Chemical Composition of Relativistic Cosmic Rays Detected above the Atmosphere

Abstract

Final results on the first satellite experiment on abundances of cosmic-ray nuclei having a mean energy of several GeV per nucleon are presented. A nuclear-emulsion detector, exposed on Gemini XI in a near-equatorial orbit ranging between geographic latitudes \ifmmode\pm\else\textpm\fi{}29\ifmmode^\circ\else\textdegree\fi{}, collected 619 high-quality tracks above the earth's atmosphere. Time resolution (within about 5 min) was provided by movement of a lower emulsion stack relative to an upper one. The detector was covered by only 0.07 g/${\mathrm{cm}}^{2}$ of aluminum and was favorably oriented for 18 h. The results on abundances, requiring no correction for secondary production in the atmosphere, are characterized by (a) a pronounced odd-even effect, with low abundances for elements of atomic number 7, 9, 11, and 13, compared to those of neighboring elements with even $Z$; (b) approximately equal fluxes of neon, magnesium, and silicon, each being about one-fourth that of oxygen; and (c) an abundance gap in the region $15\ensuremath{\le}Z\ensuremath{\le}19$. The observed ratios of Be+B, $10\ensuremath{\le}Z\ensuremath{\le}19$, and $Z\ensuremath{\ge}20$ to the medium group, $6\ensuremath{\le}Z\ensuremath{\le}9$, provide no evidence for significant variation of composition with rigidity between 3.5 and 30 GV. A primordial composition---prior to interactions of the cosmic rays with the interstellar medium---is calculated. This source composition is compared with "universal" and solar abundances.