Complete Numerical Solution of Electron-Hydrogen Model Collision Problem above the Ionization Threshold
S.Jones, A.Stelbovics et al.1999, P.R.L., 84, #9

Measurement of the Flux of Ultrahigh Energy Cosmic Rays from Monocular Observations by the High Resolution Fly's Eye Experiment
  Abbasi, R.U.; Abu-Zayyad, T.; et al.; Physical Review Letters, vol. 92 num. 15, 16 Apr. 2004