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109-519: HIPASS covers a velocity range of −1,280 to 12,700 km/s. It was the first blind HI survey to cover the entire southern sky and the northern sky up to +25°. Technical overview, calibration and imaging (Barnes et al. 2001). Observations of the southern sky started in February 1997, and were completed in March 2000, consisting of 23,020 eight-degree scans of each of 9 minutes duration. HIPASS scanned

218-451: A dimensionless quantity called z . If λ represents wavelength and f represents frequency (note, λf = c where c is the speed of light ), then z is defined by the equations: After z is measured, the distinction between redshift and blueshift is simply a matter of whether z is positive or negative. For example, Doppler effect blueshifts ( z < 0 ) are associated with objects approaching (moving closer to)

327-760: A focal-plane array of 13 individual receivers arranged in a hexagonal pattern. Built in a collaboration between numerous institutions, it was funded by the Australian Research Council (ARC) and the Australia Telescope National Facility (ATNF) to undertake the HIPASS and ZOA surveys. HIPASS discovered the Leading Arm of the Magellanic Stream. This is an extension of the Magellanic Stream beyond

436-494: A ( t ) in the whole period from emission to absorption." If the universe were contracting instead of expanding, we would see distant galaxies blueshifted by an amount proportional to their distance instead of redshifted. In the theory of general relativity , there is time dilation within a gravitational well. This is known as the gravitational redshift or Einstein Shift . The theoretical derivation of this effect follows from

545-455: A French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium . Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as "radioactivity" by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as

654-438: A crystal. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition. Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy

763-624: A decrease in wavelength and increase in frequency and energy, is known as a blueshift , or negative redshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum . The main causes of electromagnetic redshift in astronomy and cosmology are the relative motions of radiation sources, which give rise to the relativistic Doppler effect , and gravitational potentials, which gravitationally redshift escaping radiation. All sufficiently distant light sources show cosmological redshift corresponding to recession speeds proportional to their distances from Earth,

872-415: A different fundamental type. Later, in 1903, Villard's radiation was recognized as being of a type fundamentally different from previously named rays by Ernest Rutherford , who named Villard's rays "gamma rays" by analogy with the beta and alpha rays that Rutherford had differentiated in 1899. The "rays" emitted by radioactive elements were named in order of their power to penetrate various materials, using

981-441: A fact known as Hubble's law that implies the universe is expanding . All redshifts can be understood under the umbrella of frame transformation laws . Gravitational waves , which also travel at the speed of light , are subject to the same redshift phenomena. The value of a redshift is often denoted by the letter z , corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by

1090-478: A few weeks, suggesting their relatively small size (less than a few light-weeks across). Such sources of gamma and X-rays are the most commonly visible high intensity sources outside the Milky Way galaxy. They shine not in bursts (see illustration), but relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 10 watts, a small fraction of which is gamma radiation. Much of

1199-448: A formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete. On Earth , the magnetosphere protects life from most types of lethal cosmic radiation other than gamma rays. The first gamma ray source to be discovered was the radioactive decay process called gamma decay . In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation. Paul Villard ,

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1308-528: A lower frequency. A more complete treatment of the Doppler redshift requires considering relativistic effects associated with motion of sources close to the speed of light. A complete derivation of the effect can be found in the article on the relativistic Doppler effect . In brief, objects moving close to the speed of light will experience deviations from the above formula due to the time dilation of special relativity which can be corrected for by introducing

1417-455: A magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation. Rutherford and his co-worker Edward Andrade measured the wavelengths of gamma rays from radium, and found they were similar to X-rays , but with shorter wavelengths and thus, higher frequency. This was eventually recognized as giving them more energy per photon , as soon as

1526-457: A means for sources of GeV photons using lasers as exciters through a controlled interplay between the cascade and anomalous radiative trapping . Thunderstorms can produce a brief pulse of gamma radiation called a terrestrial gamma-ray flash . 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

1635-510: A new 20cm confusion-limited continuum map of the sky south of declination +25°. Its relatively high sensitivity and resolution (compared to other single-dish surveys) and low level of artefacts has made this survey invaluable, particularly for merging with interferometric data such as WALLABY to improve the coverage of extended structure. Observations for HIPASS were taken using the Parkes 21-cm Multibeam Receiver. The instrument consists of

1744-408: A nuclear power plant, shielding can be provided by steel and concrete in the pressure and particle containment vessel, while water provides a radiation shielding of fuel rods during storage or transport into the reactor core. The loss of water or removal of a "hot" fuel assembly into the air would result in much higher radiation levels than when kept under water. When a gamma ray passes through matter,

1853-554: 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. Notable artificial sources of gamma rays include fission , such as occurs in nuclear reactors , as well as high energy physics experiments, such as neutral pion decay and nuclear fusion . A sample of gamma ray-emitting material that

1962-449: A qualitative characterization of a redshift. For example, if a Sun-like spectrum had a redshift of z = 1 , it would be brightest in the infrared (1000nm) rather than at the blue-green(500nm) color associated with the peak of its blackbody spectrum, and the light intensity will be reduced in the filter by a factor of four, (1 + z ) . Both the photon count rate and the photon energy are redshifted. (See K correction for more details on

2071-447: A shift in the frequency of electromagnetic radiation, including scattering and optical effects ; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer ). The history of the subject began in the 19th century, with the development of classical wave mechanics and the exploration of phenomena which are associated with

2180-428: A single emission or absorption line. By measuring the broadening and shifts of the 21-centimeter hydrogen line in different directions, astronomers have been able to measure the recessional velocities of interstellar gas , which in turn reveals the rotation curve of our Milky Way. Similar measurements have been performed on other galaxies, such as Andromeda . As a diagnostic tool, redshift measurements are one of

2289-522: A wide scatter from the standard Hubble Law . The resulting situation can be illustrated by the Expanding Rubber Sheet Universe , a common cosmological analogy used to describe the expansion of the universe. If two objects are represented by ball bearings and spacetime by a stretching rubber sheet, the Doppler effect is caused by rolling the balls across the sheet to create peculiar motion. The cosmological redshift occurs when

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2398-686: Is a binary system and is not dense enough to form stars. HIPASS J1712-64 was probably ejected during an interaction between the Magellanic clouds and the Milky way. Ten new galaxies were identified in the Centaurus A/M83 Group , bringing the total (at the time) to 31 galaxies. Redshift In physics , a redshift is an increase in the wavelength , and corresponding decrease in the frequency and photon energy , of electromagnetic radiation (such as light ). The opposite change,

2507-403: Is also a mode of relaxation of many excited states of atomic nuclei following other types of radioactive decay, such as beta decay, so long as these states possess the necessary component of nuclear spin . When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms emit characteristic "secondary" gamma rays, which are products of the creation of excited nuclear states in

2616-620: Is also sufficient to excite the same energy state in a second immobilized nucleus of the same type. Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth's atmosphere. Instruments aboard high-altitude balloons and satellites missions, such as the Fermi Gamma-ray Space Telescope , provide our only view of the universe in gamma rays. Gamma-induced molecular changes can also be used to alter

2725-448: Is another possible mechanism of gamma ray production. Neutron stars with a very high magnetic field ( magnetars ), thought to produce astronomical soft gamma repeaters , are another relatively long-lived star-powered source of gamma radiation. More powerful gamma rays from very distant quasars and closer active galaxies are thought to have a gamma ray production source similar to a particle accelerator . High energy electrons produced by

2834-403: Is classified as X-rays and is the subject of X-ray astronomy . Gamma rays are ionizing radiation and are thus hazardous to life. They can cause DNA mutations , cancer and tumors , and at high doses burns and radiation sickness . Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and thus pose

2943-446: Is close to the edge of the visible universe . Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to alpha particles , which can be stopped by paper or skin, and beta particles , which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high atomic numbers ( Z ) and high density, which contribute to

3052-443: Is commonly attributed to stretching of the wavelengths of photons propagating through the expanding space. This interpretation can be misleading, however; expanding space is only a choice of coordinates and thus cannot have physical consequences. The cosmological redshift is more naturally interpreted as a Doppler shift arising due to the recession of distant objects. The observational consequences of this effect can be derived using

3161-427: Is defined as the probability of cancer induction and genetic damage. The International Commission on Radiological Protection says "In the low dose range, below about 100 mSv, it is scientifically plausible to assume that the incidence of cancer or heritable effects will rise in direct proportion to an increase in the equivalent dose in the relevant organs and tissues" High doses produce deterministic effects, which

3270-404: Is dominated by the more common and longer-term production of gamma rays that emanate from pulsars 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 sources (typically seen only in our own galaxy) than are quasars or

3379-419: Is followed 99.88% of the time: Another example is the alpha decay of Am to form Np ; which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g. Co / Ni ) while in other cases, such as with ( Am / Np and Ir / Pt ),

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3488-414: Is much slower in the case of a low-dose exposure. Studies have shown low-dose gamma radiation may be enough to cause cancer. In a study of mice, they were given human-relevant low-dose gamma radiation, with genotoxic effects 45 days after continuous low-dose gamma radiation, with significant increases of chromosomal damage, DNA lesions and phenotypic mutations in blood cells of irradiated animals, covering

3597-439: Is not moving away from the observer. Even when the source is moving towards the observer, if there is a transverse component to the motion then there is some speed at which the dilation just cancels the expected blueshift and at higher speed the approaching source will be redshifted. In the earlier part of the twentieth century, Slipher, Wirtz and others made the first measurements of the redshifts and blueshifts of galaxies beyond

3706-606: Is not required. The effect is very small but measurable on Earth using the Mössbauer effect and was first observed in the Pound–Rebka experiment . However, it is significant near a black hole , and as an object approaches the event horizon the red shift becomes infinite. It is also the dominant cause of large angular-scale temperature fluctuations in the cosmic microwave background radiation (see Sachs–Wolfe effect ). The redshift observed in astronomy can be measured because

3815-408: Is the severity of acute tissue damage that is certain to happen. These effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy). When gamma radiation breaks DNA molecules, a cell may be able to repair the damaged genetic material, within limits. However, a study of Rothkamm and Lobrich has shown that this repair process works well after high-dose exposure but

3924-417: Is the present-day Hubble constant , and z is the redshift. There are several websites for calculating various times and distances from redshift, as the precise calculations require numerical integrals for most values of the parameters. For cosmological redshifts of z < 0.01 additional Doppler redshifts and blueshifts due to the peculiar motions of the galaxies relative to one another cause

4033-499: Is used for irradiating or imaging is known as a gamma source. It is also called a radioactive source , isotope source, or radiation source, though these more general terms also apply to alpha and beta-emitting devices. Gamma sources are usually sealed to prevent radioactive contamination , and transported in heavy shielding. Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such as alpha or beta decay. A radioactive nucleus can decay by

4142-425: Is used instead. Redshifts cannot be calculated by looking at unidentified features whose rest-frame frequency is unknown, or with a spectrum that is featureless or white noise (random fluctuations in a spectrum). Redshift (and blueshift) may be characterized by the relative difference between the observed and emitted wavelengths (or frequency) of an object. In astronomy, it is customary to refer to this change using

4251-673: The Cygnus X-3 microquasar . Natural sources of gamma rays originating on Earth are mostly a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However, there are other rare natural sources, such as terrestrial gamma-ray flashes , which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission , such as that which occurs in nuclear reactors , and high energy physics experiments, such as neutral pion decay and nuclear fusion . The energy ranges of gamma rays and X-rays overlap in

4360-461: The Doppler effect . Consequently, this type of redshift is called the Doppler redshift . If the source moves away from the observer with velocity v , which is much less than the speed of light ( v ≪ c ), the redshift is given by where c is the speed of light . In the classical Doppler effect, the frequency of the source is not modified, but the recessional motion causes the illusion of

4469-512: The Doppler effect . The effect is named after the Austrian mathematician, Christian Doppler , who offered the first known physical explanation for the phenomenon in 1842. In 1845, the hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot . Doppler correctly predicted that the phenomenon would apply to all waves and, in particular, suggested that

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4578-588: The Friedmann–Lemaître equations . They are now considered to be strong evidence for an expanding universe and the Big Bang theory. The spectrum of light that comes from a source (see idealized spectrum illustration top-right) can be measured. To determine the redshift, one searches for features in the spectrum such as absorption lines , emission lines , or other variations in light intensity. If found, these features can be compared with known features in

4687-595: The Lorentz factor γ into the classical Doppler formula as follows (for motion solely in the line of sight): This phenomenon was first observed in a 1938 experiment performed by Herbert E. Ives and G.R. Stilwell, called the Ives–Stilwell experiment . Since the Lorentz factor is dependent only on the magnitude of the velocity, this causes the redshift associated with the relativistic correction to be independent of

4796-588: The Magellanic clouds . The existence of the Leading Arm is predicted by models of a tidal interaction between the Magellanic Clouds and the Milky Way. HIPASS J0731-69 is a cloud of gas devoid of any stars. It is associated with the asymmetric spiral galaxy NGC 2442 . It is likely that HIPASS J0731-69 was torn loose from NGC 2442 by a companion. HIPASS J1712-64 is an isolated extragalactic cloud of neutral hydrogen with no associated stars. The cloud

4905-681: The Milky Way . They initially interpreted these redshifts and blueshifts as being due to random motions, but later Lemaître (1927) and Hubble (1929), using previous data, discovered a roughly linear correlation between the increasing redshifts of, and distances to, galaxies. Lemaître realized that these observations could be explained by a mechanism of producing redshifts seen in Friedmann's solutions to Einstein's equations of general relativity . The correlation between redshifts and distances arises in all expanding models. This cosmological redshift

5014-465: The Schwarzschild geometry : In terms of escape velocity : for v e ≪ c {\displaystyle v_{\text{e}}\ll c} If a source of the light is moving away from an observer, then redshift ( z > 0 ) occurs; if the source moves towards the observer, then blueshift ( z < 0 ) occurs. This is true for all electromagnetic waves and is explained by

5123-463: The Schwarzschild solution of the Einstein equations which yields the following formula for redshift associated with a photon traveling in the gravitational field of an uncharged , nonrotating , spherically symmetric mass: where This gravitational redshift result can be derived from the assumptions of special relativity and the equivalence principle ; the full theory of general relativity

5232-722: The brightness of astronomical objects through certain filters . When photometric data is all that is available (for example, the Hubble Deep Field and the Hubble Ultra Deep Field ), astronomers rely on a technique for measuring photometric redshifts . Due to the broad wavelength ranges in photometric filters and the necessary assumptions about the nature of the spectrum at the light-source, errors for these sorts of measurements can range up to δ z = 0.5 , and are much less reliable than spectroscopic determinations. However, photometry does at least allow

5341-455: The electromagnetic spectrum , so the terminology for these electromagnetic waves varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: gamma rays are created by nuclear decay while X-rays originate outside the nucleus. In astrophysics , gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma-ray astronomy , while radiation below 100 keV

5450-451: The emission and absorption spectra for atoms are distinctive and well known, calibrated from spectroscopic experiments in laboratories on Earth. When the redshift of various absorption and emission lines from a single astronomical object is measured, z is found to be remarkably constant. Although distant objects may be slightly blurred and lines broadened, it is by no more than can be explained by thermal or mechanical motion of

5559-459: The extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectra. Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays. As in optical spectroscopy (see Franck–Condon effect)

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5668-637: The "annual Doppler effect", the yearly change in the Doppler shift of stars located near the ecliptic, due to the orbital velocity of the Earth. In 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. Arthur Eddington used the term "red-shift" as early as 1923, although the word does not appear unhyphenated until about 1934, when Willem de Sitter used it. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies , then mostly thought to be spiral nebulae , had considerable redshifts. Slipher first reported on his measurement in

5777-458: The Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by the method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines , using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered

5886-420: The K shell electrons of the atom, causing it to be ejected from that atom, in a process generally termed the photoelectric effect (external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with the internal conversion process, in which a gamma ray photon is not produced as an intermediate particle (rather, a "virtual gamma ray" may be thought to mediate

5995-444: The absorption of gamma rays by a nucleus is especially likely (i.e., peaks in a "resonance") when the energy of the gamma ray is the same as that of an energy transition in the nucleus. In the case of gamma rays, such a resonance is seen in the technique of Mössbauer spectroscopy . In the Mössbauer effect the narrow resonance absorption for nuclear gamma absorption can be successfully attained by physically immobilizing atomic nuclei in

6104-715: The annihilating electron and positron are at rest, each of the resulting gamma rays has an energy of ~ 511 keV and frequency of ~ 1.24 × 10  Hz . Similarly, a neutral pion most often decays into two photons. Many other hadrons and massive bosons also decay electromagnetically. High energy physics experiments, such as the Large Hadron Collider , accordingly employ substantial radiation shielding. Because subatomic particles mostly have far shorter wavelengths than atomic nuclei, particle physics gamma rays are generally several orders of magnitude more energetic than nuclear decay gamma rays. Since gamma rays are at

6213-441: The atmosphere. Gamma rays up to 100 MeV can be emitted by terrestrial thunderstorms, and were discovered by space-borne observatories. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds. The most effusive solar flares emit across the entire EM spectrum, including γ-rays. The first confident observation occurred in 1972 . Extraterrestrial, high energy gamma rays include

6322-470: The average 10 seconds. Such relatively long-lived excited nuclei are termed nuclear isomers , and their decays are termed isomeric transitions . Such nuclei have half-lifes that are more easily measurable, and rare nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves

6431-406: The ball bearings are stuck to the sheet and the sheet is stretched. The redshifts of galaxies include both a component related to recessional velocity from expansion of the universe, and a component related to peculiar motion (Doppler shift). The redshift due to expansion of the universe depends upon the recessional velocity in a fashion determined by the cosmological model chosen to describe

6540-482: The bombarded atoms. Such transitions, a form of nuclear gamma fluorescence , form a topic in nuclear physics called gamma spectroscopy . Formation of fluorescent gamma rays are a rapid subtype of radioactive gamma decay. In certain cases, the excited nuclear state that follows the emission of a beta particle or other type of excitation, may be more stable than average, and is termed a metastable excited state, if its decay takes (at least) 100 to 1000 times longer than

6649-496: The cancer often has a higher metabolic rate than the 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 which molecule has been labeled with

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6758-496: 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 radiolabeled sugar called fluorodeoxyglucose emits positrons that are annihilated by electrons, producing pairs of gamma rays that highlight cancer as

6867-663: The collision of pairs of neutron stars, or a neutron star and a black hole . The so-called long-duration gamma-ray bursts produce a total energy output of about 10 joules (as much energy as the Sun will produce in its entire life-time) but in a period of only 20 to 40 seconds. Gamma rays are approximately 50% of the total energy output. The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse Compton scattering and synchrotron radiation from high-energy charged particles. These processes occur as relativistic charged particles leave

6976-479: The emission of an α or β particle. The daughter nucleus that results is usually left in an excited state. It can then decay to a lower energy state by emitting a gamma ray photon, in a process called gamma decay. The emission of a gamma ray from an excited nucleus typically requires only 10 seconds. Gamma decay may also follow nuclear reactions such as neutron capture , nuclear fission , or nuclear fusion. Gamma decay

7085-562: The energy of the gamma rays, the thicker the shielding made from the same shielding material is required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half-value layer or HVL). For example, gamma rays that require 1 cm (0.4 inch) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2.5 inches) of concrete , or 9 cm (3.5 inches) of packed soil . However,

7194-425: The energy range from a few kilo electronvolts (keV) to approximately 8 megaelectronvolts (MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying radionuclides using gamma spectroscopy . Very-high-energy gamma rays in the 100–1000 teraelectronvolt (TeV) range have been observed from astronomical sources such as

7303-542: The entire southern sky five times. The southern HIPASS galaxy catalog (HICAT) contains 4315 HI sources. Northern HIPASS extended the survey into the northern sky . The entire Virgo Cluster region was observed in Northern HIPASS. NHICAT, the catalogue of the northern extension of HIPASS contains 1,002 H I sources. Archival data from HIPASS and the HI Zone of Avoidance (HIZOA) survey were reprocessed to make

7412-407: The equations from general relativity that describe a homogeneous and isotropic universe . The cosmological redshift can thus be written as a function of a , the time-dependent cosmic scale factor : In an expanding universe such as the one we inhabit, the scale factor is monotonically increasing as time passes, thus, z is positive and distant galaxies appear redshifted. Using a model of

7521-435: The expansion of the universe, redshift can be related to the age of an observed object, the so-called cosmic time –redshift relation . Denote a density ratio as Ω 0 : with ρ crit the critical density demarcating a universe that eventually crunches from one that simply expands. This density is about three hydrogen atoms per cubic meter of space. At large redshifts, 1 + z > Ω 0 , one finds: where H 0

7630-403: The expansion of the universe, which is very different from how Doppler redshift depends upon local velocity. Describing the cosmological expansion origin of redshift, cosmologist Edward Robert Harrison said, "Light leaves a galaxy, which is stationary in its local region of space, and is eventually received by observers who are stationary in their own local region of space. Between the galaxy and

7739-583: The first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays. Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by

7848-403: The full form for the relativistic Doppler effect becomes: and for motion solely in the line of sight ( θ = 0° ), this equation reduces to: For the special case that the light is moving at right angle ( θ = 90° ) to the direction of relative motion in the observer's frame, the relativistic redshift is known as the transverse redshift , and a redshift: is measured, even though the object

7957-423: The gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist. Gamma rays are produced in many processes of particle physics . Typically, gamma rays are the products of neutral systems which decay through electromagnetic interactions (rather than a weak or strong interaction). For example, in an electron–positron annihilation , the usual products are two gamma ray photons. If

8066-513: The gamma ray background produced when cosmic rays (either high speed electrons or protons) collide with ordinary matter, producing pair-production gamma rays at 511 keV. Alternatively, bremsstrahlung are produced at energies of tens of MeV or more when cosmic ray electrons interact with nuclei of sufficiently high atomic number (see gamma ray image of the Moon near the end of this article, for illustration). The gamma ray sky (see illustration at right)

8175-548: The highest photon energy of any form of electromagnetic radiation. Paul Villard , a French chemist and physicist , discovered gamma radiation in 1900 while studying radiation emitted by radium . In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter ; in 1900, he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel ) alpha rays and beta rays in ascending order of penetrating power. Gamma rays from radioactive decay are in

8284-486: The inaugural volume of the Lowell Observatory Bulletin . Three years later, he wrote a review in the journal Popular Astronomy . In it he stated that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km(/s) showed the means then available, capable of investigating not only the spectra of the spirals but their velocities as well." Slipher reported

8393-494: The intermediate metastable excited state(s) of the nuclei. Metastable states are often characterized by high nuclear spin , requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10 seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small. An emitted gamma ray from any type of excited state may transfer its energy directly to any electrons , but most probably to one of

8502-551: The latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon. Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40 , and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Natural terrestrial sources that produce gamma rays include lightning strikes and terrestrial gamma-ray flashes , which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by

8611-402: The mass of this much concrete or soil is only 20–30% greater than that of lead with the same absorption capability. Depleted uranium is sometimes used for shielding in portable gamma ray sources , due to the smaller half-value layer when compared to lead (around 0.6 times the thickness for common gamma ray sources, i.e. Iridium-192 and Cobalt-60) and cheaper cost compared to tungsten . In

8720-623: The material (atomic density) and σ the absorption cross section in cm . As it passes through matter, gamma radiation ionizes via three processes: The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionization themselves. Additionally, gamma rays, particularly high energy ones, can interact with atomic nuclei resulting in ejection of particles in photodisintegration , or in some cases, even nuclear fission ( photofission ). High-energy (from 80 GeV to ~10 TeV ) gamma rays arriving from far-distant quasars are used to estimate

8829-491: The most important spectroscopic measurements made in astronomy. The most distant objects exhibit larger redshifts corresponding to the Hubble flow of the universe . The largest-observed redshift, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background radiation; the numerical value of its redshift is about z = 1089 ( z = 0 corresponds to present time), and it shows

8938-449: The observer with the light shifting to greater energies . Conversely, Doppler effect redshifts ( z > 0 ) are associated with objects receding (moving away) from the observer with the light shifting to lower energies. Likewise, gravitational blueshifts are associated with light emitted from a source residing within a weaker gravitational field as observed from within a stronger gravitational field, while gravitational redshifting implies

9047-410: The observer, light travels through vast regions of expanding space. As a result, all wavelengths of the light are stretched by the expansion of space. It is as simple as that..." Steven Weinberg clarified, "The increase of wavelength from emission to absorption of light does not depend on the rate of change of a ( t ) [the scale factor ] at the times of emission or absorption, but on the increase of

9156-439: The opposite conditions. In general relativity one can derive several important special-case formulae for redshift in certain special spacetime geometries, as summarized in the following table. In all cases the magnitude of the shift (the value of z ) is independent of the wavelength. For motion completely in the radial or line-of-sight direction: For motion completely in the transverse direction: Hubble's law : For

9265-400: The orientation of the source movement. In contrast, the classical part of the formula is dependent on the projection of the movement of the source into the line-of-sight which yields different results for different orientations. If θ is the angle between the direction of relative motion and the direction of emission in the observer's frame (zero angle is directly away from the observer),

9374-525: The photometric consequences of redshift.) In nearby objects (within our Milky Way galaxy) observed redshifts are almost always related to the line-of-sight velocities associated with the objects being observed. Observations of such redshifts and blueshifts have enabled astronomers to measure velocities and parametrize the masses of the orbiting stars in spectroscopic binaries , a method first employed in 1868 by British astronomer William Huggins . Similarly, small redshifts and blueshifts detected in

9483-507: The precise movements of the photosphere of the Sun . Redshifts have also been used to make the first measurements of the rotation rates of planets , velocities of interstellar clouds , the rotation of galaxies , and the dynamics of accretion onto neutron stars and black holes which exhibit both Doppler and gravitational redshifts. The temperatures of various emitting and absorbing objects can be obtained by measuring Doppler broadening —effectively redshifts and blueshifts over

9592-424: The probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material. The total absorption shows an exponential decrease of intensity with distance from the incident surface: where x is the thickness of the material from the incident surface, μ= n σ is the absorption coefficient, measured in cm , n the number of atoms per cm of

9701-471: The process). One example of gamma ray production due to radionuclide decay is the decay scheme for cobalt-60, as illustrated in the accompanying diagram. First, Co decays to excited Ni by beta decay emission of an electron of 0.31  MeV . Then the excited Ni decays to the ground state (see nuclear shell model ) by emitting gamma rays in succession of 1.17 MeV followed by 1.33 MeV . This path

9810-466: The properties of semi-precious stones , and is often used to change white topaz into blue topaz . Non-contact industrial sensors commonly use sources of gamma radiation in refining, mining, chemicals, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses. Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries. Typically, these use Co-60 or Cs-137 isotopes as

9919-553: The quasar, and subjected to inverse Compton scattering, synchrotron radiation , or bremsstrahlung, are the likely source of the gamma rays from those objects. It is thought that a supermassive black hole at the center of such galaxies provides the power source that intermittently destroys stars and focuses the resulting charged particles into beams that emerge from their rotational poles. When those beams interact with gas, dust, and lower energy photons they produce X-rays and gamma rays. These sources are known to fluctuate with durations of

10028-553: The radiation source. In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These machines are advertised to be able to scan 30 containers per hour. Gamma radiation is often used to kill living organisms, in a process called irradiation . Applications of this include the sterilization of medical equipment (as an alternative to autoclaves or chemical means),

10137-638: The rarer gamma-ray burst sources of gamma rays. Pulsars have relatively long-lived magnetic fields that produce focused beams of relativistic speed charged particles, which emit gamma rays (bremsstrahlung) when those strike gas or dust in their nearby medium, and are decelerated. This is a similar mechanism to the production of high-energy photons in megavoltage radiation therapy machines (see bremsstrahlung ). Inverse Compton scattering , in which charged particles (usually electrons) impart energy to low-energy photons boosting them to higher energy photons. Such impacts of photons on relativistic charged particle beams

10246-411: The redshift, one has to know the wavelength of the emitted light in the rest frame of the source: in other words, the wavelength that would be measured by an observer located adjacent to and comoving with the source. Since in astronomical applications this measurement cannot be done directly, because that would require traveling to the distant star of interest, the method using spectral lines described here

10355-523: The region of the event horizon of a newly formed black hole created during supernova explosion. The beam of particles moving at relativistic speeds are focused for a few tens of seconds 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 at gamma ray frequencies with such intensity, that it can be detected even at distances of up to 10 billion light years, which

10464-422: The removal of decay-causing bacteria from many foods and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor. Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer , since the rays also kill cancer cells. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill

10573-556: The rest is emitted as electromagnetic waves of all frequencies, including radio waves. The most intense sources of gamma rays, are also the most intense sources of any type of electromagnetic radiation presently known. They are the "long duration burst" sources of gamma rays in astronomy ("long" in this context, meaning a few tens of seconds), and they are rare compared with the sources discussed above. By contrast, "short" gamma-ray bursts of two seconds or less, which are not associated with supernovae, are thought to produce gamma rays during

10682-419: The same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. If the same spectral line is identified in both spectra—but at different wavelengths—then the redshift can be calculated using the table below. Determining the redshift of an object in this way requires a frequency or wavelength range. In order to calculate

10791-416: The source. For these reasons and others, the consensus among astronomers is that the redshifts they observe are due to some combination of the three established forms of Doppler-like redshifts. Alternative hypotheses and explanations for redshift such as tired light are not generally considered plausible. Spectroscopy, as a measurement, is considerably more difficult than simple photometry , which measures

10900-422: The spectroscopic measurements of individual stars are one way astronomers have been able to diagnose and measure the presence and characteristics of planetary systems around other stars and have even made very detailed differential measurements of redshifts during planetary transits to determine precise orbital parameters. Finely detailed measurements of redshifts are used in helioseismology to determine

11009-416: The spectrum of various chemical compounds found in experiments where that compound is located on Earth. A very common atomic element in space is hydrogen . The spectrum of originally featureless light shone through hydrogen will show a signature spectrum specific to hydrogen that has features at regular intervals. If restricted to absorption lines it would look similar to the illustration (top right). If

11118-598: The state of the universe about 13.8 billion years ago, and 379,000 years after the initial moments of the Big Bang . Gamma ray A gamma ray , also known as gamma radiation (symbol γ ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei . It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays . With frequencies above 30 exahertz ( 3 × 10  Hz ) and wavelengths less than 10 picometers ( 1 × 10  m ), gamma ray photons have

11227-516: The three types of genotoxic activity. Another study studied the effects of acute ionizing gamma radiation in rats, up to 10 Gy , and who ended up showing acute oxidative protein damage, DNA damage, cardiac troponin T carbonylation, and long-term cardiomyopathy . The natural outdoor exposure in the United Kingdom ranges from 0.1 to 0.5 μSv/h with significant increase around known nuclear and contaminated sites. Natural exposure to gamma rays

11336-813: The top of the electromagnetic spectrum in terms of energy, all extremely high-energy photons are gamma rays; for example, a photon having the Planck energy would be a gamma ray. A few gamma rays in astronomy are known to arise from gamma decay (see discussion of SN1987A ), but most do not. Photons from astrophysical sources that carry energy in the gamma radiation range are often explicitly called gamma-radiation. In addition to nuclear emissions, they are often produced by sub-atomic particle and particle-photon interactions. Those include electron-positron annihilation , neutral pion decay , bremsstrahlung , inverse Compton scattering , and synchrotron radiation . In October 2017, scientists from various European universities proposed

11445-523: The total stopping power. Because of this, a lead (high Z ) shield is 20–30% better as a gamma shield than an equal mass of another low- Z shielding material, such as aluminium, concrete, water, or soil; lead's major advantage is not in lower weight, but rather its compactness due to its higher density. Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources. The higher

11554-446: The tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones via bone scan ). Gamma rays cause damage at a cellular level and are penetrating, causing diffuse damage throughout the body. However, they are less ionising than alpha or beta particles, which are less penetrating. Low levels of gamma rays cause a stochastic health risk, which for radiation dose assessment

11663-443: The varying colors of stars could be attributed to their motion with respect to the Earth. Before this was verified, it was found that stellar colors were primarily due to a star's temperature , not motion. Only later was Doppler vindicated by verified redshift observations. The Doppler redshift was first described by French physicist Hippolyte Fizeau in 1848, who noted the shift in spectral lines seen in stars as being due to

11772-551: The velocities for 15 spiral nebulae spread across the entire celestial sphere , all but three having observable "positive" (that is recessional) velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae", and the distances to them, with the formulation of his eponymous Hubble's law . Milton Humason worked on those observations with Hubble. These observations corroborated Alexander Friedmann 's 1922 work, in which he derived

11881-456: The wavelength ratio 1 + z (which is greater than 1 for redshifts and less than 1 for blueshifts). Examples of strong redshifting are a gamma ray perceived as an X-ray , or initially visible light perceived as radio waves . Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns . Other physical processes exist that can lead to

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