Why do different nuclei need different neutron speed for neutron capture?

  • #1

Main Question or Discussion Point

Why do different nuclei need different neutron speed to achieve fission??

My understanding is that U-235 requires a thermal neutron in order to be likely to cause a fission reaction, however Pu-239 requires the additional energy of a fast neutron in order to cause the nucleus to undergo fission rather than just absorbing the neutron and becoming Pu-240.

I also think that a U-235 nucleus is unlikely to absorb a fast neutron. (is this true?)

Is it true that the strong nuclear force in the vicinity of the nucleons is responsible for the "capture" of a passing neutron? If so why should U-235 and Pu-239 be so different in terms of the likelihood of them capturing fast neutrons?

Also, if a U-235 nucleus did absorb a fast neutron, would it be likely to split?

Thanks for any help.
 
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Answers and Replies

  • #2
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Did you find values for the cross-section of fast neutrons at U-235 and Pu-239? Maybe they are not so different, and the cross-section for thermal neutrons at U-235 is just larger.

Is it true that the strong nuclear force in the vicinity of the nucleons is responsible for the "capture" of a passing neutron?
This is true.
 
  • #3
Bill_K
Science Advisor
Insights Author
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Thermal neutron fission is observed for some even-odd nuclei such as U235 and Pu239. The compound nucleus formed is even-even, which has greater binding energy, and therefore there's plenty of excess energy available to cause instability and result in fission.

Fast neutron fission is observed in some even-even nuclei like U238. In this case the compound nucleus is even-odd and has less binding energy. Meaning at least a certain amount of excitation energy must be supplied by the incoming neutron for fission to occur.

Your statement that Pu239 requires fast neutrons is incorrect.
 
  • #4
Is fast neutron fission possible or just as likely as thermal neutron fission in the even-odd nuclei like U235 and Pu239?
 
  • #5
Did you find values for the cross-section of fast neutrons at U-235 and Pu-239? Maybe they are not so different, and the cross-section for thermal neutrons at U-235 is just larger.
From what I can see for thermal neutrons the cross-sections for fission for the two nuclei are:

U235 - 346 barns
Pu239 - 695 barns

And for fast neutrons the the cross-sections are

U235 - 1.8 barns
Pu239 - 1.7 barns

So could I conclude from these that both nuclei are much more likely to encounter, and therefore undergo fission as a result of a thermal neutron than a fast one?

Why should the speed of the neutron effect the likelihood of a collision? Is this related to the nuclear force?
 
  • #6
mathman
Science Advisor
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Is fast neutron fission possible or just as likely as thermal neutron fission in the even-odd nuclei like U235 and Pu239?
Yes - fast fission is possible. That's how bombs work. Thermal fission is for reactors.
 

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