The mass composition of cosmic rays in the range 0.5-10 PeV

A well-known technique for determining the mass composition at energies above those where direct measurements can be made is to find the 'depth of shower maximum' (Xmax ), i.e. the atmospheric depth at which the number of shower particles is a maximum, as a function of primary energy. This quantity can then be related, by way of an interaction and propagation model, to the primary mass. The paper gives an analytical form for Xmax , in terms of the primary mass and primary energy, for the model adopted by us; comparisons are given with the results of other workers for the limiting nuclei: protons and iron. Hitherto, most analyses of Xmax have yielded a value for the mean mass (actually ); although Arqueros et al (Arqueros F et al 2000 Astron. Astrophys. at press) have made a division into two components: 'light' and 'heavy'. Here we endeavour to make a finer division, into four components. It is appreciated that the experimental data so far available, from HEGRA (Arqueros F et al 2000 Astron. Astrophys. at press) and SPASE-VULCAN (Dickinson J E et al 1999 Proc. 26th Int. Conf. on Cosmic Ray vol 3 p 136, Dickinson J E et al 1999 Private communication) are somewhat disparate, but we apply the method put forward by us, with the object of demonstrating its application. A comparison has been made of the tentative mass composition with that advocated by Erlykin and Wolfendale (Erlykin A D and Wolfendale A W 1998 Astropart. Phys. 8 265) and others, namely that the mean mass increases with energy in a particular way and that the division between particular mass groups has a specific form. Although there can be no claim for close agreement, there is certainly no inconsistency. Interestingly, both sets of data imply that there is a significant increase in the iron component at the highest energies: 5-10 PeV, as predicted by Erlykin and Wolfendale. (author)