A Chopper Extension to model neutron transport with non-static surfaces and high-speed moving media in MCNPX 2.7

Neutron beamlines frequently include moving components such as neutron choppers. These components are typically wheels that rotate about a fixed axis in order to preferentially transmit or absorb neutrons of the desired energies. Capabilities were developed and implemented into MCNPX to include materials and surfaces in motion. The Chopper Extension implements rotating surfaces and media in MCNPX in order to provide a direct way to include rotating components in models for background calculation purposes.