Jiaxin Wang and Robin P. Gardner
Oct 6th 2011, CEAR at NC State University, Raleigh, NC
Motivation
 It has been a general practice to use MCNP, a computer
code developed and maintained by Los Alamos National
Laboratory, to predict nuclear tool response
 During the past decade, MCNP has been continuously
updated with more computational features and more
recent nuclear cross‐section databases. The available
literature in the public domain on using MCNP has been
more focused on those new computational features.
 The logging industry currently does not have a clear
guideline on what the impacts are of using these new
nuclear cross‐section databases.
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Overview
 To fill this gap, this technical project is to compare
among different versions of MCNP data.
 Elements/isotopes of interest for the well logging
industry are H, He, Li, B, C, N, O, Na, Mg, Al, Si, P, S,
Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Ge, Gd.
 For each element/isotope, MCNP data for gamma and
neutron interactions are to be explored in the energy
range 10 KeV to 20 MeV.
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Photon XS
 There are four photon transport libraries maintained by
X‐5 and distributed with MCNP:
--> MCPLIB (01p), MCPLIB02 (02p), MCPLIB03 (03p) and MCPLIB04 (04p).
 MCPLIB02 = MCPLIB03 (ENDF/B‐IV/89)
 MCPLIB≈ MCPLIB02 (difference exists, if any, in the range
of 10 MeV to 100 MeV or 15 MeV (for Z equal 84, 85, 87, 88,
89, 91, 93).)
 MCPLIB04 was officially released in 2002. The cross section
data are all derived from the ENDF/B‐VI.8
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Photon Examples
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Photon XS Summary
 01p and 02p/03p are confirmed to be identical when energy is less than
10 MeV.
 The Compton scattering cross section data are not significantly
different from each other for all libraries.
 The photoelectric cross section is higher (around 10% or less on
average) in 04p than others for all elements when energy is higher than
750 KeV, and is a little bit lower (less than 1% on average) in 04p than
others for some elements when less than several hundred KeV.
 The pair production cross section is smoother (convex curve versus
straight line) in 04p at the turning point, due to more data points.
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Neutron XS
 The neutron cross section data libraries are more
complicated than photon data in the following aspects:
 1). The neutron data are associated with isotopes while the photon data
are only associated with elements.
Eg. silicon, natural abundance silicon and Si-28, Si-29 and Si-30
 2). Each isotope has a set of different data libraries to be evaluated.
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Neutron XS (cont.)
 3). The most difficult and most important, different combinations of
these data libraries might be suitable for different situation.
 For example, silicon dioxide (SiO2), there are 4 (natural silicon) times
14 (Oxygen-16) different kinds of combination of neutron continuous
energy cross section. If examining all, the total combination of cross
section sets for elements of interests is astronomical figures.
 4). There are a lot of neutron reactions.
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Methodology
 Among all cross sections, total neutron cross section (MT=1),
neutron absorption cross section (MT=101, sum of MT=102-117),
neutron capture cross section (MT=102), and total photon
production cross section (MT=202) are of greatest interests.
 Only continuous energy cross section data at room temperature (both
293.6K and 300K are considered room temperature) are evaluated in
this report.
 Only the isotopes of most interest have been evaluated.
 Compare these cross section sets for isotopes of interest to try to reduce
the total number of combinations that are necessary
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Methodology (cont.)
 For elements whose cross section sets are identical, one cross section
set will be picked randomly, and will not be changed in future
evaluation.
 For elements whose cross section sets have slight differences, one cross
section set will be selected on a temporary basis.
 For elements whose cross section sets have large differences, the best
cross section sets will needed to be determined case by case through
comparing their corresponding simulated spectra with measured
spectra.
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Neutron examples
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Neutron examples (cont.)
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Neutron examples (cont.)
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Neutron examples (cont.)
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Neutron XS Summary
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Neutron XS Summary (cont.)
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Neutron XS Summary (cont.)
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Future Work
 Origin of the difference in neutron cross section.
 Result of the difference in neutron cross section in
different applications.
 Most suitable neutron cross section in different
applications.
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Thank you!
Questions and comments?
The authors are also grateful for the financial support of CEAR through
the Associates Program for Nuclear Techniques in Oil Well Logging
presently supported by Baker Hughes, Weatherford, EXXON Mobil,
Halliburton, Pathfinder, and Los Alamos National Laboratory
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