A number of particle Monte Carlo transport codes are suitable for the production of the response matrices of the atmosphere for energetic particle interactions in the atmosphere. MCNPX, FLUKA and Geant4 are three well known codes widely used by the radiation effects community.

We have evaluated the three codes to compare the predicted secondary particle
spectra and to identify any significant discrepancy between them. In the comparison,
all three codes used the same simple atmospheric model and the same incident
particle spectrum, assumed to be cosmic ray protons for 15/06/1997 as predicted
by the cosmic ray spectra model of MAIRE, which corresponds to the solar minimum
condition. Protons between 100 MeV and 100 GeV were fired at the top (100km)
of the atmosphere following the "cosine-law" angular distribution
(i.e. varying as *sinθcosθ* to the normal to the atmosphere).
Secondary particles crossing the boundary at 15 km were tallied into flux spectra.

The particle fluxes calculated by all three codes are summarised in the table below,

*A summary of the particle fluxes at 15 km altitude predicted
by the three MC codes, using the same atmospheric model and incident cosmic
ray proton spectrum. The calculated fluxes are in agreement within ~30%.*

The calculated particle fluxes are in reasonable agreement, < 30%, for the important particle species over the broad energy range covered in the simulations. The agreement for charged hadrons is notably better, with deviations less than ~ 10%. The neutron spectra are of particular interest in this comparison because of their importance in radiation effects and the fact that complex physical processes are involved in the simulations. In the Figure below are the neutron spectra calculated by the three codes. These are plotted in lethargy format to better reveal the discrepancies. In general the spectra do follow a similar shape, and the integral fluxes are in agreement to within ~ 26 %. However, larger discrepancies, up to a factor of 2, can be detected in some energy bands. For example, the thermal neutron flux calculated by Geant4 is about 50% lower than that predicted by MCNPX, and the MCNPX calculated neutron flux is consistently above that of the other two codes.

*Calculated atmospheric neutron spectra at 15 km altitude: solid line: MCNPX,
dashed line: FLUKA, dotted line: Geant4. The incident cosmic ray proton spectrum
is for solar min with 1 GV rigidity cut-off.
*