What is a gamma ray? What produces these rays? Are there gamma rays in space? Are there gamma rays here on Earth?
In the following NASA video, the creation of anti matter is revealed, from an unlikely source, right here on Earth.. Behold the mystery…
The sun creates Solar flares, which produce gamma rays by several processes, one of which is illustrated in the video above. The energy released in a solar flare rapidly accelerates charged particles. When a high-energy proton strikes matter in the sun’s atmosphere and visible surface, the result may be a short-lived particle — a pion — that emits gamma rays when it decays. Credit: NASA’s Goddard Space Flight Center
According to Wikipedia; “Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency and therefore high energy. Gamma rays are ionizing radiation and are thus biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium during its gamma decay. Villard’s radiation was named “gamma rays” by Ernest Rutherford in 1903.
Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes, and secondary radiation from atmospheric interactions withcosmic ray particles. Rare terrestrial natural sources produce gamma rays that are not of a nuclear origin, such as lightning strikes and terrestrial gamma-ray flashes. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced, that in turn cause secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere and must be detected by spacecraft.
Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV. In astronomy, gamma rays are defined by their energy, and no production process need be specified. The energies of gamma rays from astronomical sources range over 10 TeV, at a level far too large to result from radioactive decay. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay. These bursts of gamma rays, thought to be due to the collapse of stars called hypernovas, are the most powerful events so far discovered in the cosmos.
Natural sources of gamma rays on Earth include gamma decay from naturally occurringradioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere and must be detected by spacecraft. Notable artificial sources of gamma rays include fission such as occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.
Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. On the other side of the decay energy range, there is effectively no lower limit to gamma energy derived from radioactive decay. By contrast, the energies of gamma rays from astronomical sources can be much higher, ranging over 10 TeV, at a level far too large to result from radioactive decay.
The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubesalmost invariably had a longer wavelength than the radiation (gamma rays) emitted byradioactive nuclei. Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays. However, with artificial sources now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types, now completely overlap.
Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus. Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but other high energy processes known to involve other than radioactive decay are still classed as sources of gamma radiation.
A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay. These bursts of gamma rays, thought to be due to the collapse of stars called hypernovas, are the most powerful events so far discovered in the cosmos.
Naming conventions and overlap in terminology
The Moon as seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. These are produced by cosmic ray bombardment of its surface. The Sun, which has no similar surface of high atomic number to act as target for cosmic rays, cannot usually be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion (though occasionally the Sun produces gamma rays by cyclotron-type mechanisms, during solar flares). Gamma rays have higher energy than X-rays.
In the past, the distinction between X-rays and gamma rays was based on energy, with gamma rays being considered a higher-energy version of electromagnetic radiation. However, modern high-energy X-rays produced by linear accelerators formegavoltage treatment in cancer often have higher energy (4 to 25 MeV) than do most classical gamma rays produced by nuclear gamma decay.
One of the most common gamma ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of the same energy (140 keV) as that produced by diagnostic X-ray machines, but of significantly lower energy than therapeutic photons from linear particle accelerators. In the medical community today, the convention that radiation produced by nuclear decay is the only type referred to as “gamma” radiation is still respected.
Because of this broad overlap in energy ranges, in physics the two types of electromagnetic radiation are now often defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produceBremsstrahlung-type radiation), while gamma rays are emitted by the nucleus or by means of other particle decays or annihilation events.
There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet or lower energy photons produced by these processes would also be defined as “gamma rays”. The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is always referred to as “gamma rays,” and never as X-rays. However, in physics and astronomy, the reverse convention that all gamma rays are considered to be of nuclear origin is frequently violated.
In astronomy, higher energy gamma and X-rays are defined by energy, since the processes which produce them may be uncertain and photon energy, not origin, determines the required astronomical detectors needed. High energy photons occur in nature which are known to be produced by processes other than nuclear decay but are still referred to as gamma radiation. An example is “gamma rays” from lightning discharges at 10 to 20 MeV, and known to be produced by the Bremsstrahlung mechanism.
Another example is gamma ray bursts, now known to be produced from processes too powerful to involve simple collections of atoms undergoing radioactive decay. This has led to the realization that many gamma rays produced in astronomical processes result not from radioactive decay or particle annihilation, but rather in much the same manner as the production of X-rays.
Although gamma rays in astronomy are discussed below as non-radioactive events, in fact a few gamma rays are known in astronomy to originate explicitly from gamma decay of nucleus (by their spectra and half-life). A classic example is that of supernova SN 1987A, which emits an “afterglow” of gamma-ray photons from the decay of newly made radioactive cobalt-56. Most gamma rays in astronomy, however, arise by other mechanisms. Note that, astronomical literature tends to write “gamma-ray” with a hyphen, by analogy to X-rays, rather than in a way analogous to alpha rays and beta rays. This notation tends to subtly stress the non-nuclear source of most astronomical gamma rays.
Gamma rays from sources other than radioactive decay
A few gamma rays in astronomy are known to arise from gamma decay (see discussion ofSN1987A) but most do not.
Gamma radiation, like X-radiation, can be produced by a variety of phenomena. When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms within emit characteristic “secondary” gamma rays, which are products of the temporary creation of excited nuclear states in the bombarded atoms (such transitions form a topic in nuclear spectroscopy). Such gamma rays are produced by the nucleus, but not as a result of nuclear excitement from radioactive decay.
Energy in the gamma radiation range, often explicitly called gamma-radiation when it comes from astrophysical sources, is also produced by sub-atomic particle and particle-photon interactions. These include electron-positron annihilation, neutral pion decay, bremsstrahlung, inverse Compton scattering and synchrotron radiation.
The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010. Credit: NASA/Goddard Space Flight Center.
In a terrestrial gamma-ray flash a brief pulse of gamma radiation can occur in the Earth’s atmosphere, inside thunderstorms. These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays by bremsstrahlung as they collide with and are slowed by atoms in the atmosphere.
Gamma rays up to 100 MeV can be emitted by terrestrial thunderstorms, as first discovered by space-borne observatories. Current theories hold that the strong electric fields within thunderclouds unleash avalanches of electrons that reach relativistic speeds before colliding with air molecules to release powerful gamma rays. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds.
High energy gamma rays in astronomy include the gamma ray background produced whencosmic rays (either high speed electrons or protons) interact with ordinary matter, producing pair-production gamma rays at 511 keV. Alternatively bremsstrahlung at energies of tens of MeV or more are produced when cosmic ray electrons interact with nuclei of sufficiently high atomic number (see gamma ray image of the Moon at the beginning of this article, for illustration).
Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGROspacecraft. Bright spots within the galactic plane arepulsars while those above and below the plane are thought to be quasars.
Pulsars and magnetars. The gamma ray sky (see illustration at right) is dominated by the more common and longer-term production of gamma rays in beams that emanate frompulsars within the Milky Way. Sources from the rest of the sky are mostly quasars. Pulsars are thought to be neutron stars with magnetic fields that produce focused beams of radiation, and are far less energetic, more common, and much nearer (typically seen only in our own galaxy) than are quasars or the rarer sources of gamma ray bursts.
In a pulsar, which produces gamma rays for much longer than a burst, the relatively long-lived magnetic field of the pulsar produces focused beams of relativistic speed charged particles, which produce gamma rays (bremsstrahlung) when these charged particles strike gas or dust in the nearby medium, and are decelerated. This is a similar mechanism to the production of high energy photons in megavoltage radiation therapymachines (see bremsstrahlung).
The “inverse Compton effect“, in which charged particles (usually electrons) scatter from low-energy photons to convert them to higher energy photons is another possible mechanism of gamma ray production from relativistic charged particle beams. Neutron stars with a very high magnetic field (magnetars) are thought to produce astronomical soft gamma repeaters, which are another relatively long-lived star-powered source of gamma radiation.
Quasars and active galaxies. More powerful gamma rays from more distant quasars and active nearby galaxies probably have a roughly similar linear particle accelerator-like method of gamma ray production. High energy electrons produced by the quasar, followed again by inverse Compton scattering, synchrotron radiation, or bremsstrahlung, likely produce the gamma rays.
As the black hole at the center of such galaxies intermittantly destroys stars and focuses charged particles derived from them into beams, these beams interact with gas, dust, and lower energy photons to produce X-ray and gamma ray radiation. These sources are known to fluctuate with durations of a few weeks, indicating their relatively small size (less than a few light-weeks across). The particle beams emerge from the rotatational poles of the supermassive black hole at a galactic center, which is thought to form the power source of the quasar.
Such sources of gamma and X-rays are the most commonly visible high intensity sources outside our own galaxy, since they shine not as bursts (see illustration), but instead relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 1040 watts, of which only a small fraction is emitted as gamma radiation, and much of the rest is emitted as electromagnetic waves at all frequencies, including radio waves.
A hypernova. Artist’s illustration showing the life of amassive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a black hole. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration gamma-ray burst.
The most intense sources of gamma rays known, are also the most intense sources of any type of electromagnetic radiation presently known. They are rare compared with the sources discussed above. These intense sources are the “long duration burst” sources of gamma rays in astronomy (“long” in this context, meaning a few tens of seconds).
By contrast, “short” gamma ray bursts, which are not associated with supernovae, are thought to produce gamma rays during the collision of pairs of neutron stars, or a neutron star and black holeafter they spiral toward each other by emission of gravitational waves; such bursts last two seconds or less, and are of far lower energy than the “long” bursts (they are often seen only in our own galaxy for this reason).
The so-called long-duration gamma ray bursts produce events in which energies of ~ 1044joules (as much energy as our Sun will produce in its entire life-time) but over a period of only 20 to 40 seconds, accompanied by high-efficiency conversion to gamma rays (on the order of 50% total energy conversion).
The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse Compton scattering and synchrotron radiation production of gamma rays from high-energy charged particles. These processes occur as relativistic charged particles leaving the region near the event horizon of the newly formed black hole during the supernova explosion, and focused for a few tens of seconds into a relativistic beam by the magnetic field of the exploding hypernova.
The fusion explosion of the hypernova drives the energetics of the process. If the narrowly directed beam happens to be pointed toward the Earth, it shines with high gamma ray power even at distances of up to 10 billion light years—close to the edge of the visible universe.
Gamma-ray image of a truck with two stowaways taken with a VACIS (vehicle and container imaging system)
Gamma rays travel to Earth across vast distances of the universe, only to be absorbed by Earth’s atmosphere. Different wavelengths of light penetrate Earth’s atmosphere to different depths. Instruments aboard high-altitude balloons and such satellites as the Compton Observatory provide our only view of the gamma spectrum sky.
Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz.
Non-contact industrial sensors used in the Refining, Mining, Chemical, Food, Soaps and Detergents, and Pulp and Paper industries, in applications measuring levels, density, and thicknesses commonly use sources of gamma. Typically these use Co-60 or Cs-137 isotopes as the radiation source.
In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These US $5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to screen merchant ship containers before they enter US ports.
Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor.
Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer, since the rays kill cancer cells also. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.
Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in a PET scan a radiolabled sugar called fludeoxyglucose emits positrons that are converted to pairs of gamma rays that localize cancer (which often takes up more sugar than other surrounding tissues).
The most common gamma emitter used in medical applications is the nuclear isomer technetium-99m which emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, a gamma camera can be used to form an image of the radioisotope’s distribution by detecting the gamma radiation emitted (see also SPECT).
Depending on what molecule has been labeled with the tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones in a bone scan).