Conditions for thermal neutron induced fission

[dtsormpa]

Does anyone know what the necessary conditions are for a nucleus to undergo fission with a thermal neutron? I have found something for the chain reactions, but not very helpful. I want to find out the conditions for ONE nucleus to undergo fission with a thermal neutron.

 

[malawi_glenn]

Well on a very fundamental basis, it has to do with the activation energy and excitation energy. And that is related to if the nuclei you want to fission is an Even-even, or odd-odd or even-odd. But when we talk about the resonances, it gets a more complicated.

I believe that the chapter about fission in Krane's book "introductory nuclear physics" have a good explantion.

 

[JeffKoch]

A particular fission reaction has a neutron energy-dependent cross section, regardless of whether or not the neutron is "thermal". Are you asking for links to these energy-dependent cross sections?

 

[dtsormpa]

A particular fission reaction has a neutron energy-dependent cross section, regardless of whether or not the neutron is "thermal". Are you asking for links to these energy-dependent cross sections?

No, actually. I want to know just about the thermal neutrons and the conditions that a nucleus must follow, in order to fission by it.

 

[Astronuc]

Nuclides that are readily fissionable, of fissile, by thermal neutrons are:
U-233, U-235, Pu-239, and Pu-241, as malawi_glenn indicated.

The condition for fission is that the fissile nucleus absorbs the neutron, just the same condition which is necessary for a neutron of any energy.

Even then, there is stil a finite probability that a gamma-ray will be emitted and the nucleus will simply increase in mass by ~ 1amu.

With the absorption of a thermal neutron, there is ~ 84% chance of fission and 16% chance of gamma-decay and U235 becomes U236.

 

[dtsormpa]

Actually, what I meant by the question, was if the nucleus has a low activation energy, be heavy (A > 210) and has a specific ratio N/Z. It seems that only even N nuclei can fission by thermal neutron. Is that correct?

 

[malawi_glenn]

Actually, what I meant by the question, was if the nucleus has a low activation energy, be heavy (A > 210) and has a specific ratio N/Z. It seems that only even N nuclei can fission by thermal neutron. Is that correct?

And the reason for that is the pairing-term in the semi emperical mass formula.

 

[Astronuc]

Actually, what I meant by the question, was if the nucleus has a low activation energy, be heavy (A > 210) and has a specific ratio N/Z. It seems that only even N nuclei can fission by thermal neutron. Is that correct?

Well, the fissile nuclei have an odd mass number intially, but they form an even numbered nucleus up absorption of a neutron, e.g. U-233 + n -> U-234*, U-235 + n -> U-236*, Pu-239 + n -> Pu-240*, where the * indicates an excited state of the nucleus. The excited nucleus can fission or it can release a gamma-ray, in which case it forms a more stable nucleus. The point is that fissile nuclei are more likely to form an excited nucleus which does fission, as opposed to simply releasing a gamma ray (gamma decay).

As malawi_glenn indicated, there is a forumula for nuclear stability that more or less indicates which nuclei are prone to fission (instability).

One can also look at the binding energy per nucleon for the isotopes of U and Pu.