DHS Written Report

Abstract

Recently, a new galactic cosmic-ray (GCR) source option [1] was implemented in the all-energy, all-particle transport code MCNP6 [2]. In this paper, we made use of this feature to calculate surface neutron and photon background fluxes on a terrestrial grid around the earth. These spectra have been incorporated into Release 2 of the 'background.dat' file, which is read and sampled by MCNP6 whenever a user invokes the background source option [2]. Cosmic radiation bombards Earth with various particles, such as protons and {alpha} particles, some of which are deflected by the Earth's shielding magnetic field. Particles that carry sufficient energy can overcome the deflection and penetrate into the atmosphere. The sufficient energy is dependent on the terrestrial coordinates due to the magnetic force's proportionality to the sine of the angle between the velocity vector of the incoming particle and the magnetic field. As the particles propagate through the atmosphere, collisions with atmospheric molecules generate new particles such as neutrons, protons, photons, muons, pions, and other exotic particles. These secondary often have sufficient energy to undergo additional nuclear interactions, and so on, forming what is known as a cascade shower. The tabulation of background particle fluxes on the surface of the earth is important for a variety of reasons, one of which is the design of nuclear material detection systems. The simulations used various models and formulations for the cosmic source spectra, atmosphere, and terrestrial conditions to correctly model the propagation of GCR particles through the atmosphere to surface level.