Modelling the detection of radiation sources in low earth orbits with Geant4

The monitoring of particle radiation sources in low earth orbits (LEO) can not only rely on the detection of neutral particles (gamma-rays/neutrons) as in earth-bound scenarios, but also on detecting charged particles (electrons/positrons) induced in the outer materials of the satellite carrying the radiation source. While neutral particle fluxes decrease as 1/r2 , electrons and positrons trapped in the geomagnetic field can travel thousands of km. Neutral and charge particles from reactor-powered satellites were detected in the 1980’s by the Solar Maximum Mission (Rieger et al. Science, Vol-244, 1989). The temporal and spectral features of these man-made signals allow to differentiate them from LEO backgrounds like gamma-ray bursts and solar flares.

We present the simulations of a space reactor particle flux propagated from selected source satellites to selected observer satellite orbits. The SAFE-400 space fission reactor was simulated using Monte Carlo N-Particle code. For the radiation transport of the reactor-generated neutral and charged particles, a Geant4 model of the earth atmosphere and geomagnetic field was built, where the motion of the charged particles in the geomagnetic field was implemented using the guiding center approximation following the approach of Tuneu et al. (A&A ,Vol-654, 2021). The accuracy of guiding-center Geant4 trajectories was checked with an ultra-fast C++ code that only implements the equation of motion. While Geant4 is slower in solving the charged particle motion, it models particle interactions with atmosphere and observer detectors. To verify our formalism, we simulate the detected reactor-generated positron signal discussed by Hones (Science, Vol-244, 1989).