MCNP modelling of radiation effects of the Dragonfly mission's RTG on Titan

Abstract

The Dragonfly mission to Titan is proposed to use a radioisotope thermoelectric generator to supply electrical and thermal power to the spacecraft during flight and once it arrives at its destination. As no spacecraft continuously powered by an RTG has been flown down through Titan's atmosphere before, the effects of such a generator's radiation field on the surrounding environment are unknown. MCNP (Monte Carlo N-Particle) simulations were run on a simplified model of the generator in its protective hull section in a coarse-grained Titan environment with both neutron and photon transport modelled, the former to determine the degree to which the surface was likely to become activated, and the latter to determine the energy likely to be deposited in a surface covering of tholin-analogue hydrocarbons, and consequently the potential for damage to chemical bonds found therein. Tests on the model also provided data which allowed for a determination of the degree to which the insulation surrounding the generator spacecraft might be activated by stray neutrons, how much heating might be expected in the hull section surrounding the generator and how much attenuation the beam from the spacecraft's pulsed neutron generator is likely to experience due to Titan's thick nitrogen atmosphere.