Talk:Neutron cross section

very bad article
The definitions are unclear, the statements are misleading. I would remove the contents and redirect to a more general article, like nuclear data.


 * Agree. Truly bad. I'm for removal. If there is anything useful in there, it should be transfered to the nuclear cross section stub.


 * As wonderful and deep a topic neutron cross sections are (no joking), I have to agree here. The decay information does not belong on this page, and any mechanisms of specific behavior of the cross sections is better presented in the nuclear cross section article, critical mass article, etc. ... How do you move/merge/delete pages?  THaskin (talk) 06:01, 20 August 2008 (UTC)


 * There may be a case of merging some content elsewhere, however the subject of this article is a topic in its own right relevant, for example, in neutron scattering science which forms a very significant field of condensed matter research. Polyamorph (talk) 19:53, 13 May 2010 (UTC)

What is needed
I think what is needed here is a table of neutron cross sections. In particular zirconium, and other nuclear structural materials should be discussed. -G (talk) 14:11, 21 October 2009 (UTC)
 * Agreed. A couple of sources are given at the end of the article, but there are better ones, especially for non-thermal neutrons. --JWB (talk) 21:14, 21 October 2009 (UTC)

Table of neutron cross sections
Hi, unfortunately I have had to remove the table of neutron cross-sections. This is because the source http://environmentalchemistry.com/yogi/periodic/crosssection.html clearly states that the information cannot be reproduced with the following notice: NOTICE: While linking to articles is encouraged, OUR ARTICLES MAY NOT BE COPIED TO OR REPUBLISHED ON ANOTHER WEBSITE UNDER ANY CIRCUMSTANCES.

I think it is very useful to provide a table of neutron scattering cross sections but obviously we cannot use this source because that would be a clear copyright violation. However, we could use a different source. Sears V.F. Neutron News 3 (1992) 26-37 for example is a commonly used reference in the neutron scattering community. It also provides error values on the scattering lengths / cross sections which the previous table did not. Polyamorph (talk) 19:49, 13 May 2010 (UTC)

IAEA seems to have many tables, and allows their use with appropriate attribution, and as long as there is no charge for the use. Gah4 (talk) 21:12, 12 November 2016 (UTC)

preservation of momentum
Cross section absorbtion or scattering is the interaction of atomic nuclear wave function and wave function of neutron. Cross sections vary with fast neutrons and slow neutrons.

The successful fusion of one particle and another particle generates particles and radiation which preserves momentum and energy (although sometimes it remains in a higher energy metastable state). It is exceedingly rare for a reaction not to emit a particle in addition to the original target particle. Boron-10 absorbs a neutron (and its energy and momentum) and excess energy in the Boron-11 metastable state decays it into Lithium-7 and Alpha particle.

Most of this article is irrelevant to cross section, should go to nuclear decay.

In the stellar environment H+H=2He is reversible until it (very rarely) decays to Deuterium. Same with D+H=3He. These Solar reactions are very very very rare. The Sun however is very very very big.

Not mentioned that before star burns Helium, it enters carbon-cycle which is more efficient and burns Hydrogen one proton at a time. Shjacks45 (talk) 01:08, 24 July 2010 (UTC)
 * Absorption usually means emits a gamma, which otherwise doesn't change the nuclear configuration. I am not sure what the table counts as scattering. Gah4 (talk) 13:34, 23 December 2021 (UTC)
 * Absorption usually means emits a gamma, which otherwise doesn't change the nuclear configuration. I am not sure what the table counts as scattering. Gah4 (talk) 13:34, 23 December 2021 (UTC)

Major improvement
I tried to compile all the remarks and added some new sections recently. it results in a major change (and I hope improvement) of the article. Belromain (talk) 07:45, 3 November 2011 (UTC)


 * Great job, thanks! Polyamorph (talk) 07:55, 3 November 2011 (UTC)

Janis
I'm not familiar with the Janis software and after trying it out for a bit I'm lost on how to get the Scattering/Capture/Fission numbers. Could someone with practice please add data for fuel isotopes 232Th and 233U? -- Limulus (talk) 05:46, 22 December 2013 (UTC)
 * Th232 is not fissile, but fertile, breeding U233 the same way U238 breeds Pu239. Otherwise for U233 they are 46 and 530, absorption and fission, respectively. Gah4 (talk) 13:37, 23 December 2021 (UTC)
 * Th232 is not fissile, but fertile, breeding U233 the same way U238 breeds Pu239. Otherwise for U233 they are 46 and 530, absorption and fission, respectively. Gah4 (talk) 13:37, 23 December 2021 (UTC)

Remove "Common light element moderators, reflectors and absorbers.svg" picture
Remove "Common light element moderators, reflectors and absorbers.svg" picture for inaccuracy. Looks like its supposed to be two graphs but half the information is missing (Legend confusing). Shjacks45 (talk) 15:12, 9 January 2014 (UTC)

Nuclides vs. isotopes.
It seems that the right name is nuclide, while people are more used to isotope. I suggest using nuclide throughout the article, with an explanation at the top explaining the difference. Gah4 (talk) 21:16, 12 November 2016 (UTC)

External links modified (February 2018)
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Puzzled by need fo modulator
I had read the article on the "Demon core" (subcritical mass, killed a bunch of people when set near graphite and again when placed near a neutron reflector.) However the critical mass for nucear weapons is similar to reactor critical mass (vis a vis enrichment percentage) however atomic weapons don't have moderation so all fissions in weapons are by fast neutrons? TaylorLeem (talk) 01:38, 16 June 2020 (UTC)
 * It seems that the idea was to get very close, but not critical. Designed for a bomb, it should still be subcritical with usual reflector and temper around it. In the tests, some reflectors were already placed such that only a little more was needed. Anything that will reflect neutron, which includes graphite and human hands, could be enough to do it. Moderators do increase the fission cross-section, which still make the neutrons at least a little more effective. Gah4 (talk) 05:04, 16 June 2020 (UTC)
 * It seems that the idea was to get very close, but not critical. Designed for a bomb, it should still be subcritical with usual reflector and temper around it. In the tests, some reflectors were already placed such that only a little more was needed. Anything that will reflect neutron, which includes graphite and human hands, could be enough to do it. Moderators do increase the fission cross-section, which still make the neutrons at least a little more effective. Gah4 (talk) 05:04, 16 June 2020 (UTC)


 * Moderators and neutron reflectors lower the critical mass (and that's what happened with the demon core, more than once). If the fissionable mass in a nuclear reactor is similar to the fissionable mass in a nuclear weapon, the reason is due to geometry: reactors tend to be much larger than nuclear weapon cores (where the critical mass is also lowered by compressing the core).  Anton Ertl (talk) 18:47, 16 December 2021 (UTC)

Tables
The IAEA has some tables on neutron cross section, one for actinides and the other for fission products. I have programs to convert their HTML into Wikitables, if they are useful here. (And before I forget how to do it, would be better.) Gah4 (talk) 01:29, 23 June 2021 (UTC)
 * Any thoughts on more tables? I can probably still remember how to make them. Gah4 (talk) 18:01, 21 March 2023 (UTC)
 * Any thoughts on more tables? I can probably still remember how to make them. Gah4 (talk) 18:01, 21 March 2023 (UTC)

Fast Cross-Sections of U238
The table gives the capture cross-section for fast neutrons as 0.07barn and the fission cross-section as 0.3barn. Maybe my understanding of these values is wrong, but it seems to me that this would mean that U238 would fission with ~80% probability with fast neutrons, whereas elsewhere on Wikipedia I read 10%. The 10% number looks more plausible, because 80% would make it pretty likely that you can reach a critical mass with unmoderated U238. Is my understanding of cross-sections wrong, or are the cross-section numbers for U238 in the table wrong? Anton Ertl (talk) 18:37, 16 December 2021 (UTC)
 * You can't ask the question without giving an energy. U238 fission neutrons have an energy range, where the upper energy is enough for fission, but not so many have that energy. And even if they do, if they hit anything else first, they will lose too much energy. As well as I know, the U238 tamper in many fission bombs supplies maybe 10% of the yield. Gah4 (talk) 06:27, 17 December 2021 (UTC)
 * The fast numbers in the table are given "using the uranium-235 fission spectrum". I expect that the neutron spectrum for U238 fission is similar (correct?).  The scattering cross-section is given as 5 barns, i.e., 14 times as much as the sum of the other cross-sections, so in pure U238 a neutron will hit on average 14 U238 nuclei before either fissioning one or being captured.  Given the mass disparity, these 14 hits do not slow the neutrons much (correct?). Anton Ertl (talk) 22:46, 21 December 2021 (UTC)
 * I am trying to get the tables from IAEA, but it seems that this combination is not usual. This is the one I have so far. From the Los Alamos Primer: 3/4 of the neutrons are above threshold. 1/4 of those escape being slowed down below 1MeV. So, only 3/16 of them cause fission. Times 2.2 neutrons/fission, it isn't enough. Gah4 (talk) 14:41, 22 December 2021 (UTC)
 * Thanks, the Los Alamos Primer indicates that I was wrong in assuming that the slowdown from scattering does not make a big difference (the IAEA table seems to be about thermal neutrons); the slowdown makes the decisive difference; in addition, U238 absorbs too many neutrons to be a moderator good enough to sustain a chain reaction in natural or low-enriched Uranium. Anton Ertl (talk) 14:00, 27 December 2021 (UTC)
 * I haven't thought about this so much lately. I was looking for a good description of resonance integral, but I think it is that there are narrow but strong absorption peaks at some energies. They then integrate over them. Getting the energy down fast reduces the chance of hitting one. There should be, someone, a better description of resonance integral though. Gah4 (talk) 18:11, 27 December 2021 (UTC)
 * I am trying to get the tables from IAEA, but it seems that this combination is not usual. This is the one I have so far. From the Los Alamos Primer: 3/4 of the neutrons are above threshold. 1/4 of those escape being slowed down below 1MeV. So, only 3/16 of them cause fission. Times 2.2 neutrons/fission, it isn't enough. Gah4 (talk) 14:41, 22 December 2021 (UTC)
 * Thanks, the Los Alamos Primer indicates that I was wrong in assuming that the slowdown from scattering does not make a big difference (the IAEA table seems to be about thermal neutrons); the slowdown makes the decisive difference; in addition, U238 absorbs too many neutrons to be a moderator good enough to sustain a chain reaction in natural or low-enriched Uranium. Anton Ertl (talk) 14:00, 27 December 2021 (UTC)
 * I haven't thought about this so much lately. I was looking for a good description of resonance integral, but I think it is that there are narrow but strong absorption peaks at some energies. They then integrate over them. Getting the energy down fast reduces the chance of hitting one. There should be, someone, a better description of resonance integral though. Gah4 (talk) 18:11, 27 December 2021 (UTC)
 * I haven't thought about this so much lately. I was looking for a good description of resonance integral, but I think it is that there are narrow but strong absorption peaks at some energies. They then integrate over them. Getting the energy down fast reduces the chance of hitting one. There should be, someone, a better description of resonance integral though. Gah4 (talk) 18:11, 27 December 2021 (UTC)
 * I haven't thought about this so much lately. I was looking for a good description of resonance integral, but I think it is that there are narrow but strong absorption peaks at some energies. They then integrate over them. Getting the energy down fast reduces the chance of hitting one. There should be, someone, a better description of resonance integral though. Gah4 (talk) 18:11, 27 December 2021 (UTC)

Attenuation coefficient
There is mention in the article, and recent edit summary, about macroscopic cross section. It is usual for cross section to be an area, and what is called macroscopic cross section sounds more like Attenuation coefficient. There are complications in both cases, when they depend on energy, but other than that ... Gah4 (talk) 02:06, 3 July 2022 (UTC)

Nuclear weapon design
There is discussion in Nuclear weapon design on U-233, how well it might work, and whether any bombs were designed or built. Knowing the cross sections would be nice, but they aren't here. Especially the fast neutron cross section. Gah4 (talk) 22:56, 28 July 2023 (UTC)