Final answer:
Primary cosmic rays mainly consist of very fast protons, which are hydrogen nuclei without electrons, constituting around 90% of cosmic rays. Helium and heavier nuclei form about 9%, while electrons and positrons make up the remainder.
Step-by-step explanation:
Primary cosmic rays are composed largely of very fast protons. These cosmic rays are mostly high-speed atomic nuclei, predominantly hydrogen nuclei, which are protons that have been stripped of their electrons. Almost 90% of cosmic rays are composed of these protons, thereby making them the primary constituents. Additionally, helium and heavier nuclei account for about 9% of cosmic ray components while electrons, including positrons, make up about 1%. Positrons are positively charged particles with the same mass as electrons, and they represent a minor but very interesting part of cosmic radiation due to being a form of antimatter. Cosmic rays can travel as fast as 90% of the speed of light and are likely produced in events such as supernova shocks.