The Charge Spectrum of Very Heavy Cosmic Ray Nuclei

Several examples of highly charged (Z > 40) cosmic ray primaries have been observed in a photographic emulsion detector of area 4.5 m2. The detector was flown by balloon over Texas where the geomagnetic cutoff ensures that all primaries are relativistic at the top of the atmosphere. Approximately 2 x 105 tracks of cosmic ray primaries with charge Z > 20 were observed. Among these we found nine outstandingly heavy tracks which penetrated the entire assembly without measurable change of track density. This shows that the range of each track was in excess of ca. 6 g/cm2; these tracks, therefore, could not have been produced by slow iron nuclei. The two heaviest tracks detected have a charge of 90 ± 4, which suggests that the source is relatively rich in elements which have been formed by rapid neutron synthesis-the 'r’ process.