Evaluation of the Fluence to Dose Conversion Coefficients for High Energy Neutrons Using a Voxel Phantom Coupled with the GEANT4 Code

Crews working on present-day jet aircraft are a large occupationally exposed group with a relatively high average effective dose from galactic cosmic radiation. Crews of future high-speed commercial flying at higher altitudes would be even more exposed. To help reduce the significant uncertainties in calculations of such exposures, the male adult voxels phantom MAX, developed in the Nuclear Energy Department of Pernambuco Federal University in Brazil, has been coupled with the Monte Carlo simulation code GEANT4. This toolkit, distributed and upgraded from the international scientific community of CERN/Switzerland, simulates thermal to ultrahigh energy neutrons transport and interactions in the matter. The high energy neutrons are pointed as the component that contribute about 70% of the neutron effective dose that represent the 35% to 60% total dose at aircraft altitude. In this research calculations of conversion coefficients from fluence to effective dose are performed for neutrons of energies from 100 MeV up to 1 GeV, irradiating MAX with mono-energetic beams in the mode Anterior-Posterior. An alternative methodology is developed too, using the atmospheric neutrons spectrum simulated with GEANT4 code at aircraft altitude instead of the traditional method that uses monoenergetic beams. To obtain the neutrons spectrum 1.5× 105 extensive atmospheric showers are simulated by cosmic rays interactions with atmospherics atoms. The main characteristics of the spectrum are in agreement with literature confirming the validity of GEANT4. For 100 MeV energy the conversion coefficient calculated with spectrum shows a decrease of 8%, pointing out the importance of the environment influence.