Implementation of Doppler Broadening in MCNP5
L Bartol*, L DeWerd, University of WI-Madison/ADCL, Madison, WISU-E-T-39 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: To evaluate an error found in the implementation of Doppler broadening (DB) in the incoherent scattering model used in MCNP5.
Methods: Simulations were run with three versions of MCNP5: v1.51, v1.60, and a modified form of v1.60 (v1.60m). All simulations used the MCPLIB04 photon data library. This library presents the electron subshell data for incoherent scattering in probability distribution form. In v1.60m, the source code was altered to sample the electron subshell from a cumulative probability distribution instead. Each version of the code was tested using an identical set of simulations that investigated DB at scattering angles of 15, 30, and 45 degrees. In all cases, the simulation geometry consisted of a 4-mm-thick slab of silicon surrounded by vacuum. A monoenergetic point source emitted photons normal to the slab. For each angle, simulations were run for multiple energies (between 200 and 800 keV). The spectrum of singly-scattered photons at the exit of the slab was scored for each angle within +/-0.01 degrees.
Results: Each version of MCNP5 produced different Doppler-broadened spectra. In v1.51, broadening was modeled for energies below 760, 385, and 260 keV at scattering angles of 15, 30, and 45 degrees, respectively. Above these energy thresholds no broadening occurred. The spectra found using v1.60 and v1.60m exhibited energy broadening for all energy-angle combinations; however, v1.60m, which sampled the electron subshell more accurately, exhibited more broadening than v1.60.
Conclusions: MCNP5 v1.51 does not model energy broadening for all energy-angle combinations. While v1.60 models partial broadening in all cases, it does not appropriately model DB when used with the MCPLIB04 data library. MCNP5 v1.60m appropriately models DB due to the form of the electron subshell data. It is recommended that MCNP5 v1.60 be used with a data library that presents the electron subshell data in cumulative distribution form.