Suggestions for Identifying the Primary of Post-GZK Air Showers
  A procedure is suggested for systematically narrowing the choice of possible primaries for (UHECR) air showers with energies beyond the Greisen-Zatsepin-Kuz'min cut-off of $4 \times 10^{19} eV$. 
J Bordes (Valencia), HM Chan (Rutherford Appleton Lab) and ST Tsou   (Oxford)