Abstract
Based on the observed results on the source composition of cosmic rays, a possible mechanism for the formation of this composition is considered by taking into account the fractionation of the elements in the interstellar molecular clouds, in which there may exist dusts and grains enriched with the elements whose condensation temperature is higher than about 1000K. Most of these nuclei enhanced in the source composition are identified as heavy and ultra-heavy ones, which must have been synthesized in the r-process initiated with the explosions of type II supernovae and/or supergiant stars. It seems that these nuclei in atomic states may have been relatively efficiently condensed into dusts and grains in the interstellar clouds, which are formed in supernova ejectae while being cooled off.
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