# Capture cross-section of neutrons

How can I calculate the capture cross-section of neutrons in a given target material?

In principle, you can get all the information you need on the FLUKA neutron cross sections by setting a printing flag in option LOW-NEUT. From the manual:

``````WHAT(4) = printing flag: from 0.0 to 3.0 increases the amount of
output about cross-sections, kerma factors, etc.
Default: 0.0 (minimum output)
``````

However, the information is given in a form that is difficult to understand unless one is familiar with multigroup neutron transport codes. Anyway, one will get a table of which a few lines are reproduced here:

``````1                     CROSS SECTIONS FOR MEDIA     1
(RESIDUAL NUCLEI INFORMATIONS AVAILABLE)
GROUP    SIGT    SIGST   PNUP   PNABS  GAMGEN NU*FIS   EDEP
DOWNSCATTER MATRIX
barn     barn  (PNEL    PXN   PFISS  PNGAM)  GeV/col
1 5.826E+00 9.287E+00  .0000 1.5939 1.2904  .3886 1.536E-02   .3228
.0079   .0021   .0023   .0020   .0013   .0011   .0035
.......................................................................

26 6.861E+00 6.697E+00  .0000  .9761 1.0609  .0181 1.458E-03
.......................................................................
``````

Explanation of the relevant quantities:

``````Group 1 (the highest): Total cross section (SIGT) = 5.826E+00 barn
"Scattering" cross. s. (SIGST) = 9.287E+00 barn
Probability of Non Absorption (PNABS) = 1.5939

Group 26             : Total cross section (SIGT) = 6.861E+00 barn
"Scattering" cross. s. (SIGST) = 6.697E+00 barn
Probability of Non Absorption (PNABS) = .9761
``````

The data for the first groups will probably look strange (scattering cross section larger than total cross section): the reason is that there is neutron production through (n,xn) reactions (fission is accounted for separately), and here “scattering” means “number of outgoing neutrons times cross section” or also “changing energy group”. Since more than one neutron on average is exiting a collision, the probability of non absorption is larger than 1.

But looking at the lower groups (group 26 has been copied here as an example) one will see that the data make more sense. The absorption cross sections (all of them included) will be = total - scattering = 6.861 - 6.697 = 0.164 barn