First picture of an ultraheavy cosmic ray primary, compared with a cosmic ray iron nucleus. The tracks of both nuclei are recorded in photographic emulsion exposed during balloon flights high in the atmosphere over Texas in 1966. Iron has atomic number 26: it has 26 protons. The ultraheavy nucleus had about 90 protons - the number of thorium. The more protons in a nucleus, the greater its electric charge & its ability to ionise; the more it ionises, the denser the track it produces in emulsion. The wispy side-shoots are delta rays" - electrons knocked out of atoms by the passage of the nuclei."