Talk:Spontaneous fission

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I modified the spontaneous fission rates reported, some of which weren't quite right, based on the data on half-lives and branching ratios provided by Brookhaven National Laboratory in their handy-dandy chart of the nuclides ( I also modified the statement that spontaneous fission can't provide the flux needed for a chain reaction, since, if a critical mass is present, it is possible for an INITIAL spontaneous fission to provide the neutrons to initiate a chain reaction.

The question about a code error below seems already to have been resolved before I got here; I fixed the other question below (explaining what Z and A are, and pointing out that it is possible to have spontaneous fission if Z^2/A>45 and not just if it is approximately equal). ---

Is there a code error? The following appears: \hbox{Z}^2/\hbox{A}\approx45. when a simple number or item or something would be expected. I'm not sure what exactly should be here, or how it should be coded, though.

Mathematical criterion[edit]

It would be nice to explain what Z and what A means in the equation (I guess A is atomic mass and Z is number of protons but I'm not sure). Additionaly it seems to me that if Z^2/A is greater than 45 then spontaneous fission can occur (but again it is my guess).

Mathematical criterion[edit]

It is interesting to note the formula Z2/A ≥ 45, as for Uranium, this doesn't work. Take U-238, which undergoes spontaneous fission. 922 = 8464. Divided by 238, this gives 35.6, which doesn't stack up.

This has been flagged with a citation needed tag. Piperh (talk) 18:17, 9 July 2009 (UTC)
I suspect the formula should be ≤ instead of ≥. The larger the atomic mass, the smaller the term Z2/A, which would mean that for any given element, there would be a maximum atomic mass that could undergo spontaneous fission, which doesn't sound right to me. In fact, for most elements, there are no isotopes that would fit that formula! So, there must be something wrong with that formula XinaNicole (talk) 11:29, 19 April 2011 (UTC)
It really is ≥, but the value is too strict; the only known nuclide that passes 47 is 294Og, which frankly would probably rather not exist; comically, it has only been seen to decay through alpha decay, because it is getting some "benefits" from being near the closed neutron shell at N = 184 (it has 176 neutrons). 250Cm decays primarily through SF, and Z2/A for it is only about 37. Spontaneous fission is kind of a "catastrophic structural failure" of the nucleus; it does not usually happen even in fissile nuclides like 235U, because there is a significant activation energy. The fission products will of course have lower energy than the original nucleus, but not only must the nuclear binding energy be overcome, but also the fragments must be endowed with enough speed that they get far enough away from each other to recover the Coulomb energy.
Do note that for some nuclei, the structural failure is not as catastrophic as my colourful language might lead us to expect. If it were, you would expect the nucleus to break approximately into half. This does sometimes happen in very unstable cases like 258Fm, but in more stable cases like 256Fm the "demolition" is "controlled" and the fragments have some shell structure before separating, resulting in a preference for one of the fragments to be a handful of protons and neutrons above the doubly magic 132Sn, and the other to pick up the rest. But in general, the picture of SF as "structural failure" is a good one. Double sharp (talk) 16:44, 8 April 2017 (UTC)

Atomic fission is a failure rate characteristic of atoms with relation to their size and structure. The 2 competing failure modes are alpha particle emission and fission. The fission possibility occurs in the structural area that is more stable than alpha particle emission prone, but less than the neutron values required for a stable element. This implies the existence of a structural condition in heavier isotopes that permits their being fractured by impacting neutrons. At lesser energy levels, the more probable alpha particle emission occurs such as to get rid of the excess energy content.WFPM (talk) 14:58, 30 April 2011 (UTC)

Rates of spontaneous fission[edit]

I have put a citation needed tag in this section because the number for U-235 disagrees with what I have read elsewhere (althought I don't have all that much faith in other sources). It would be nice to have a reliable source. Man with two legs 09:54, 16 October 2006 (UTC)

Spontaneous fission occurrences[edit]

Spontaneous fission is a random failure rate characteristic of atomic nuclides. Evidently nature more or less indiscriminately produced a lot of them and then, over time, the more stable ones stayed in existence longer than the rest. A measure of the relative stability of each isotope is its halflifetime period, when half of the remaining ones will have decayed. Rather than a mishmash of time period values, the value of the halflifetime period is better understood as the base 10 logsecond halflifetime value, which can only vary from negative values (for less than 1 second) to say a value of 18 or more (the life of the universe). The occurrence of spontaneous fission is noted to occur to nuclides that are close to structural conditions that are stable, such that they don't have individual nucleon condition problems or alpha emission tendency problems, but are nevertheless not structurally enough stable to be able to survive a random variation of conditions that can result in their splitting into fragments.WFPM (talk) 19:06, 8 April 2011 (UTC)

Neutrons per g.s.[edit]

What's that then? — Preceding unsigned comment added by (talk) 10:51, 25 July 2011 (UTC)

I am moderately knowlegeable w.r.t. physics. I have no clue WTF "g.s" is supposed to mean. I wonder why it needs to be said to explain jargon which is OBVIOUSLY not in common usage in an article explaining a technical term to a lay audience. Lame. And oh, gee, you think someone could actually write down specific decay processes as examples? Or would that be too informative? (talk) 22:29, 10 August 2012 (UTC)

Is 92 mass unit?[edit]

Why 92 mass unit? (92Zr), not 58Ni? or 116Sn??? — Preceding unsigned comment added by (talk) 15:15, 15 July 2012 (UTC)


(Z^2)/A>45, but "Nb-93": (41^2)/93=18.0752688<45 — Preceding unsigned comment added by (talk) 15:17, 15 July 2012 (UTC)


I am not sure why this article is NOT classified as a stub. I got here from cluster decay. In that article, it was stated that the decay fragments of S.F. are normally distributed. Yet here, NO mention is made of what the products are. Yet an almost completely USELESS sentence is included that cluster decay is super asymmetric spontaneous fission. Anybody really think giving something a name informs? The claimed distinction between S.F. and C.D. is that C.D. results in the same daughter nuclei every time while S.F. results in a distribution of isotopes. I am (obviously) not qualified to decide the accuracy of these statements. But I can say that the best articles answer the Who, What, Why, When, How, and Where questions; both by inclusion AND by exclusion. After reading this, I still don't know whether the process results in 2 fragments, 3 fragments, a variable number of fragments, a variable number of neutrons, or what. As a minimum it should DEFINE THE PROCESS(es) IT IS in a complete way. (talk) 23:07, 2 March 2013 (UTC)

It can vary, but it is usually 2 fragments, plus a variable amount of neutrons. If there is a third, it is probably an alpha particle.
Perhaps it is not so good to describe cluster decay as a subset of spontaneous fission. Spontaneous fission is more like a complete structral failure of the nucleus; alpha decay and cluster decay just involve small clusters of protons and neutrons tunnelling out of the nucleus. Double sharp (talk) 16:49, 8 April 2017 (UTC)

SF for small nuclides[edit]

According to Isotopes of helium, Helium-8 can undergo fission into Helium-5 and Tritium (and an electron), with a 0.9% probability. I'm not sure what the source for that claim is there. But if it's true, would that not be spontaneous fission, and if so, wouldn't that be noteworthy for the article here? Same holds for Lithium-11. --Thogo 17:40, 23 May 2015 (UTC)

It's fission after the beta decay, which releases a lot of energy, possibly more than these weakly bound nuclei can bear. But it is not really spontaneous because the energy is coming in from some source. Double sharp (talk) 16:50, 8 April 2017 (UTC)