The difference between primary and secondary cosmic rays is essentially one of convenient classification rather than of fundamental type: primary cosmic rays are those which are created by external sources like our own Sun, while secondary cosmic rays are those which are created when primary cosmic rays interact with matter in the interstellar medium, resulting in the emission of new rays.
– About Cosmic Rays –
Cosmic rays are extremely high-energy streams of particles which originate in outer space and strike the Earth’s atmosphere in large numbers, where most are dispersed. They consist almost entirely of protons, along with a smaller number of hydrogen nuclei (two protons fused together, with neutrons) and an even smaller number of electrons, along with very small trace amounts of heavier nuclei. These tend to travel through space more or less unmolested because, although they are physical matter, the interplanetary medium, and far more so the interstellar medium, is of very low density (i.e. it is nearly, though not quite, a vacuum).
Cosmic rays were first theorized to exist around 1900, and then confirmed to exist over the coming decades through monitoring of their ionizing effects upon Earth’s atmosphere. (Unlike outer space, Earth’s atmosphere is quite dense, and so large numbers of collisions are inevitable.) Today there are a number of important cosmic ray observatories both around the world and in orbit, and cosmic ray detectors are installed on several space probes, including the two Voyager probes, the Cassini-Huygens Saturn probe, and the Solar and Heliospheric Observatory (SOHO) probe.
– Primary and Secondary Cosmic Rays –
Primary cosmic rays are emitted by stars; our own Sun produces cosmic rays, particularly via solar flares. Among these primary cosmic rays, the proportion of the contents is essentially the same as the ratio of hydrogen and helium in the universe: 90% of primary cosmic rays are single protons (which can form hydrogen), about 9% are fused protons in the form of helium nuclei, and about 1% are electrons. Primary cosmic rays contain much smaller amounts of heavier nuclei like oxygen.
In contrast, secondary cosmic rays are those which are produced from the collisions of the primary cosmic rays, above, with the small amounts of matter in the interstellar medium. Secondary cosmic rays are unlike primary cosmic rays because they bear the remains of the interstellar matter with which they collided. For this reason, secondary cosmic rays may also contain trace amounts of heavier-nucleus material, like lithium and beryllium.