is fissionable but not fissile, indicating that it cannot fission with a zero kinetic energy neutron, which is also attributed to the pairing force.
  • One participant points out that Np-237 is fissile despite having an odd number of protons and an even number of neutrons, which raises questions about the pairing force's applicability.
  • A participant expresses interest in a paper discussing binding energy and nucleon interactions, particularly the concept of neutron repulsion, and seeks introductory resources on the topic.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the role of the pairing force in fissionability, and there is no consensus on the implications of Np-237's properties. The discussion remains unresolved with competing ideas about the factors influencing fissionability.

Contextual Notes

Some claims rely on specific definitions of fissionability and fissility, and the discussion includes references to binding energy and nucleon interactions that may not be fully explored or agreed upon.

CRGreathouse
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Can anyone explain this to me? It piqued my interest, especially since I nominally work with radioactive materials (though admittedly, not fissile material) and didn't know where this came from.

"It is not a coincidence, for example, that the three nuclei which are fissionable with slow neutrons, U233, U235, and Pu239, all contain an even number of protons and an odd number of neutrons."
- Shanks, Solved and Unsolved Problems in Number Theory (5th ed.), p. 137
 
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As far as I know, the reason for this is the pairing force. For example when a U235 nucleus absorbs a neutron , it forms (as an intermediate stage before fission) U236. Now U326 is excited by an amount of energy equal to the binding energy of the neutron and its kinetic energy. But since U236 has even number of neutrons , the pairing force will lower its ground state so much which in turn renders the excitation energy due to the nuetron binding energy (without the kinetic energy part) high eneough to exceed the fission activation energy. So this nucleus can fission ,in principle, by absorping a zero kinetic energy neutron. This is actually why U235, U233 ,.. are called fissile.

On the other hand U238 is fissionable but non fissile. This because it cannot fission by a zero kinetic energy neutron. An explation based on the paring force is valid here also
 
Useful nucleus said:
As far as I know, the reason for this is the pairing force.
Useful nucleus,

You are correct. Anyone can look up the semi-empirical mass formula - which actually gives
the binding energy

http://www.phys.washington.edu/users/savage/Class_560/lec560_5/node2.html

http://www.phy.uct.ac.za/courses/phy300w/np/ch1/node22.html

The pairing forces influence the binding energy.

Dr. Gregory Greenman
Physicist
 
Last edited by a moderator:
It's rather surprising then that Np-237 is fissile.

It has it exactly opposite: odd number of protons (93) and an even number of neutrons (144). And yet...
 
i'm by no means well versed, so mentor kindly erase this one if it's utterly out to lunch.

i am still working to digest the thoughts in section 2 of this paper
http://www.omatumr.com/abstracts/jfeinterbetnuc.pdf
about binding energy and nucleons and just what holds them together.
"Cradle of nuclides" in fig 3 looks a lot like the black line of stable nuclides that runs up the middle of "chart of nuclides"

His concept of neutron repulsion peaked my interest, it seems counter-intuitive being as they have no electric charge, but the guy works more from observations in nature than from computer studies.

maybe you know of some introductory (low level) writings on the subject? I'm in over my head.

old jim
 

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