DOE OSTI2014
MCNP6, due to the heritage of MCNPX, has always possessed the capability to transport the light ions, such as alpha particles. Two types of competing physics take place during the particle transport: atomic and nuclear physics. Charged particles lose energy to media due to Coulombic interactions. MCNP6 models Coulombic interactions through the use of the continuous slowing down approximation. Stopping powers are calculated at a variety of energies using the Bethe-Bloch formula with Sternheimer and Peierls density effect correction. For energies less than 1.31 MeV/AMU, a low-energy SPAR model is used. When the energy of the particle is high enough, there is a transition from nuclear interactions to nucleon interactions. MCNP6 possess various physics models to account for these types of intra-nuclear cascades. These models are less accurate at low energies. Even though MCNP6 will transport these light ions to low energies (i.e. less than 20 MeV), most of the available model physics in MCNP6, for nuclear interactions, has a suggested minimum energy of ∼20-200 MeV. The result of the poorer prediction at low energies causes important light ion events, such as F-19(a,n)Na-22 from U-234 decay in UF{sub 6}, to produce orders of magnitude fewer neutrons in MCNP6 than should be emitted. As with neutrons, ACE (A Compact ENDF) formatted light ion libraries have been developed from several different evaluations. These libraries possess better physics at low energies where the physics models break down. The TENDL 2012 evaluations possess alpha transport libraries, in ACE format, such that they can easily be used with codes like MCNP6 that read ACE formatted cross sections. As a result, a capability was developed for MCNP6 to read and implement these library cross sections when the light ion reached energies below the library upper energy cutoff. Perfectly modelling an experiment for benchmark comparisons to a code calculation can be tricky. For complex setups, systematic error is always possible; and usually a hefty uncertainty analysis is required to quantify just the geometry setup and material combination choices. For an (a,n) interaction, a thick target yield is defined as the amount of neutrons emitted given that the alpha ranges out in the material. For alpha transport at low energy <10 MeV, in reasonably dense materials (>1-2 g/cc), with a material dimension length larger than a few hundred microns, alpha particles will tend to completely stop and range out in the material. For Li, Be, B, C, N, O, F, Na, Mg and Al thick target yields exist in 200 keV intervals from 1 to 9.8 MeV (most of the elements do have (a,n) thresholds above 1 MeV). Because modelling a thick target yield in MCNP6 is much easier than attempting to account for the complexity of systematic error from a full out experiment, we present here comparisons of (a,n) thick target yields to those calculated in MCNP6 using the TENDL 2012 alpha library evaluations.
61 RADIATION PROTECTION AND DOSIMETRY↗