# Question about neutron cross sections

I am looking for thermal neutron scattering cross sections (0.025eV) for individual isotopes, having found non isotopic data for most elements in a textbook. The most comprehensive cross section database appears to be http://www.nndc.bnl.gov/sigma/index.jsp?as=238&lib=endfb7.0&nsub=10". Unfortunately I am unable to identify which plots if any could give me the scattering cross section.

Presumably all (n,elastic) collisions must be scattering but n(inelastic) could either be a scattering or absorption event. When the scattering cross section is usually quoted (as typically in textbooks) does that usually include elastic and inelastic collisions? Am I also correct in thinking that neutron scattering data will generally be poor for isotopes with very high absorption cross sections such as boron-10?

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Thanks.

I'm assuming the desired quantity from the site above is the total bound scattering cross section. The value for hydrogen quoted above is 82.02b and appears to be taken from a 1992 publication. However two textbooks (one from 1977, one from 1994) both put the thermal value for hydrogen at 38b. It seems unlikely there would be such a large discrepancy between values of the same parameter.

The link above does not give the neutron energy for the scattering cross sections but I believe that unless there any resonances, it should be lower at higher energies. Indeed the 'epithermal scattering cross section' of hydrogen is given at 20b.

vanesch
Staff Emeritus
Gold Member
Thanks.

I'm assuming the desired quantity from the site above is the total bound scattering cross section. The value for hydrogen quoted above is 82.02b and appears to be taken from a 1992 publication. However two textbooks (one from 1977, one from 1994) both put the thermal value for hydrogen at 38b. It seems unlikely there would be such a large discrepancy between values of the same parameter.

The link above does not give the neutron energy for the scattering cross sections but I believe that unless there any resonances, it should be lower at higher energies. Indeed the 'epithermal scattering cross section' of hydrogen is given at 20b.
Normally, these values are given for 2200 m/s ("thermal" neutrons).

You're right. On the NNDC plot, you find rather 30.14 barn at 0.0253 eV.
I have to say that I'm puzzled too as to why there is this difference.

Ah ok. Do you know whether or not when the scattering cross section is normally quoted, is that the elastic scattering cross section or does it include inelastic scattering events aswell?

vanesch
Staff Emeritus
Gold Member
Ah ok. Do you know whether or not when the scattering cross section is normally quoted, is that the elastic scattering cross section or does it include inelastic scattering events aswell?
At low (thermal) energies, I don't think there are inelastic scattering events.

BTW, don't confuse "inelastic" scattering and "incoherent" scattering.

Normally, these values are given for 2200 m/s ("thermal" neutrons).

You're right. On the NNDC plot, you find rather 30.14 barn at 0.0253 eV.
I have to say that I'm puzzled too as to why there is this difference.
Also curiously looking at the http://www.nndc.bnl.gov/sigma/getInterpreted.jsp?evalid=4208&mf=3&mt=2" [Broken] for hydrogen, the data point for 0.0253eV has a cross section of 20.44b with all the low energy cross sections being in this area. However it appears to be 30.14b in the plot. There is an explanation on the site

why the cross sections from the "interpreted" table are different from those in the plot. Cross section data is kept in 2 sections of the ENDF file, one contains resonance parameters and the other non-resonant cross section at a temperature of 0 Kelvins. The plot shows reconstructed cross sections, that is combining the results of those 2 sections, and then doppler-broadened, at a temperature of 300 Kelvins. The rise of the cross section at the lower energy end is due to the doppler broadening. This rise disappears as one decreases the temperature.
Hope this helps, if not, let me know.
Presumably tabulated values are usually quoted without doppler broadening?

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