MCNP/X Form Factor Upgrade for Improved Photon Transport

The angular distribution of scattered photons is incorrect in MCNPX and MCNP5 because the incoherent and coherent form factors are obsolete. The obsolete data affect all photon transport problems with E > 74 keV. Elastic backscatter for E > 105 keV is completely missing. Consequently, a new ACE-format photoatomic data library, tentatively named MCPLIB05 and referred to herein as MCPLIB05T, has been developed for MCNP/X. Data in MCPLIB05T other than form factors are identical to that in its predecessor photoatomic library, MCPLIB04. The new form factor data in MCPLIB05T come directly from ENDF/B-VII (rev. 0) and are in a format incompatible with older versions of MCNP/X. Consequently, a new version of MCNP/X has been developed to identify and use the new MCPLIB05T data and yet retain backward compatibility, including tracking, when MCPLIB04 is used. The NJOY nuclear data processing system is undergoing development to enable future generations of photoatomic data libraries with modern form factor data in the new format.