Talk:Cross section (physics)

Weird Disambiguation
Why is it "not to be confused with Radar scattering cross section"? Radar scattering cross section is just a scattering cross section with radar. Same math, same physics. Radar scattering and light scattering literatures especially borrow from each other extensively. 70.181.67.17 (talk) 07:51, 26 October 2020 (UTC)

Untitled
I made an attempt at basic cleanup by adding an intro paragraph and letting the rest be in sections below. I am not a physicist however, so the efforts of anyone who can make sure I didn't oversimplify or otherwise screw it up would be appreciated. Knotnic 15:14, 23 March 2006 (UTC)

I guess I'll take a stab at this one too after I get the scattering article. The concept of event cross section isn't restricted to nuclear and particle physics. It's a much more generalized idea than that. Tarchon 00:49, 5 April 2006 (UTC)

I was doing my Physics homework and this page popped up. I noticed that it claimed the Nuclear Diameter to be 10&minus;12 cm. Nuclear Diameter is actually measured in fm (femto-meters) which is 10&minus;15 m, or 10&minus;13 cm. Nuclear Radius is 10&minus;12 cm. This unfortunatly makes the next sentence confusing: $$\pi r^2$$ to give the correct size of a barn as 10&minus;24 cm2 or 10&minus;28 m2. Karplusan 03:28, 22 May 2006 (UTC)

I agree with Tarchon. Cross section is a general concept used in many areas and has it's own page to which we can add explanation of the phenomonon, physical meaning, mysteries of, and the like. Nuclear cross section is a specific type of cross section which I think should have it's own page because in the nuclear field this parameter is given a certain symbol (lowercase sigma), and even within that there are several divisions and sub-divisions of nuclear cross sections which could be subheadings on this page or have their own page, such as neutron cross section (which exists as its own page already), absorbtion cross section (separate page merged with general absorbtion spectrum information), gamma cross section... maybe even proton cross sections (which is why I found this page because I was trying to find information on it for a research project). I added a few things to the page on nuclear cross section which are commonly used in this field, and it could definitely be expanded much more. ~Aquamarine19 (~AK 11:02, 14 December 2006 (UTC))

It seems to me, that this definition of the cross section is not exactly what is understood in nuclear physics. If we speak about differential cross section - we mean some fix value (for selected: projectile, target, energy, outgoing angle). It should not depend on detector. Lok for example to hyperphysics: http://hyperphysics.phy-astr.gsu.edu/Hbase/nuclear/nucrea.html The cross section connects number of reactions (per unit time) with the number of incidents ( per unit time per area). Morover, in the first definition should not be dsigma/do but total sigma.

Jaromrax 1:21, 29 January 2008 (CET)

flux
please, specify what do you mean by flux --WISo 16:48, 13 April 2007 (UTC)

barns
Is there a reason there is no mention of barns as a unit of measure in the page? The absence is conspicuous, so I thought I'd ask before editing the article. modify 03:39, 11 September 2015 (UTC)
 * Never mind — I see that they are mentioned in a section by reference to "b". modify 03:47, 11 September 2015 (UTC)

Cross-section or Cross section?
The lemma of this article is cross section, however, both variants are used in the text, each in many places. Which one is correct? --85 [?!] 09:47, 22 April 2016 (UTC)
 * Use hyphen only in adjective form, e.g., "the cross section was big" and "the cross-section diameter was big". fgnievinski (talk) 21:31, 22 April 2016 (UTC)
 * Yes, that's what I'm used to but not what is done in the article. --85 [?!] 18:48, 25 April 2016 (UTC)
 * Be bold. fgnievinski (talk) 21:27, 25 April 2016 (UTC)
 * By the way, some random source on the Internet I found on this topic says that for "common compounds", one would not put a hyphen. Perhaps someone has a more reputable source? --85 [?!] 09:32, 26 October 2017 (UTC)

Three sections needs smoothing
In the sections


 * Attenuation
 * Scattering of light
 * Scattering of light on extended bodies

the same things is said in different ways with some repetition. It would be a good idea to rewrite these three sections more tightly. 178.39.122.125 (talk) —Preceding undated comment added 05:40, 12 September 2016 (UTC)

Add a table of typical cross section values?
It seems to me the article could use a small table that shows some typical values for cross section. Its my understanding the cross section of a barn, 10^-24 cm^2, is huge, while the cross section for a neutrino and proton interacting is 10^-44 cm^2. Would it not be useful to have a table that indicates some various well-known cross sections? Bdushaw (talk) 18:01, 28 August 2018 (UTC)
 * The barn is conveniently close to the geometric cross-section for many nuclei. The high-energy neutron cross section for many nuclei is close to one. For quantum mechanical reasons, slow (thermal) neutrons have a large cross section for many nuclear reactions. That is, much larger than the geometric area would explain. Cross section might also be used in, for example, shooting metal balls at each other, in which case the geometric cross section should apply. For charges particles interacting with matter, it depends on the charge distribution around the atom, and the energy of the incoming particle. One might copy over some values from neutron cross section articles. Gah4 (talk) 21:28, 13 May 2020 (UTC)
 * The barn is conveniently close to the geometric cross-section for many nuclei. The high-energy neutron cross section for many nuclei is close to one. For quantum mechanical reasons, slow (thermal) neutrons have a large cross section for many nuclear reactions. That is, much larger than the geometric area would explain. Cross section might also be used in, for example, shooting metal balls at each other, in which case the geometric cross section should apply. For charges particles interacting with matter, it depends on the charge distribution around the atom, and the energy of the incoming particle. One might copy over some values from neutron cross section articles. Gah4 (talk) 21:28, 13 May 2020 (UTC)

Wrong explanation in "4 Quantum scattering"
Cross section is propability over target particle areal density, not just total number, This is obvious from unit inconsistency. The error in:
 * $$\begin{align}

I_\text{r} &= I_\text{i}N\sigma, \\ \sigma &= \frac{I_\text{r}}{I_\text{i}} \frac{1}{N} \\ &= \text{probability of interaction} \times \frac{1}{N}. \end{align}$$:

logs
Optical absorption is commonly described using a base 10 log. That is, for visible and near visible light. I suspect the section on light is intended to mean EM radiation from far IR through gamma rays, in which case natural logs might be used for many of those. Since there is a lot of work done using visible and near visible light, especially in photography, it might be that base 10 logs are, over all, more common. Gah4 (talk) 19:15, 13 May 2020 (UTC)

Scattering of light
Regarding Scattering of light, and more generally from far-IR through X-rays, and maybe gamma rays, it is often described in density of material, instead of atoms or molecules. That is, in macroscopic terms assuming uniform density. In these cases, an absorption coefficient is used, instead of a cross section. Should the article mention or explain this? Gah4 (talk) 19:29, 13 May 2020 (UTC)
 * In light scattering, "cross section" is typically used for single scattering by discrete scattering centers like particles and even molecules, but it is used quite extensively in that area, and the calculation of attenuation (NOT absorption, the distinction is very important) coefficients from particle distributions and cross-sections is well developed.70.181.67.17 (talk) 04:51, 26 October 2020 (UTC)

Effective area/surface?
Not explained in the article. Fundamental for good understanding.