Primary Heavy Nuclei

Primary cosmic-ray nuclei of z≥2 were observed in a balloon-borne cloud chamber. The chamber was triggered by events with ionization loss {≥}6 minimum in each of three proportional counters, arranged in a telescope. Proportional counter pulse amplitudes were recorded by a neon bulb system.Two Skyhook daytime balloon flights at geomagnetic latitude 41^∘{} maintained average altitudes of 17.6 and 14.3 g/cm². In the 0.77 hour sensitive time, in two flights, there were observed in the cloud chamber (charge determined from both cloud chamber and proportional counter data) 3-5 Li, 1 Be, 0 B, and 21 z≥6 nuclei. A primary flux of 10.2_-3.4+4.5 particles per m² sec sterad is found for the z≥6 nuclei. An unusually large fraction of this group is indicated to be heavier than oxygen. The primary flux of Li, Be, and B nuclei is found to be 5_-5⁺³² percent of the primary flux of z≥6 nuclei, from which we conclude that heavy nuclei have traversed less than 7 g/cm² interstellar hydrogen [=(4/n)×10⁶ light years, where n=numberofHatoms per cm³].The proportional counter data alone showed a 14±{}16 percent decrease in intensity of particles of $z>8$ from morning to afternoon.