I.5                 Atmospheric albedo neutron model

The QinetiQ Atmospheric Radiation Model (QARM) uses a response function approach based on Monte Carlo radiation transport codes to generate directional fluxes of atmospheric secondary radiation. The model has been widely validated against a range of measurements [RD.17], [RD.18]. It can be applied to give the upward (albedo) neutron flux at 100 km altitude and this can then be scaled to any spacecraft altitude using the inverse square law:

(I-11)

where

ϕ        is the particle flux,

h        is the altitude in km and

Re      is the earth’s radius in km

This method compares favourably with the limited experimental measurements [RD.81], [RD.82].

Results for three cut-off rigidities are given for solar minimum and maximum respectively in Figure I-6 and Figure I-7.