Numerical simulation and analytical description of solar neutron transport in the Earth's atmosphere

Abstract

We present here results of a numerical simulation of small‐angle neutron multiscattering and attenuation in the Earth's atmosphere. A range of initial zenith angles, θo, and different atmospheric depths h are considered. We show that the angular distribution of neutrons remains symmetrical only for vertical arrival, θo = 0. For inclined arrival the distribution becomes asymmetric; this asymmetry grows with increasing initial zenith angle. The asymmetry of the angular distribution enhances the solar neutron refraction effect. It follows from our simulation that this effect is caused by the stronger attenuation of neutrons scattered to zenith angles larger than the arrival angle. This effect makes the effective zenith angle become smaller as the solar neutrons propagate downwards. The refraction angle of our simulation is not constant, but it is a growing function of θo and h. An analytical description of the problem is also performed starting from simple assumptions of the parameters involved in the solution. We modify this simple model in successive steps to take into account additional details of the solar neutron propagation, until it reaches reasonable agreement with our numerical simulation estimates.