# Talk:Absorption cross section

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## Comment about proposed deletion

I meant to correct the misspelling and neglected to do so because the link from which I created this page was misspelled, namely, at nuclear cross section. From that point of view, the proper thing to do would be to move this page to Absorption cross section and then correct the misspelling at nuclear cross section. And the redirect of absorption cross section to absorption spectrum should be eliminated. I base this argument entirely on the consideration of content. A redirection to absorption spectrum is definitely inappropriate in the context of the probability of neutrons or other particles being absorbed by the nucleus, because absorption spectrum deals entirely with electromagnetic radiation. As applied to photons being absorbed by ozone (for example), "absorption cross section" also refers to a probability of absorption (as with neutrons) rather than the inverse of emission spectrum -- which is the meaning of absorption spectrum.

My preference is to correct the misspelling (and make the move) and the redirection rather than have the proposed deletion. But I will leave the matter open to discussion for now. --Ben Best 16:12, 7 January 2007 (UTC)

I've moved this article to Absorption cross section, per the request at WP:RM. Cheers. -GTBacchus(talk) 18:54, 8 January 2007 (UTC)

## Reverting removal of reference to photons

I am reverting the edit that removed the preference to photons [1] made by Shahbaznihal. In the context of ozone shielding of ultraviolet light I cannot see how the particles would be anything other than photons. Please explain if I am missing something. --Ben Best 16:19, 21 February 2007 (UTC)

## Units

I received the message below on my talk page.

### Absorption Cross Section

Hello, I am writing to inform you that there is an error on the Absorption Cross section page. The dimensional analysis does not add up.

${\displaystyle \sigma =(\mu /\rho )\cdot um_{a}}$

I use SI units for simplicity. using the dimensions:

${\displaystyle \mu }$ = [m-1]
${\displaystyle \rho }$-1 = [m3/g]
u = [g]
Ma= [g/mol] (mol is dimensionless not a problem)

gives the absorption cross section as [gm-2]

Absorption cross section units should be = [m-2]

So this comes from the mistake of using Ma which is not correct, I believe the number should be 1/Avagadros constant i.e. dimensionless or per mol.

As you seem a dab hand at editing and I have no idea. I hope you do not mind me bringing this to your attention with the hope it should be corrected. Such a wonderful resource Wikipedia ChoppyDunbar (talk) 08:45, 18 February 2011 (UTC)

What is the best way to resolve this and make it as clear as possible? --Bduke (Discussion) 08:31, 19 February 2011 (UTC)

I looked this over, and I believe the correct equation should be ${\displaystyle \sigma =(\mu /\rho )m_{a}N_{A}}$ where ${\displaystyle N_{A}}$ is Avogadro's number. The conversion is solely a density conversion from mass per unit volume to molecules per unit volume. The molar mass is needed to go from mass to moles, and Avogradro's constant is needed to go from moles to molecules. The final units will be length squared as expected for a cross section. I'll make that change now, but if I'm incorrect please discuss further here. ronningt (talk) 18:39, 19 February 2011 (UTC)
I think you are right, but cross sections is not something that chemists use very often and I wanted to make sure that the equation used the correct terms. --Bduke (Discussion) 21:15, 19 February 2011 (UTC)

Hello, Yes I make it this too. The reasoning is as follows: Beer Lambert law is I=Io exp (- mu L) or I=Io exp (- sigma N L). mu is the attenuation coefficient (cm-1). Sigma is the absorption cross section (cm2/molecule) N is the density of the absorber (molecules/cm3), L is the distance of interaction (cm). To rewrite N = rho/Na.ma (g/cm3.g) or in words = density/(number of molecules per mole (avagadros's number) times the atomic mass). A long winded way to say that sigma = mu/N = mu Na ma /rho. ChoppyDunbar (talk) 10:08, 21 February 2011 (UTC)

I edited this equation because there was still some unit mismatch. ChoppyDunbar, I believe you are correct up to a point: "mu is the attenuation coefficient (cm-1). Sigma is the absorption cross section (cm2/molecule) N is the density of the absorber (molecules/cm3), " However ${\displaystyle N=\rho /(m_{a}N_{A})}$ causes a violation in units. Avogadro's number is in mol-1. cm-3 ≠ (g/cm3)/( (g/mol mol-1). ${\displaystyle \rho /(m_{a}N_{A})}$ would give units of moles squared per volume, or mol2/cm3. The corrected equation is ${\displaystyle N=(\rho N_{A})/(m_{a})}$ Getting back to sigma, ${\displaystyle \sigma =\mu /N=(\mu /\rho )m_{a}/N_{A}}$ Weimdog (talk) 02:26, 26 February 2011 (UTC)

Agreed, Weimdog. Thanks. ronningt (talk) 14:25, 26 February 2011 (UTC)

### Links to other articles

I feel that this article should link somewhere to the Beer–Lambert law article. — Preceding unsigned comment added by 129.215.139.65 (talk) 10:48, 14 October 2011 (UTC)