I Is nuclear fission a quantum fluctuation

[rootone]

Nuclei of unstable isotopes spontaneously fission in a way that is measured in half-life.
So for a particular nucleus at a given time, it is a probability of some amount, thus is a QM fluctuation?
Then what is going on in reactors which in effect modify the nucleus half life by introducing extra neutrons.
Will an atom of a fissile isotope never actually fission if were contained in a field free of neutrons, (and neutrinos, etc)

 

[PeterDonis]

Nuclei of unstable isotopes spontaneously fission in a way that is measured in half-life.

No, they don't. They are radioactive in a way that is measured in half-life. But the term "fission" does not refer to radioactivity. It refers to something else. See below.

what is going on in reactors which in effect modify the nucleus half life by introducing extra neutrons.

The neutrons interact with the nucleus and cause it to fission. There is no half-life associated with this because it is not a spontaneous process; it would not happen in the absence of the extra neutrons.

Will an atom of a fissile isotope never actually fission if were contained in a field free of neutrons, (and neutrinos, etc)

No, it would not.

 

[rootone]

So it's not statistical probability that a fissile nucleus just does it's thing every now and then.
There has to be interaction with other particles of some kind?

 

[PeterDonis]

So it's not statistical probability that a fissile nucleus just does it's thing every now and then.
There has to be interaction with other particles of some kind?

Yes.

 

[Nugatory]

Is that true even of the spontaneous fission that is observed in, for example, californium?

 

[PeterDonis]

Is that true even of the spontaneous fission that is observed in, for example, californium?

For some of the artificial trans-uranic elements, spontaneous fission is actually possible with a non-negligible probability. But none of these are used in reactors, and the term "fissile isotope" is not used (AFAIK) to refer to them.

 

[Nugatory]

For some of the artificial trans-uranic elements, spontaneous fission is actually possible with a non-negligible probability. But none of these are used in reactors, and the term "fissile isotope" is not used (AFAIK) to refer to them.

And as far as the question in the thread title is concerned... we wouldn't describe spontaneous fission as "a quantum fluctuation".

 

[PeterDonis]

we wouldn't describe spontaneous fission as "a quantum fluctuation".

I wouldn't, but the OP might have simply meant "a quantum mechanical effect". Although even that is not cut and dried, since some of the nuclear models that are used to predict which nuclei are fissile (and which can spontaneously fission) are not really quantum models (the liquid drop model, for example).

 

[jtbell]

So it's not statistical probability that a fissile nucleus just does it's thing every now and then.
There has to be interaction with other particles of some kind?

Yes.

Note, however, that there is statistical probability involved in the interaction between a fissile nucleus and a neutron that passes by. This is where the concept of cross section comes in. It's not a geometrical concept, even though it has units of area and has a geometrical analogue. It's a probabilistic concept.

 

[PeterDonis]

there is statistical probability involved in the interaction between a fissile nucleus and a neutron that passes by. This is where the concept of cross section comes in

Hm, good point. Even here, though, I don't know that the models used to predict cross sections are quantum models. The ones I remember from nuclear engineering classes in college were basically classical.

 

[Jilang]

Isn't it a tunnelling problem?

 

[PeterDonis]

Isn't it a tunnelling problem?

Theoretically I suppose you could try modeling it that way; I just don't know that anyone has ever been able to actually do that and make predictions that match what we actually see in experiments.

 

[fresh_42]

Isn't it a tunnelling problem?

Theoretically I suppose you could try modeling it that way; I just don't know that anyone has ever been able to actually do that and make predictions that match what we actually see in experiments.

This made me curious for I also had tunneling in mind which probably dates back to schooldays. Therefore I had a closer look into my textbook about nuclear physics and this starts right away with nuclei wave functions for both forms of decays (spontaneous and due to excitation) and exposes why the transition probabilities are well-defined.

 

[Jilang]

It's the way I always understood it.
http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/alptun2.html#c1

 

[PeterDonis]

It's the way I always understood it.

This is for radioactive decay. I thought you were asking if fission could be modeled this way.

 

[PeterDonis]

http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/alptun2.html#c1

Also note that this page says the tunneling model does not give close agreement to measured half-lives for all nuclei. Part of the problem could be that the predicted half-life is very sensitive to the shape of the barrier, and the shape of the barrier depends on details of the strong interaction that we don't fully understand.

 

[Jilang]

This is for radioactive decay. I thought you were asking if fission could be modeled this way.

That's fission.

 

[PeterDonis]

That's fission.

No, it isn't. Fission is not the emission of an alpha particle by a nucleus (unless the nucleus were, say, beryllium, but beryllium does not emit alpha particles). Fission is the splitting of a nucleus into two pieces which are fairly close to each other in atomic weight (e.g., U-235 fissions into Kr-92 and Ba-141, plus 3 neutrons). Many fissile nuclei also can emit alpha particles (U-235 does), but that is a different process from fission.

 

[jtbell]

Even here, though, I don't know that the models used to predict cross sections are quantum models.

For interactions involving only fundamental particles, you can calculate cross sections from first principles using QED or weak-interaction theory. When I was in grad school forty years ago IIRC, we did this for electron-electron elastic scattering in our third-semester quantum course which was basically QED. For more complicated interactions, even electron-nucleon or neutrino-nucleon scattering, let alone neutron-nucleus interactions, you have to use phenomenological models and fit stuff to experimental data.

 

[rootone]

Thanks for all the replies and for interesting related subjects I can look further in to.