Cosmic Radiation
Cosmic radiation, commonly referred to as cosmic rays, consists of high-energy particles originating from beyond Earth's atmosphere. These rays primarily include protons, alpha particles, and heavier nuclei, such as carbon and iron, with their composition varying depending on their source. The main categories of cosmic rays are solar cosmic rays, which are emitted during solar flares; galactic cosmic rays, originating from supernovae within the Milky Way; and anomalous cosmic rays, ionized by solar radiation. Cosmic rays interact with Earth's atmosphere, resulting in cascades of secondary particles, including muons and neutrinos, some of which reach the surface.
While cosmic rays contribute to the background radiation that humans are exposed to—approximately 0.3–0.5 mSv annually—they can also pose risks, particularly for astronauts and high-altitude pilots, due to increased exposure. Cosmic radiation has practical applications in science, such as aiding in carbon dating and contributing to particle physics research. Despite their harmful potential, cosmic rays have beneficial roles, providing insights into high-energy astrophysical processes and enhancing our understanding of the universe.
Cosmic Radiation
FIELDS OF STUDY: Particle Physics; Nuclear Physics
ABSTRACT: Cosmic radiation, or cosmic rays, is ultra-high-energy radiation that originates beyond Earth’s atmosphere. This article describes its discovery, its composition, its origins, its effect on human life, and ongoing observational experiments.
Principal Terms
- active galactic nucleus: the region in the center of certain types of galaxies that emits massive amounts of energy across most, if not all, of the electromagnetic spectrum; believed to result from a supermassive black hole.
- alpha particle: a particle consisting of two protons and two neutrons, identical to a helium nucleus, which is emitted from an unstable nucleus via radioactive decay.
- background radiation: the total amount of ionizing radiation to which Earth is constantly exposed from both natural and artificial sources.
- beta particle: an electron that is emitted from an unstable nucleus via radioactive decay.
- Cherenkov radiation: electromagnetic radiation emitted when a charged particle, such as an electron, travels through a given medium faster than light would in that same medium.
- Compton scattering: the collision of a high-energy photon with a lower-energy electron, causing energy to be transferred from the photon to the electron and changing the angle of the photon’s trajectory.
- electromagnetic field: a physical field consisting of a combined electric field (generated by stationary electric charges) and magnetic field (generated by moving electric charges) that affects the behavior of charged objects in its vicinity.
- gamma ray: a high-energy photon that is usually emitted by an unstable nucleus via radioactive decay; it may also be produced by various other means, including particle-antiparticle annihilation and the interaction of atmospheric particles with cosmic rays.
Mysterious "Extra" Particles
Humans are exposed to constant background radiation from both natural and artificial sources. Common sources include radon gas, foods such as bananas and carrots (which produce certain isotopes of potassium), medical imaging technology, and consumer products such as cigarettes. Background radiation is ionizing, meaning it has the potential to damage DNA and cause subsequent health problems. However, the average annual worldwide effective dose is 3.01 millisieverts (mSv); one sievert is equal to one joule of energy per kilogram of body mass. This is well below the lowest annual dose linked to a measurable increase in the incidence of cancer among a population (100 mSv).
Radioactivity was discovered in the late nineteenth century by French physicist Henri Becquerel (1852–1908). Subsequent research by him and others, including physicists Marie Curie (1867–1934), Ernest Rutherford (1871–1937), and Paul Villard (1860–1934), revealed that certain elements emit different types of radiation. The researchers identified three distinct types: positively charged alpha particles, negatively charged beta particles, and neutral gamma rays. All three are forms of ionizing radiation.
In 1912, Austrian physicist Victor Hess (1883–1964) identified another source of ionizing radiation: cosmic radiation, or cosmic rays. Hess observed that the amount of ionizing radiation in Earth’s atmosphere increased with altitude. This implied that it originated somewhere beyond the atmosphere. Later experiments determined that cosmic rays vary with latitude as well because their paths are affected by Earth’s electromagnetic field, meaning that they are electrically charged.
Over the next decade, further experiments found that cosmic rays consist of about 83–90 percent protons, 9–15 percent helium nuclei (alpha particles), and 1 percent heavier nuclei such as carbon, iron, and lead. These heavy nuclei are also called HZE particles due to their high atomic number (Z) and high energy (E). Proportions vary depending on the source of the rays. Cosmic radiation accounts for about 0.3–0.5 mSv of the average annual background radiation exposure. Commercial airline pilots and astronauts experience higher doses due to the amount of time they spend at high altitudes.
Stellar Origins
Cosmic rays can be divided into several categories. The main types are solar cosmic rays, or solar energetic particles (SEPs); galactic cosmic rays (GCRs); and anomalous cosmic rays (ACRs). SEPs come from the sun and are believed to result from solar flares, with a smaller amount produced by coronal mass ejections. GCRs originate outside the solar system but generally within the Milky Way galaxy. They are most likely produced in supernovas. ACRs are believed to be atoms of neutral interstellar gas that were ionized by solar ultraviolet radiation.
Cosmic rays are extremely high-energy particles. Their energies generally range from 10 megaelectronvolts (MeV; 107 eV), the lower range of SEPs and ACRs, to 1 petaelectronvolt (PeV; 1015 eV), the upper range of GCRs. Another, rarer type of cosmic radiation is ultra-high-energy cosmic rays (UHECRs), which have energies greater than 1015 eV. Their origin is unknown, but many astronomers believe that they come from outside the Milky Way. They may be the product of active galactic nuclei at the centers of certain types of galaxies, such as radio galaxies and Seyfert galaxies. The most energetic UHECR ever observed had an energy of 3.2 × 105 PeV (3.2 × 1020 eV)—about the same kinetic energy as a baseball traveling 100 kilometers per hour (62 mph), concentrated in a single atomic nucleus.
SEPs, GCRs, ACRs, and UHECRs are all considered primary cosmic rays because they come from outside Earth’s atmosphere. Primary cosmic rays are deflected by the magnetic fields of other bodies in space, so they appear to arrive at Earth from all directions. When they enter Earth’s upper atmosphere, they collide with atoms and molecules, producing a "shower" of secondary cosmic rays. These include lower-energy particles such as protons, electrons, neutrons, muons, pions, neutrinos, and alpha particles, as well as electromagnetic radiation such as x-rays and gamma rays. Because muons are the most resistant to energy loss and the effects of Earth’s electromagnetic field, they make up more than half of the cosmic rays that reach sea level. Muons can penetrate Earth’s surface down to the level of deep underground mines. Neutrinos also easily reach Earth from the upper atmosphere, but most pass through it without detection.
The collision of high-energy cosmic rays with much slower atmospheric particles can result in Compton scattering. This is when a photon collides with a charged particle, transferring some of its energy to that particle. If the charged particle gains enough energy to travel faster than the speed of light in atmosphere, it creates a shock wave that produces a flash of blue light. This light is called Cherenkov radiation. It can be used to determine the source and intensity of primary cosmic rays.
Other cosmic-ray detection methods include extensive air shower (EAS) arrays, cloud chambers, and nitrogen fluorescence. An EAS array is a collection of small ground-based devices that detect the passage of charged particles. A cloud chamber is a chamber filled with supersaturated air. When a charged particle passes through the chamber, the water vapor condenses around it. Nitrogen fluorescence occurs when charged particles excite atmospheric nitrogen, causing it to emit radiation.
In April 2016, researchers at NASA's Goddard Space Flight Center reported that the majority of cosmic rays detected in the vicinity of Earth were generated relatively recently, by supernovas in nearby massive star clusters, according to data gathered by NASA's Advanced Composition Explorer (ACE) spacecraft. ACE was able to identify the source of the cosmic rays by observing a rare type of GCR that contains the radioactive isotope iron-60, which has a half-life of about 2.6 million years—a relatively short period of time on a galactic scale, indicating an origin somewhere within about three thousand light-years of Earth. The researchers cited the Upper Scorpius, Upper Centaurus–Lupus, and Lower Centaurus–Crux star clusters (all subgroups of the Scorpius–Centaurus stellar association, approximately 380–470 light-years from Earth) as probable major sources of the iron-60-containing GCRs detected by ACE.
High Energy, High Stakes
Although most cosmic rays are rendered harmless by Earth’s atmosphere, some have enough energy to disrupt electronic circuits at ground level. A 1996 study by IBM estimated that cosmic rays cause one error per 256 megabytes (MB) of random-access memory (RAM) per month. The problem is greatly intensified for electronics beyond Earth’s atmosphere, such as satellites and spacecraft. Cosmic rays also pose a danger to humans who spend long periods in space. Thus, cosmic-ray shielding will be an important consideration for future space travel.
Cosmic rays have beneficial aspects as well. They served as the original source of high-energy particles for early physics experiments, before the widespread construction of particle accelerators. They are also responsible for the production of unstable isotopes in Earth’s atmosphere, including carbon-14, which is used for carbon dating in archaeology. Ground-based and satellite research projects, such as the IceCube Neutrino Observatory and the Fermi Gamma-Ray Space Telescope, rely on cosmic rays to produce neutrinos and to identify sources of high-energy gamma rays, respectively.
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