Muons in extensive air showers: III. Primary mass composition studies

The extensive air shower muon-density and time-spread measurements made over many years at Haverah Park are discussed in relation to the mass composition of primary cosmic rays covering the primary energy range from 5x1014 to more than 1019 eV. At energies above 3x1015 eV the measurements seem to indicate an unchanging primary-mass composition up to 1019 eV. At energies below the 'knee' however the results indicate a move from heavier to lighter primaries with increasing energy. There is support for this picture from other muon-content studies; however other techniques for inferring the primary-mass composition seem to come to somewhat different conclusions. (author)