Cosmic rays are high-energy particles that rain down on Earth from outer space. When cosmic rays enter Earth’s atmosphere, they collide with atomic nuclei in the air and produce secondary particles. Of these, muons are the representative particles that reach the surface.
Types of Cosmic Rays #
Primary Cosmic Rays #
Particles traveling directly from space, observed above the atmosphere.
- Protons (hydrogen nuclei) — about 90%
- Helium nuclei (alpha particles) — about 9%
- Heavy element nuclei (carbon, iron, etc.) — about 1%
Secondary Cosmic Rays #
Particles produced when primary cosmic rays collide with atmospheric nuclei in the upper atmosphere (about 20 km altitude).
- Muons — the main particles that reach the surface
- Pions — parent particles of muons
- Gamma rays, electrons, positrons — electromagnetic component
Journey to Earth #
Cosmic rays travel through space at near-light speed and enter Earth’s atmosphere. Primary cosmic rays (mainly protons) collide with air and produce large numbers of secondary particles. Of these secondary particles, muons with their high penetrating power survive to reach the surface.
History of Discovery #
In 1912, Victor Hess discovered that radiation increased with altitude during balloon experiments. This revealed that some radiation comes from space, leading to the term “cosmic rays.”
Hess’s Balloon Experiment #
It was believed that radiation originated from rocks and soil on the ground, but Hess confirmed that radiation levels increased even at 5,000m altitude, proving the existence of radiation of cosmic origin (Nobel Prize in Physics, 1936).
References #
- “Victor F. Hess – Facts,” The Nobel Prize
- De Angelis, A., “Atmospheric ionization and cosmic rays: studies and measurements before 1912.” arXiv:1208.6527