Cosmic ray

Cosmic ray. They are high-energy particles, primarily originating from outside the Solar System, that travel through space at nearly the speed of light. Upon entering the Earth's atmosphere, they collide with atomic nuclei, producing cascades of secondary particles detectable at the surface. Their study, a key part of astrophysics and particle physics, has revealed fundamental information about the universe and high-energy processes.

Discovery and history

The investigation began in the early 20th century with experiments on atmospheric electricity by scientists like Charles Thomson Rees Wilson. In 1912, Victor Francis Hess conducted daring balloon flights, proving the ionizing radiation increased with altitude, a discovery for which he later received the Nobel Prize in Physics. Subsequent work by Robert Andrews Millikan, who coined the term, and Bruno Rossi, who pioneered coincidence circuit techniques, solidified their extraterrestrial origin. Key discoveries, such as the latitude effect by Jacob Clay and the identification of the positron by Carl David Anderson, were directly linked to their study. Major projects like the Chicago Cosmic Ray Symposium helped coordinate global research efforts.

Composition and sources

Primary cosmic rays are about 99% atomic nuclei and 1% solitary electrons. The nuclear component is roughly 90% protons (hydrogen nuclei), 9% alpha particles (helium nuclei), and 1% heavier nuclei, including traces of elements like iron. A tiny fraction consists of other particles like antimatter positrons. Their exact origins are a subject of active research, with likely sources including energetic events within our Milky Way such as supernova remnants like the Crab Nebula and pulsar wind nebulae. The highest-energy particles, with energies exceeding those produced at the Large Hadron Collider, may originate from extragalactic sources like active galactic nuclei or gamma-ray bursts.

Detection and measurement

Detection relies on observing the extensive air showers they create in the Earth's atmosphere. Ground-based observatories use arrays of particle detectors, such as scintillation counters or Cherenkov telescopes, to sample these showers. Famous installations include the Pierre Auger Observatory in Argentina, the Telescope Array Project in Utah, and the High Altitude Water Cherenkov Experiment in Mexico. Historically, instruments like the cloud chamber and later the spark chamber were crucial for visualizing particle tracks. Space-based detectors, such as those on the International Space Station like the Alpha Magnetic Spectrometer, measure the primary flux directly before atmospheric interaction.

Effects and interactions

When interacting with the atmosphere, they produce showers of secondary particles including pions, muons, neutrinos, and gamma rays. These secondary muons contribute significantly to background radiation at sea level. They are also responsible for creating unstable isotopes like carbon-14 through interactions with nitrogen nuclei, which is the basis for radiocarbon dating developed by Willard Libby. In technology, they can cause single-event upsets in microelectronics, a concern for satellites and aircraft, and are a primary radiation hazard for astronauts on missions beyond Low Earth orbit, such as those planned for the Artemis program.

Research and significance

Research is a interdisciplinary field bridging astrophysics, solar physics, and particle physics. It has been instrumental in the discovery of new subatomic particles, including the muon and the pion, leading to Nobel Prizes for scientists like Cecil Frank Powell. Studies help probe the magnetic fields of the Milky Way and the heliosphere. Current mysteries focus on the nature of the highest-energy events, sometimes called Oh-My-God particles, and the search for sources through multimessenger astronomy combining data with observations from facilities like the IceCube Neutrino Observatory and the Fermi Gamma-ray Space Telescope. Understanding their acceleration mechanisms remains a primary goal of modern astrophysics.

Category:Astrophysics Category:Particle physics Category:Radiation