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R ECENT high altitude flights in free balloons have given evidence for the existence of nuclei of atomic number up to about 40 and kinetic energies of about ^ Bev per nucleon as a component of cosmic radiation above 90,000 feet.Tracks of such particles were observed both in a cloud chamber and in Ilford nuclear emulsions,The cloud chamber and associated equipment was enclosed in a sphere of aluminum 30 inches in diameter and 0.040-inch thick, which was kept at atmospheric pressure and within a temperature range between 58 and 98 degrees F throughout the flight. The balloon reached an altitude of 94,000 feet (14 g/cm 2 below the top of the atmosphere), spent three hours above 90,000 feet, and four hours above 65,000 feet.Stacks of photographic plates in groups of twelve were placed above and below the chamber, the emulsion lying in the vertical plane. Tracks which entered the photographic plates below the chamber within 30° from the vertical had to pass through all or part of the four J-inch lead plates placed in the chamber.In the photographic plates we observed tracks many times denser and heavier than those which are obtained from fragments produced in nuclear explosions. A further feature which clearly distinguishes these tracks from those produced by ordinary nuclear fragments is the very large number of slow electrons (5-rays) issuing from the dense and completely solid filament of silver grains which constitutes the core of the tracks. Figures 1, 2 and 3 show examples of such tracks. For comparison, Fig. 4 shows tracks of a wmeson, three protons, and a Li 8 nucleus disintegrating into two slow a-particles. All tracks are reproduced at the same magnification.Most of these tracks pass through the entire stack of plates and can be followed from one emulsion to the next after penetrating the 1.4-mm thick glass backing of the plates. Not all of the penetrating tracks are equally heavy; some are only slightly heavier than those of slow a-particles, but their ranges are about 1000 times larger.In cases where the angle is favorable, the track may be pursued through all twelve plates, and tracks have been observed whose range is certainly greater than 7.5 cm of glass or 20 g/cm 2 . If one combines this information with the observation that these tracks are everywhere much heavier than a-particles near the end of their range and that, therefore, at no point of the track is the energy loss smaller than 0.6 Mev/(mg/cm 2 ), one obtains for such a track a minimum charge Z = 16 and a minimum kinetic energy E = 13 Bev. Actually, since for many of these tracks the central core of developed silver grains is more than three times as wide as the corresponding core for slow aparticles, the minmum energy loss is probably in the neighborhood of nine times 0.6 Mev/ (mg/cm 2 ) or 5.4 Mev/(mg/cm 2 ).As an example, we now consider a track which enters plate No. 3 at an angle of 37° with the vertical and stops in the glass between plates No. 10 and 11. This particle has a range of 1.5 FIG. 1. A very heavy track produced by a particle ...

Further evidence is presented for the existence of heavy nuclei as components of the primary cosmic radiation. Preliminary results are given for the distribution in atomic numbers of these components. Lower limits of the energies of the particles on entrance into the atmosphere are calculated. These are, in general, above the cut-off imposed by the earth's magnetic field. The mean free path for nuclear collisions is of the order of 14 cm of photographic emulsion. This is longer than that expected from the geometrical cross section and may indicate velocity dependence of nuclear forces. An example of a track that stops in emulsion is shown. This particle gives further evidence for the nuclear character of the rays, because as it slows down it captures planetary electrons and decreases its rate of energy loss. An approximate value of the hydrogen-helium ratio of 4 is reported.

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