Confused on (neutron) cross-sections

[Jkay]

Hello, I am looking at scattering cross sections for neutrons.

From basic scattering theory and the partial wave expansion I have understood that the reaction and scattering cross sections are dependant of each other. There can be scattering without reactions but only 1:1 the other way around - ie. scattering cs's are at least as large as that of absorption. There is a graph in blatt & wigner on page 322 showing this

http://books.google.com/books?id=R3...ner&pg=PA322#v=onepage&q=blatt wigner&f=false

BUT when i view data of neutron cross-sections such as
http://www.ncnr.nist.gov/resources/n-lengths/elements/b.html

for boron, i notice the thermal capture cross-section is more than a thousand times larger.

Have i understood something fundamentally wrong, or am I just missing something?

 

[Astronuc]

One may be comparing fast neutrons (or other particles) in the Blatt, Wigner book, with thermal neutrons.

Look at the cross-section of B-10 for neutrons. The absorption cross-section is huge, but decreases with energy. Part of that is the mass of B-10 compared to the neutron, as opposed to a nuclide of an element like Fe or U.

In the MeV range, one is likely to get more scattering than absorption. One also has to look at the aspects of potential scattering vs resonance aborption, and elastic scattering versus inelastic scattering vs absorption.

 

[Jkay]

One may be comparing fast neutrons (or other particles) in the Blatt, Wigner book, with thermal neutrons.

Look at the cross-section of B-10 for neutrons. The absorption cross-section is huge, but decreases with energy. Part of that is the mass of B-10 compared to the neutron, as opposed to a nuclide of an element like Fe or U.

In the MeV range, one is likely to get more scattering than absorption. One also has to look at the aspects of potential scattering vs resonance aborption, and elastic scattering versus inelastic scattering vs absorption.

Hello and thanks for the answer.

B-10 is pretty much resonance free what I've understood. And it has a huge absorption cross-section indeed in the thermal area.

However, I do not understand where the logic of the basic partial wave expansion used in Blatt & Wigner (on the pages before the one i pasted in google books) breaks. As it is truly just scattering theory without any mentions of the speed of the neutron.

Also, is it assumed in the nist data that the scattering cs is independent of energy? (as it only says thermal, for the absorbption one)

I am still quite confused about how this works. The explanation of the 1/v law in the book is also based on the same results which i don't think fit together with data.

 

[Astronuc]

I don't have access to my primary texts at the moment. I'll have to check on the 1/v nuclides.

Meanwhile - from previous discussions:

Physics Forums > Engineering > Nuclear Engineering
Question about neutron cross sections
https://www.physicsforums.com/showthread.php?t=329069

http://www.nndc.bnl.gov/sigma/search.jsp

n,B10 x-sections

nndc has some useful tools.

 

[Jkay]

Hmm, I still can't really get my head around it, both krane, bölatt&wigner and debenedetti state through a partial wave expansion that the scattering cross section has to be at least as big as the reaction one. I don't understand how data can be so totally different then.

I also feel the explanations for the one over v law to be pretty weird in these books.