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NEDDERMEYER 1937

Neddermeyer, S.H.; Anderson, C.D.;
Note on the Nature of Cosmic-Ray Particles
Phys. Rev. 51 (1937) 884;

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Reprinted in
R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 32.
The Physical Review - the First Hundred Years, AIP Press (1995) CD-ROM.

Motivation
Measurements (Anderson and Neddermeyer, Phys. Rev. 50 (1936) 263) of the energy loss of the particles occurring in the cosmic-ray showers have shown that this loss is proportional to the incident energy and within the range of the measurements, up to about 400 MeV is in approximate agreement with values calculated theoretically for electrons by Bethe and Heitler. These measurements were taken using a thin plate of lead (0.35 cm), and the observed individual losses were found to vary from an amount below experimental detection up to the whole initial energy of the particle, with a mean fractional loss of about 0.5. If these measurements are correct it is evident that in a much thicker layer of heavy material multiple losses should become much more important, and the probability of observing a particle loss less than a large fraction of its initial energy should be very small. For the purpose of testing this inference and also for checking our previous measurements (Anderson and Neddermeyer, Report of London Conference, V.1 (1934) 179) which had shown the presence of some particles less massive than protons but more penetrating than electrons obeying the Bethe-Heitler theory, we have taken about 6000 counter-tripped photographs with a 1 cm plate of platinum placed across the center of the cloud chamber. (Extracted from the introductory part of the paper.).

Accelerator COSM Detectors CLOUD

Related references
More (earlier) information appears in
C. D. Anderson and S. H. Neddermeyer, Report of London Conference, 1 (1934) 179;