  Under current conditions, the cosmic ray spectrum incident on the Earth is
dominated by particles with energies < 1 GeV. Astrophysical sources including
high energy solar flares, supernovae and gamma ray bursts produce high energy
cosmic rays (HECRs) with drastically higher energies. The Earth is likely
episodically exposed to a greatly increased HECR flux from such events, some of
which lasting thousands to millions of years. The air showers produced by HECRs
ionize the atmosphere and produce harmful secondary particles such as muons and
neutrons. Neutrons currently contribute a significant radiation dose at
commercial passenger airplane altitude. With higher cosmic ray energies, these
effects will be propagated to ground level. This work shows the results of
Monte Carlo simulations quantifying the neutron flux due to high energy cosmic
rays at various primary energies and altitudes. We provide here lookup tables
that can be used to determine neutron fluxes from primaries with total energies
1 GeV - 1 PeV. By convolution, one can compute the neutron flux for any
arbitrary CR spectrum. Our results demonstrate that deducing the nature of
primaries from ground level neutron enhancements would be very difficult.
