A new approach to inferring the mass composition of cosmic rays at energies above 10¹8 eV

We describe a new approach to establishing the mass composition at high energies. Based on measuring both the vertical and inclined shower rates, it has the potential to distinguish heavy nuclei from light nuclei. We apply the method to Haverah Park data above 10^18 eV to show that, under the assumption that the Quark Gluon String Jet Model correctly describes the high energy interactions, the inclined shower measurements favour a light composition at energies above 10^19 eV. The same conclusion is obtained using a variety of assumptions about the cosmic ray spectrum. To the extent that precise spectral measurements will be possible by forthcoming experiments such as the Auger observatories, the method will further constrain data on composition of the ultra high energy cosmic rays.