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Forum: Hadronic Processes
Re: Question Cross Sections used to determine Elastic Scattering (Evan Askanazi)
Date: 24 Jun, 2011
From: Joel Long <Joel Long>

On Thu, 23 Jun 2011 19:51:02 GMT, Evan Askanazi wrote:
> I have been looking at a lot of the Geant4 source code and I have been
> looking at the date files read in by Geant4 to use for Elasitc and other
> kinds of scattering. It is because I need to understand how Geant4 uses
> the data in order to determine the Elastic Scattering Cross Sections of
> neutrons in Aluminium and other variosu materials and thus see how
> Geant4 determines the rate at which Elastic Scattering of neutrons
> occurs in a given material. I have been looking at given data files, for
> example, the elastic cross sectiond that Geant4 uses for Aluminium :
> /geant4.9.4/data/G4NDL3.14/Elastic/CrossSection/13_27_Aluminum . For the
> Elastic Cross Section files, such as this one, there are a set of three
> numbers at the top and then a set of columns below it. For
> 13_27_Aluminum, for example, the data format in geant4 looks like this :
> This is what part of the data format looks like. There is a total of 771
> lines worth of data from the 6 columns so this is not all of it but it
> is part of what the data formal looks like. I was wondering what about
> what the numbers represent. I had thought that for the six columns the
> first, third and fifth represent the energy of the particle, in this
> case the neutron since the neutron is being launched into it. And that
> the 2nd, 4th and 6th represent the cross sections for a given energy.
> Questions are, first off, is that correct, what I assumed above ? What
> are the units of each of the six columns ? Are the units of the energy
> values in eV and the units of cross sections, assuming that is what the
> three sets of columns are, or are different units used ? And also, I
> noticed that there are 3 numbers in the top, 2, 0 and 2306. What do
> those numbers represent ?
> Thanks for any help anyone can provide.

Hello Evan,

As far as I have ever been able to find, the Geant4 data format has never been published, mostly because they don't want it used anywhere else, I think, I have spent some time trying to sort it out for much the same reasons you have :). Here is what I have pieced together:

1. Yes, the columns of data are energy-cross section pairs. 2. Units are eV and barns 3. The third line of those initial three is the number of points in the data set. 4. As I recall, I could never find that the second line of the initial three had any meaning. I dug through the code to find where the data was read in, and it looked like it was read and discarded, with some comment in the code about doing it to allow for a bug on some particular platform. Sorry I can't recall more specifically, I did this a while back. 5. I have a strong suspicion that the first number in the initial three has something to do with the reaction type - you'll note that it is the same for all the elastic data - but it does not appear to be the standard MT reaction values, so I was never entirely sure.

As a side note, if you haven't already, you should look at the readme (and possibly the license information) that comes with the ENDL data. It will tell you about where the data came from, as well as the rules for what you can and can't legally do with it. In principle, it all comes from the evaluated data libraries of various countries (ENDF/B-VI, JENDL, BRONTE, CENDL, etc), but to be perfectly frank, the more I looked at the data, the less confidence I had in it. For example, check out the high energy portion of the O-16 capture cross-section, which magically starts increasing with increased energy, contrary to all the physics I know, as well as the ENDF/B-VII data I compared it to.

As a side note, Al-27 isn't too bad. I've attached a graphical comparison between GNDL3.13 (did this before the upgrade to 3.14, perhaps they fixed Oxygen) and ENDF/B-VII.

Hope this helps! Joel


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1 Question: Re: Cross Sections used to determine Elastic Scattering   (e askan - 26 Jun, 2011)
(_ None: Re: Cross Sections used to determine Elastic Scattering   (Joel Long - 27 Jun, 2011)
(_ Feedback: Re: Cross Sections used to determine Elastic Scattering   (Evan Askanazi - 27 Jun, 2011)
(_ Feedback: Re: Cross Sections used to determine Elastic Scattering   (Joel Long - 28 Jun, 2011)
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