Fermi, Enrico. On the Origin of Cosmic Radiation. American Physical Society, 1949. 1st edition. Physical Review, 75, pp. 1169-1174  Original wrappers. Fine condition. $1500.
The reason for the expense here is that this paper is in the same issue as the epochal Bardeen/Brattain transistor action paper*** Besides naming the neutron, bringing about the first self-sustaining nuclear reaction ('42), and other works in high energy physics, Fermi (Doctorate U Pisa 1922 at 21 years old, Nobel Prize 1938)developed this (above) theory of cosmic ray origin.
From the APS website, the abstract for the article: "A theory of the origin of cosmic radiation is proposed according to which cosmic rays are originated and accelerated primarily in the interstellar space of the galaxy by collisions against moving magnetic fields. One of the features of the theory is that it yields naturally an inverse power law for the spectral distribution of the cosmic rays. The chief difficulty is that it fails to explain in a straight-forward way the heavy nuclei observed in the primary radiation."