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 5 x 10(14) to more than 10(19) eV. At energies above 3 x 10(15) eV the measurements seem to indicate an unchanging primary-mass composition up to 10(19) 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.