Which particles make up most galactic cosmic rays?

Galactic cosmic rays (GCRs) consist primarily of high-speed atomic nuclei. While these rays also include electrons, the vast majority of the primary particles are nuclei.12

Research into the elemental composition of these nuclei indicates that they represent a mixture of material, with approximately 80 percent sharing a composition similar to solar-system matter and about 20 percent originating from massive, evolved stars such as those found in supernova remnants. Although hydrogen and helium are present, their abundances in GCRs differ from those found in general solar-system matter. Additionally, light elements such as lithium, beryllium, and boron are found in GCRs as a result of heavier nuclei fragmenting during collisions with interstellar gas.2