POWELL FOWLER PERKINS / SCIENCE PHOTO LIBRARY POWELL FOWLER PERKINS / SCIENCE PHOTO LIBRARY
Emulsion photograph showing the tracks of various cosmic ray nuclei, ranging from a hydrogen nucleus to an iron nucleus. The more protons a nucleus contains, the higher its positive electric charge & the greater its ability to ionise; the more it ionises, the denser its track in emulsion. Hydrog- en has 1 proton & produces a barely visible track. Ionisation depends on the square of the charge; so lithium's 3 protons have 9 times hydrogen's charge & produce a track 9 times denser. Iron, with 26 protons, produces a track 676 times denser than hydrogen. The wispy strands that adorn the denser tracks are "delta rays" - electrons knocked out of atoms in the emulsion by the passing nucleus.
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