1. Jiaxin Wang and Robin P. Gardner Oct 6 th 2011, CEAR at NC State University, Raleigh, NC

2. 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. 2

3. 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. 3

4. 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 4

5. Photon Examples 5

6. 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. 6

7. 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. 7

8. 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. 8

9. 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 9

10. 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. 10

11. Neutron examples 11

12. Neutron examples (cont.) 12

13. Neutron examples (cont.) 13

14. Neutron examples (cont.) 14

15. Neutron XS Summary 15

16. Neutron XS Summary (cont.) 16

17. Neutron XS Summary (cont.) 17

18. 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. 18

19. Thank you! Questions and comments? 19 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