Cosmic Rays - Primary Cosmic Rays

Primary Cosmic Rays

Primary cosmic rays originate mainly outside the Solar system, i. e. in the Galaxy or even in other galaxies. When they interact with Earth's atmosphere, they are converted to secondary particles. Almost 90% of primary cosmic rays are protons, about 9% are helium nuclei (alpha particles) and nearly 1% are electrons. The mass ratio of hydrogen to helium nuclei (28%) is about the same as the primordial elemental abundance ratio of these elements (24%).

The remaining fraction is made up of the other heavier nuclei that are nuclear synthesis end products, products of the Big Bang, primarily lithium, beryllium, and boron. These light nuclei appear in cosmic rays in much greater abundance (~1%) than in the solar atmosphere, where their abundance is about 10−9% that of helium.

This abundance difference is a result of the way secondary cosmic rays are formed. Carbon and oxygen nuclei collide with interstellar matter to form lithium, beryllium and boron in a process termed cosmic ray spallation. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter. See Natural Environmental Radioactivity.

Satellite experiments have found evidence of a few antiprotons and positrons in primary cosmic rays, although there is no evidence of complex antimatter atomic nuclei, such as anti-helium nuclei (anti-alpha) particles. Antiprotons arrive at Earth with a characteristic energy maximum of 2 GeV, indicating their production in a fundamentally different process from cosmic ray protons.