Monte Carlo simulation of atmospheric neutron transport at high altitudes using MCNP

Neutron transport calculations were performed using the Monte Carol code, MCNP. The transport problem considered has a point source at high altitude (40 km). Since atmospheric density decreases with altitude, 2-D effects (cylindrical coordinates) can be important. Results were compared to those obtained with the SMAUG-II computer code, which performs mass-integral scaling of approximate fits to 1-D spherical discrete ordinates solutions. These comparisons show the importance of 2-D computations. Practical issues are discussed of applying MCNP to this problem, without code modifications and includes example input files for MCNP and for SMAUG-II. Computational modelling of neutron transport in the atmosphere is complicated by the variation of air density with altitude.