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' (X-max), 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 X-max 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, must analyses of X-max, have yielded a value for the mean mass (actually (In A)); 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 er 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 barn 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 then 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.