Measurement of the elongation rate of extensive air showers produced by primary cosmic rays of energy above 2×1017eV

Abstract

A measurement of the elongation rate, the rate of change of the depth of shower maximum with energy, has been made in air showers initiated by cosmic-ray primaries with energy in the range 2*10¹⁷-10²⁰ eV. The measurement is based on the study of the rise times of over 13000 pulses recorded from the four 34 m² water Cerenkov detectors of the Haverah Park array and on an application of the elongation rate theorem. The elongation rate is determined to be 70+or-5 g cm^-2 per decade, averaged over the whole energy range, while for 35 events of primary energy greater than 5*10¹⁸ eV the corresponding value is 40+or-20 g cm^-2 per decade. These results are independent of assumptions about features of high-energy interactions. The changes in mass composition, consistent with the authors' measurement and with different assumptions about nuclear interactions, are derived.