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We here argue that the "knee" of the cosmic ray energy distribution at $E_c\sim 1$ PeV represents a second order phase transition of cosmic proportions.The discontinuity of the heat capacity per cosmic ray particle is given by$\Delta c=0.450196\ k_B$. However the idea of a deeper critical pointsingularity cannot be ruled out by present accuracy in neither theory norexperiment. The quantum phase transition consists of cosmic rays dominated bybosons for the low temperature phase E<E_c and dominated by fermions for hightemperature phase $E > E_c$. The low temperature phase arises from those nucleidescribed by the usual and conventional collective boson models of nuclearphysics. The high temperature phase is dominated by protons. The transitionenergy $E_c$ may be estimated in terms of the photo-disintegration of nuclei.

A Quantum Phase Transition in the Cosmic Ray Energy Distribution