R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 35.
The Physical Review - the First Hundred Years, AIP Press (1995) CD-ROM.
Anderson and Neddermeyer have shown that, for energies up to 300 and 400 MeV, the cosmic-ray shower particles have energy losses in lead plates corresponding to those predicted by theory for electrons. Recent studies of range and energy loss indicate that the singly occurring cosmic-ray corpuscles, even in the energy range below 400 MeV, are more penetrating than shower particles of corresponding magnetic deflection. Thus the natural assumption have been expressed: the shower particles
are electrons, the theory describing their energy losses is satisfactory, and the singly occurring particles are not electrons. The experiments cited above have shown from consideration of the specific ionization that the penetrating rays are not protons. The suggestion has been made that they are particles of electronic charge, and of mass intermediate between those of the proton and electron. If this is true, it should be possible to distinguish clearly such a particle from an electron or proton
by observing its track density and magnetic deflection near the end of its range, although it is to be expected that the fraction of the total range in which the distinction can be made is very small. (Extracted from the introductory part of the paper.).
Related references See also C. D. Anderson and S. H. Neddermeyer, Phys. Rev. 50 (1936) 263;
S. H. Neddermeyer and C. D. Anderson, Phys. Rev. 51 (1937) 885;
J. C. Street and E. G. Stevenson, Phys. Rev. 51 (1937) 1005;