A STUDY OF THE SKYSHINE NEUTRON DOSES WITH THE CODE GEANT4

The skyshine doses for point source neutrons with initial energies 100 MeV and 1 GeV were investigated through simulation with the code GEANT4. The direct component of the radiation field exhibits an exponential falling down with a 1/r(2) behavior and has attenuation lengths 582.7 in for the initial energy 100 MeV and 947.8 m for 1 GeV. In the case of the scattered component the dose distribution falls down exponentially and has a 1/r behavior. The attenuation length is 420.16 m for the initial energy 100 MeV and 696.8 m for 1GeV. The importance functions of the neutrons, calculated in 7 angular intervals of the vertical emission angle (0 degrees - 5 degrees, 10 degrees - 20 degrees, 25 degrees - 35 degrees, 40 degrees - 50 degrees, 55 degrees - 65 degrees, 70 degrees - 80 degrees and 80 degrees - 90 degrees) are described with two exponential terms with a 1/r behavior. The second exponential is due to the hardening of the neutrons spectra with the distance from the source and has a higher attenuation length.