Talk:Neutron

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Former good article nominee Neutron was a Natural sciences good articles nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
June 16, 2012 Good article nominee Not listed
WikiProject Physics (Rated C-class, Top-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 Top  This article has been rated as Top-importance on the project's importance scale.
 

This article has comments here.

Wikipedia Version 1.0 Editorial Team / v0.5 / Vital
WikiProject icon This article has been reviewed by the Version 1.0 Editorial Team.
Taskforce icon
This article has been selected for Version 0.5 and subsequent release versions of Wikipedia.

The EM force in fission[edit]

This sentence was removed: "Ultimately, the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for the energy that makes nuclear reactors or bombs possible." I put it back in because that's what happens. It could be clarified, but ultimately nearly 180 MeV of energy is produced in the fission fragments because they repel each other due to being positively charged. That's not true of where the NEUTRON energy comes from (of course), but it certainly describes where the fission fragment kinetic energy comes from. It's where most of the energy of a fission bomb comes from, in fact. SBHarris 02:47, 11 December 2014 (UTC)

Just to check my physical picture: The nuclear force binds nucleons together much like billiard balls held together by strings. If you cut the strings, the balls will fly apart, but only insofar as the kinetic energy they had within the nuclei. The electromagnetic force is like also connecting these billiard balls with a long-range, compressed, repulsive spring. When the string is cut the balls fly apart from both the kinetic energy they had within the nuclei and from the spring releasing its potential energy. The nuclear force may be strong, but it is only short range and at short range it balances the EM repulsion. The EM force may be weaker, but not so weak at short range, and it stores a great amount of energy in forcing charged billiard balls together.

I've been puzzling over neutron scattering, e.g., neutrons slowed by paraffin. I had viewed them as billiard balls, that is, hard spheres bouncing off one another, but that's not right. A neutron striking a proton must essentially be bound briefly to it, only to fly apart by the kinematics - a neutron scattered to the right, must have passed the proton on the left (or exchanged identities with the proton). Its quite a bizzaro little world...(who's in charge of this place???) Bdushaw (talk) 23:20, 11 December 2014 (UTC)

Related to this recent discussion above, a question: The article notes that neutrons are produced in nuclear fission (c.f. the new figure). Why aren't protons produced, or are they? One would think with their positive charge they would be more inclined to escape the nucleus than neutrons. The only thing I can think of is that the nature of the quantum mechanical system of the nucleus makes them more tightly bound, or more likely to be bound to the fragments from the fission process. Bdushaw (talk) 01:24, 15 December 2014 (UTC)
Proton emission does occur. I'll speculate handwavingly as to why this does not appear to occur frequently (or at all?) during fission (which I'll define as fragmentation with at least two fragments having more than one nucleon): The stable ridge of neutron/proton ratio is very narrow and curves with atomic number, so that the resulting fragments would almost inevitably be neutron-rich. This would suggest that further loosing free proton would be energetically unfavourable. —Quondum 02:37, 15 December 2014 (UTC)
The handwaving argument above sounds good, but (alas) proton emission does occur quite frequently in ternary fission, which happens in a small fraction of fissions both neutron induced and spontaneous. In that case, the 3rd positive particle is most often an alpha, but it can be everything from a proton to an argon. In binary fission, rarely is anything below Z=30 seen.

The processes that drive out delayed protons are quite different from neutrons, of course. The Coulomb potential drives out protons just like alphas, whenever the nuclear binding potential has been penetrated by tunneling, or else surmounted by some kind of "shockwave" from a nuclear breakup. The last is something like the loss of billard balls from the racked group when hit by the cue ball after a "break". Anything can happen as one ball hits another and that one hits one next to it. If you think about it, that's really the only way a neutron ever gets kicked out, as neutrons have every reason to stay and none to leave a nucleus. Near the neutron drip line where there's a huge excess of neutrons, little shocks like beta decay or inverse beta decay can liberate a loosely-bound neutron. But fission fragments are far from either the neutron or proton drip line, so they all need something a lot more energetic.

I don't know if anybody really understands this (I certainly don't), but something like the focused shock wave in a rack of billiard balls must happen, and it piles up on one nucleon and kicks it out. We know that nucleon emission happens very fast after the nucleus is struck by a neutron, or else undergoes the energetic inward collapse of a new proton, after beta-decay of a neutron. The time is only the time it takes the neutron to cross the nucleus and get out. Thus, a new neutron or proton emitted far from the drip-line, comes out immediately, with no waiting, because it's been kicked out.

By contrast, the only time you get (delayed) proton emission with a half life, is near the proton drip line, and it happens much like alpha decay, by tunneling of a particle that wants to escape the Coulomb repulsion anyway. But in fission that's not the mechanism for loss of either single neutrons or protons. They don't tunnel-- they are shoved out by a sort of knock-on shock, and it happens instantly. As for neutrons, there is no true "delayed neutron emission" by any mechanism. The delay for "delayed" neutron emission is beta decay and then the neutron emission follows promptly. The same is true of proton emission in ternary fission, since you are too far from the proton drip line for protons to tunnel with a delay; indeed as pointed out above, these nuclei are all proton-poor, since they are fragments of heavier elements. Thus, fission fragments don't generally undergo positron decay and electron capture and the kinds of things that happen to proton-rich nuclei in other circumstances. Proton emission in fission is like neutron emission in fission-- a far more energetic event than radioactive decay emissions. SBHarris 03:53, 15 December 2014 (UTC)

That's a nice description and presumably fully answers Bdushaw's question. From the premise that there is no mechanism of free neutron emission that does not involve some form of excitation (excess energy), there is no energy decrease in a neutron being emitted: i.e., that cold neutrons would endlessly be absorbed by any nucleus, with the possibility of decay through some other mechanism such as beta decay. This should be testable as the liberated energy of (electron) beta decay (including rest mass energy) being less than that for a free neutron, further minus the energy of forcing another proton into the nucleus. —Quondum 14:41, 15 December 2014 (UTC)
Not really that clear to me, other than its clearly complicated. I suspect the answer may be that if protons want to leave, they likely do so (energetically favorable) in the form of an alpha particle. It does seem clear that individual neutrons break free more readily than individual protons, yes? Bdushaw (talk) 04:23, 17 December 2014 (UTC)
You're right, that bit didn't get a clear answer. I suppose another one would be to figure out why it can be energetically favourable for a neutron to decay into a proton despite the high local positive charge. All rather more complicated than my understanding level. —Quondum 07:04, 17 December 2014 (UTC)

A physics professor I know has waded in on the question of why neutrons are scattered about, but not protons:

I think the tendency for individual, unbound neutrons to be emitted in fission, as opposed to protons, is pretty well explained, qualitatively at least, by the "valley of beta-stability" of nuclei, i.e., the excess of neutrons over protons in stable nuclei, which grows more than linearly with atomic number or mass number. (Specifically, the neutron excess, N - Z, grows approximately proportional to the 5/3 power of A, the total number of nucleons.) Hence, when fission occurs, the resulting fission product nuclei tend to be neutron rich, and, in particular, tend to be unstable against beta decay in which the excess neutrons convert to protons. (This is why nuclear fission reactors are such prolific sources of anti-neutrinos, rather than neutrinos, which are emitted in the reverse process: protons converting to neutrons.) But, for the same reason, the fission process can give rise to slightly more stable nuclei (in terms of the relative proportion of protons and neutrons) with the excess neutrons being emitted as unbound particles, singly or multiply.

The "valley of beta stability" is itself the result of the point you make, that, because of Coulomb repulsion the nucleus should tend to be unstable against breaking up, with the protons flying apart. This is why heavier nuclei increasingly need more of the electrically neutral neutrons to provide sufficient attractive strong nuclear force to keep the nuclei bound. So, for stability, the excess of neutrons over protons has to grow more than linearly with A.

I think that makes sense - those heavy nuclei undergoing fission have many more neutrons than protons. Bdushaw (talk) 22:29, 9 February 2015 (UTC)

It explains, empirically, that excess neutrons are eliminated in association with fission. It does not explain the process of neutron emission, and in particular, where the energy comes from to overcome the nuclear binding force holding the neutrons in place. After all, there is an alternate neutron reduction mechanism: beta decay. Clearly, there is some mechanism that makes it energetically favourable for excess neutrons to be expelled from neutron-rich nuclei, and we are not seeing what this is. At first blush we have a powerful binding force (the nuclear force), and no apparent mechanism of repulsion. I'll make a suggestion: the Pauli exclusion principle ensures that some of the neutrons in neutron-rich nuclei have high momentum and hence kinetic energy, which liberates them.
There is a related observation (really diverging here): it is rather curious that the range of stable nuclides for each atomic number is so very narrow, yet that there the range of atomic numbers over which there are stable nuclides is so large, despite meandering so much, with so many complex mechanisms at play. After all, one would expect that as Coulomb repulsion grows, we'd just get instability. Yet somehow, the nucleus's tolerance for high neutron density paradoxically grows at just the right rate to balance this instability. The "need more of the electrically neutral neutrons to provide sufficient attractive strong nuclear force to keep the nuclei bound" mentioned above doesn't explain it; it merely says that if it isn't there, larger nuclei would be unstable. The whole precarious balance seems kinda weird. —Quondum 00:46, 10 February 2015 (UTC)
Yes weird - our cosmos seems to be built on miraculous chances! I note that the neutron excess after fission is alleviated by two mechanisms: the departure of a neutron, which I assume occurs as you say because it gets sufficient kinetic energy out of the violence of fission, and the beta-decay of the neutron as noted above (why reactors have an abundance of antineutrinos). Bdushaw (talk) 01:34, 10 February 2015 (UTC)
"Weird" and "miraculous". I think you guys are heading in the direction of the anthropic principle. Dirac66 (talk) 02:01, 10 February 2015 (UTC)
Amen, brother... :) (When we start to imbue the article with faith-based science, please correct us appropriately.) Bdushaw (talk) 02:46, 10 February 2015 (UTC)
Yeah, that's not part of my normal vocabulary; I was getting a bit whimsical. My normal term would be "counterintuitive" or "curious". This particular observation doesn't even fit with the anthropic principle, since it seems that would involve tuning more than the limited number of degrees of freedom avaiable.
Back to my earlier statement: I did not mean kinetic energy from the fission event. I meant that when identical fermions are packed too tightly, some of the quantum states are necessarily high-momentum states, and in the neutron-rich nucleus, the Fermi energy may be higher than the binding energy. The neutron pressure could simply be so high that it expels neutrons, without the need for any violent events. See Neutron emission. —Quondum 04:46, 10 February 2015 (UTC)

Archive of talk page[edit]

I've archived this Talk page, as you see, since this seemed a good place/time to do that. Much of the existing discussion was old or related to the Discovery of the Neutron sections, which are now a different article. The section above relates to the Valley of Beta Stability, which I think needs a bit of development in the article (to explain why neutrons, rather than protons, get released during the fission process.) Dearchive other sections as desired, perhaps retaining the chronological order. Bdushaw (talk) 19:43, 9 April 2015 (UTC)