In nuclear physics, beta decay is a type of radioactive decay in which a beta ray is emitted from an atomic nucleus. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino, or conversely a proton is converted into a neutron by the emission of a positron with a neutrino, thus changing the nuclide type. Neither the beta particle nor its associated neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons; the probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. For either electron or positron emission to be energetically possible, the energy release or Q value must be positive. Beta decay is a consequence of the weak force, characterized by lengthy decay times.
Nucleons are composed of up quarks and down quarks, the weak force allows a quark to change type by the exchange of a W boson and the creation of an electron/antineutrino or positron/neutrino pair. For example, a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks. Decay times for many nuclides that are subject to beta decay can be thousands of years. Electron capture is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. In electron capture, an inner atomic electron is captured by a proton in the nucleus, transforming it into a neutron, an electron neutrino is released; the two types of beta decay are known as beta beta plus. In beta minus decay, a neutron is converted to a proton, the process creates an electron and an electron antineutrino. Β+ decay is known as positron emission. Beta decay conserves a quantum number known as the lepton number, or the number of electrons and their associated neutrinos.
These particles have lepton number +1, while their antiparticles have lepton number −1. Since a proton or neutron has lepton number zero, β+ decay must be accompanied with an electron neutrino, while β− decay must be accompanied by an electron antineutrino. An example of electron emission is the decay of carbon-14 into nitrogen-14 with a half-life of about 5,730 years: 146C → 147N + e− + νeIn this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation; this new element has an unchanged mass number A, but an atomic number Z, increased by one. As in all nuclear decays, the decaying element is known as the parent nuclide while the resulting element is known as the daughter nuclide. Another example is the decay of hydrogen-3 into helium-3 with a half-life of about 12.3 years: 31H → 32He + e− + νeAn example of positron emission is the decay of magnesium-23 into sodium-23 with a half-life of about 11.3 s: 2312Mg → 2311Na + e+ + νeβ+ decay results in nuclear transmutation, with the resulting element having an atomic number, decreased by one.
The beta spectrum, or distribution of energy values for the beta particles, is continuous. The total energy of the decay process is divided between the electron, the antineutrino, the recoiling nuclide. In the figure to the right, an example of an electron with 0.40 MeV energy from the beta decay of 210Bi is shown. In this example, the total decay energy is 1.16 MeV, so the antineutrino has the remaining energy: 1.16-0.40=0.76 MeV. An electron at the far right of the curve would have the maximum possible kinetic energy, leaving the energy of the neutrino to be only its small rest mass. Radioactivity was discovered in 1896 by Henri Becquerel in uranium, subsequently observed by Marie and Pierre Curie in thorium and in the new elements polonium and radium. In 1899, Ernest Rutherford separated radioactive emissions into two types: alpha and beta, based on penetration of objects and ability to cause ionization. Alpha rays could be stopped by thin sheets of paper or aluminium, whereas beta rays could penetrate several millimetres of aluminium.
In 1900, Paul Villard identified a still more penetrating type of radiation, which Rutherford identified as a fundamentally new type in 1903 and termed gamma rays. Alpha and gamma are the first three letters of the Greek alphabet. In 1900, Becquerel measured the mass-to-charge ratio for beta particles by the method of J. J. Thomson used to identify the electron, he found that m/e for a beta particle is the same as for Thomson's electron, therefore suggested that the beta particle is in fact an electron. In 1901, Rutherford and Frederick Soddy showed that alpha and beta radioactivity involves the transmutation of atoms into atoms of other chemical elements. In 1913, after the products of more radioactive decays were known and Kazimierz Fajans independently proposed their radioactive displacement law, which states that beta emission from one element produces another element one place to the right in the periodic table, while alpha emission produces an element two places to the left; the study of beta decay provided the first physical evidence for the existence of the neutrino.
In both alpha and gamma decay, the resulting particle has a narrow energy distribution, since the particle carries the energy from the diffe
The antiproton, p, is the antiparticle of the proton. Antiprotons are stable, but they are short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy; the existence of the antiproton with −1 electric charge, opposite to the +1 electric charge of the proton, was predicted by Paul Dirac in his 1933 Nobel Prize lecture. Dirac received the Nobel Prize for his previous 1928 publication of his Dirac equation that predicted the existence of positive and negative solutions to the Energy Equation of Einstein and the existence of the positron, the antimatter analog to the electron, with positive charge and opposite spin; the antiproton was first experimentally confirmed in 1955 at the Bevatron particle accelerator by University of California, Berkeley physicists Emilio Segrè and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics. In terms of valence quarks, an antiproton consists of two up one down antiquark; the properties of the antiproton that have been measured all match the corresponding properties of the proton, with the exception that the antiproton has electric charge and magnetic moment that are the opposites of those in the proton.
The questions of how matter is different from antimatter, the relevance of antimatter in explaining how our universe survived the Big Bang, remain open problems—open, in part, due to the relative scarcity of antimatter in today's universe. Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more by satellite-based detectors; the standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction, where A represents a nucleus: p + A → p + p + p + A The secondary antiprotons propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium, antiprotons can be lost by "leaking out" of the galaxy; the antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.
These experimental measurements set upper limits on the number of antiprotons that could be produced in exotic ways, such as from annihilation of supersymmetric dark matter particles in the galaxy or from the Hawking radiation caused by the evaporation of primordial black holes. This provides a lower limit on the antiproton lifetime of about 1-10 million years. Since the galactic storage time of antiprotons is about 10 million years, an intrinsic decay lifetime would modify the galactic residence time and distort the spectrum of cosmic ray antiprotons; this is more stringent than the best laboratory measurements of the antiproton lifetime: LEAR collaboration at CERN: 0.08 years Antihydrogen Penning trap of Gabrielse et al.: 0.28 years APEX collaboration at Fermilab: 50000 years for p → μ− + anything APEX collaboration at Fermilab: 300000 years for p → e− + γThe magnitude of properties of the antiproton are predicted by CPT symmetry to be related to those of the proton. In particular, CPT symmetry predicts the mass and lifetime of the antiproton to be the same as those of the proton, the electric charge and magnetic moment of the antiproton to be opposite in sign and equal in magnitude to those of the proton.
CPT symmetry is a basic consequence of quantum field theory and no violations of it have been detected. BESS: balloon-borne experiment, flown in 1993, 1995, 1997, 2000, 2002, 2004 and 2007. CAPRICE: balloon-borne experiment, flown in 1994 and 1998. HEAT: balloon-borne experiment, flown in 2000. AMS: space-based experiment, prototype flown on the space shuttle in 1998, intended for the International Space Station, launched May 2011. PAMELA: satellite experiment to detect cosmic rays and antimatter from space, launched June 2006. Recent report discovered 28 antiprotons in the South Atlantic Anomaly. Antiprotons were produced at Fermilab for collider physics operations in the Tevatron, where they were collided with protons; the use of antiprotons allows for a higher average energy of collisions between quarks and antiquarks than would be possible in proton-proton collisions. This is because the valence quarks in the proton, the valence antiquarks in the antiproton, tend to carry the largest fraction of the proton or antiproton's momentum.
Their formation requires energy equivalent to a temperature of 10 trillion K and this does not tend to happen naturally. However, at CERN, protons are accelerated in the Proton Synchrotron to an energy of 26 GeV, smashed into an iridium rod; the protons bounce off the iridium nuclei with enough energy for matter to be created. A range of particles and antiparticles are formed, the antiprotons are separated off using magnets in vacuum. In July 2011, the ASACUSA experiment at CERN determined the mass of the antiproton to be 1836.1526736 times more massive than an electron. This is the same as the mass of a proton, within the level of certainty of the experiment. Antiprotons have been shown within laboratory experiments to have the potential to treat certain cancers, in a similar method used for ion therapy; the primary difference between antiproton therapy and proton therapy is that following ion energy deposition the antiproton annihilates depositing additional energy in the cancerous region. In October 2017, scientists working on the BASE experiment at CERN reported a measurement of the antiproton magnetic moment to a precision of 1.5 parts per billion.
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The Antiproton Decelerator is a storage ring at the CERN laboratory near Geneva. It was built as a successor to the Low Energy Antiproton Ring and started operation in the year 2000. Antiprotons are created by impinging a proton beam from the Proton Synchrotron on a metal target; the AD decelerates the resultant antiprotons to an energy of 5.3 MeV, which are ejected to one of several connected experiments. ELENA is a 30 m hexagonal storage ring situated inside the AD complex, it is designed to further decelerate the antiproton beam to an energy of 0.1 MeV for more precise measurements. The first beam circulated ELENA on 18 November 2016; the ring is expected to be operational in 2018. GBAR will be the first experiment to use a beam from ELENA, with the rest of the AD experiments following suit in 2019-2020. ATHENA was an antimatter research project. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in Nature. In 2005, ATHENA was disbanded and many of the former members worked on the subsequent ALPHA experiment.
The ATHENA apparatus comprises four main subsystems: the antiproton catching trap, the positron accumulator, the antiproton/positron mixing trap, the antihydrogen annihilation detector. All traps in the experiment are variations on the Penning trap, which uses an axial magnetic field to transversely confine the charged particles, a series of hollow cylindrical electrodes to trap them axially; the catching and mixing traps are adjacent to each other, coaxial with a 3 T magnetic field from a superconducting solenoid. The positron accumulator has its own magnetic system a solenoid, of 0.14 T. A separate cryogenic heat exchanger in the bore of the superconducting magnet cools the catching and mixing traps to about 15 K; the ATHENA apparatus features an open, modular design that allows great experimental flexibility in introducing large numbers of positrons into the apparatus. The catching trap slows, traps and accumulates antiprotons. To cool antiprotons, the catching trap is first loaded with 3×108 electrons, which cool by synchrotron radiation in the 3 T magnetic field.
The AD delivers 2×107 antiprotons having kinetic energy 5.3 MeV and a pulse duration of 200 ns to the experiment at 100 s intervals. The antiprotons are trapped using a pulsed electric field; the antiprotons lose equilibrate with the cold electrons by Coulomb interaction. The electrons are ejected before mixing the antiprotons with positrons; each AD shot results in about 3×103 cold antiprotons for interaction experiments. The positron accumulator slows and accumulates positrons emitted from a radioactive source. Accumulation for 300 s yields 1.5×108 positrons, 50% of which are transferred to the mixing trap, where they cool by synchrotron radiation. The mixing trap has the axial potential configuration of a nested Penning trap, which permits two plasmas of opposite charge to come into contact. In ATHENA, the spheroidal positron cloud can be characterized by exciting and detecting axial plasma oscillations. Typical conditions are: 7×107 stored positrons, a radius of 2 – 2.5 mm, a length of 32 mm, a maximum density of 2.5×108 cm−3.
Key to the observations reported here is the antihydrogen annihilation detector, situated coaxially with the mixing region, between the trap outer radius and the magnet bore. The detector is designed to provide unambiguous evidence for antihydrogen production by detecting the temporally and spatially coincident annihilations of the antiproton and positron when a neutral antihydrogen atom escapes the electromagnetic trap and strikes the trap electrodes. An antiproton annihilates into a few charged or neutral pions; the charged pions are detected by two layers of double-sided, position sensitive, silicon microstrips. The path of a charged particle passing through both layers can be reconstructed, two or more intersecting tracks allow determination of the position, or vertex, of the antiproton annihilation; the uncertainty in vertex determination is 4 mm and is dominated by the unmeasured curvature of the charged pions’ trajectories in the magnetic field. The temporal coincidence window is 5 microseconds.
The solid angle coverage of the interaction region is about 80% of 4π. A positron annihilating with an electron yields three photons; the positron detector, comprising 16 rows each containing 12 scintillating, pure cesium-iodide-crystals, is designed to detect the two-photon events, consisting of two 511 keV photons which are always emitted back-to-back. The energy resolution of the detector is 18% FWHM at 511 keV, the photo-peak detection efficiency for single photons is about 20%; the maximum readout rate of the whole detector is about 40 Hz. Ancillary detectors include large scintillator paddles external to the magnet, a thin, position sensitive, silicon diode through which the incident antiproton beam passes before entering the catching trap. To produce antihydrogen atoms, a positron well in the mixing region is filled with about 7×107 positrons and allowed to cool to the ambient temperature; the nested trap is formed around the positron well. Next 104 antiprotons are launched into the mixing region by pulsing the trap from one potential configuration to another.
The mixing time is 190 s, after which all particles are dumped and the process repeated. Events triggering the imaging silicon detector initiate readout of both the silicon and the CsI modules. Using this method, ATHENA could produce -
Cosmic rays are high-energy radiation originating outside the Solar System and from distant galaxies. Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed of high-energy protons and atomic nuclei, they are originated either from the sun or from outside of our solar system. Data from the Fermi Space Telescope have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018; the term ray is somewhat of a misnomer due to a historical accident, as cosmic rays were at first, wrongly, thought to be electromagnetic radiation. In common scientific usage, high-energy particles with intrinsic mass are known as "cosmic" rays, while photons, which are quanta of electromagnetic radiation are known by their common names, such as gamma rays or X-rays, depending on their photon energy.
In current usage, the term cosmic ray exclusively refers to massive particles – those that have rest mass – as opposed to photons, which have no rest mass, neutrinos, which have negligible rest mass. Massive particles have additional, mass-energy when they are moving, due to relativistic effects. Through this process, some particles acquire tremendously high mass-energies; these are higher than the photon energy of the highest-energy photons detected to date. The energy of the massless photon depends on frequency, not speed, as photons always travel at the same speed. At the higher end of the energy spectrum, relativistic kinetic energy is the main source of the mass-energy of cosmic rays; the highest-energy fermionic cosmic rays detected to date, such as the Oh-My-God particle, had an energy of about 3×1020 eV, while the highest-energy gamma rays to be observed, very-high-energy gamma rays, are photons with energies of up to 1014 eV, the highest energy neutrinos detected so far have energies of several 1015 eV.
Hence, the highest-energy detected fermionic cosmic rays are about 3×106 times as energetic as the highest-energy detected cosmic photons. Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei of well-known atoms, about 1% are solitary electrons. Of the nuclei, about 90% are simple protons; these fractions vary over the energy range of cosmic rays. A small fraction are stable particles of antimatter, such as positrons or antiprotons; the precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them. Cosmic rays attract great interest due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, scientifically, because the energies of the most energetic ultra-high-energy cosmic rays have been observed to approach 3 × 1020 eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider.
One can show that such enormous energies might be achieved by means of the centrifugal mechanism of acceleration in active galactic nuclei. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic energy of a 90-kilometre-per-hour baseball; as a result of these discoveries, there has been interest in investigating cosmic rays of greater energies. Most cosmic rays, however, do not have such extreme energies. After the discovery of radioactivity by Henri Becquerel in 1896, it was believed that atmospheric electricity, ionization of the air, was caused only by radiation from radioactive elements in the ground or the radioactive gases or isotopes of radon they produce. Measurements of increasing ionization rates at increasing heights above the ground during the decade from 1900 to 1910 could be explained as due to absorption of the ionizing radiation by the intervening air. In 1909, Theodor Wulf developed an electrometer, a device to measure the rate of ion production inside a hermetically sealed container, used it to show higher levels of radiation at the top of the Eiffel Tower than at its base.
However, his paper published in Physikalische Zeitschrift was not accepted. In 1911, Domenico Pacini observed simultaneous variations of the rate of ionization over a lake, over the sea, at a depth of 3 metres from the surface. Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth. In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers to an altitude of 5,300 metres in a free balloon flight, he found the ionization rate increased fourfold over the rate at ground level. Hess ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes, he concluded that "The results of the observations seem most to be explained by the assumption that radiation of high penetrating power enters from above into our atmosphere." In 1913–1914, Werner Kolhörster confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km.
Hess received the Nobel Prize in Physics in 1936
Andrei Dmitrievich Sakharov was a Russian nuclear physicist and activist for disarmament and human rights. He became renowned as the designer of the Soviet Union's RDS-37, a codename for Soviet development of thermonuclear weapons. Sakharov became an advocate of civil liberties and civil reforms in the Soviet Union, for which he faced state persecution; the Sakharov Prize, awarded annually by the European Parliament for people and organizations dedicated to human rights and freedoms, is named in his honor. Sakharov was born in Moscow on May 21, 1921, his father was a private school physics teacher and an amateur pianist. His father taught at the Second Moscow State University. Andrei's grandfather Ivan had been a prominent lawyer in the Russian Empire who had displayed respect for social awareness and humanitarian principles that would influence his grandson. Sakharov's mother was Yekaterina Alekseyevna Sakharova, a great-granddaughter of the prominent military commander Alexey Semenovich Sofiano.
Sakharov's parents and paternal grandmother, Maria Petrovna shaped his personality. His mother and grandmother were churchgoers; when Andrei was about thirteen, he realized that he did not believe, but in life he unequivocally described his religious feeling. Sakharov entered Moscow State University in 1938. Following evacuation in 1941 during the Great Patriotic War, he graduated in Aşgabat, in today's Turkmenistan, he was assigned to laboratory work in Ulyanovsk. In 1943, he married Klavdia Alekseyevna Vikhireva, with whom he raised a son. Klavdia would die in 1969, he returned to Moscow in 1945 to study at the Theoretical Department of FIAN. He received his Ph. D. in 1947. After World War II, he researched cosmic rays. In mid-1948 he participated in the Soviet atomic bomb project under Igor Tamm. Sakharov's study group at FIAN in 1948 came up with a second concept in August–September 1948. Adding a shell of natural, unenriched uranium around the deuterium would increase the deuterium concentration at the uranium-deuterium boundary and the overall yield of the device, because the natural uranium would capture neutrons and itself fission as part of the thermonuclear reaction.
This idea of a layered fission-fusion-fission bomb led Sakharov to call it the sloika, or layered cake. The first Soviet atomic device was tested on August 29, 1949. After moving to Sarov in 1950, Sakharov played a key role in the development of the first megaton-range Soviet hydrogen bomb using a design known as Sakharov's Third Idea in Russia and the Teller–Ulam design in the United States. Before his Third Idea, Sakharov tried a "layer cake" of alternating layers of fission and fusion fuel; the results were disappointing. However the design was seen to be worth pursuing because deuterium is abundant and uranium is scarce, he had no idea how powerful the US design was. Sakharov realised that in order to cause the explosion of one side of the fuel to symmetrically compress the fusion fuel, a mirror could be used to reflect the radiation; the details had not been declassified in Russia when Sakharov was writing his memoirs, but in the Teller–Ulam design, soft X-rays emitted by the fission bomb were focused onto a cylinder of lithium deuteride to compress it symmetrically.
This is called radiation implosion. The Teller–Ulam design had a secondary fission device inside the fusion cylinder to assist with the compression of the fusion fuel and generate neutrons to convert some of the lithium to tritium, producing a mixture of deuterium and tritium. Sakharov's idea was first tested as RDS-37 in 1955. A larger variation of the same design which Sakharov worked on was the 50 Mt Tsar Bomba of October 1961, the most powerful nuclear device detonated. Sakharov saw "striking parallels" between his fate and those of J. Robert Oppenheimer and Edward Teller in the US. Sakharov believed that in this "tragic confrontation of two outstanding people", both deserved respect, because "each of them was certain he had right on his side and was morally obligated to go to the end in the name of truth." While Sakharov disagreed with Teller over nuclear testing in the atmosphere and the Strategic Defense Initiative, he believed that American academics had been unfair to Teller's resolve to get the H-bomb for the United States since "all steps by the Americans of a temporary or permanent rejection of developing thermonuclear weapons would have been seen either as a clever feint, or as the manifestation of stupidity.
In both cases, the reaction would have been the same – avoid the trap and take advantage of the enemy's stupidity." Sakharov never felt that by creating nuclear weapons he had "known sin", in Oppenheimer's expression. He wrote: After more than forty years, we have had no third world war, the balance of nuclear terror... may have helped to prevent one. But I am not at all sure of this. What most troubles me now is the instability of the balance, the extreme peril of the current situation, the appalling waste of the arms race... Each of us has a responsibility to think about this in global terms, with tolerance and candor, free from ideological dogmatism, parochial interests, or national egotism." In 1950 he proposed an idea for a controlled nuclear fusion reactor, the tokamak, still the basis for
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
A gamma ray or gamma radiation, is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. 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 strong penetration of matter. Gamma rays from radioactive decay are in the energy range from a few keV to ~8 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 TeV range have been observed from sources such as the Cygnus X-3 microquasar. Natural sources of gamma rays originating on Earth are as 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, that produce gamma rays from electron action upon the nucleus. 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. Gamma rays and X-rays are both electromagnetic radiation and they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay, while in the case of X-rays, the origin is 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 is classified as X-rays and is the subject of X-ray astronomy; this convention stems from the early man-made X-rays, which had energies only up to 100 keV, whereas many gamma rays could go to higher energies.
A large fraction of astronomical gamma rays are screened by Earth's atmosphere. Gamma rays are thus biologically hazardous. Due to their high penetration power, they can damage internal organs. Unlike alpha and beta rays, they pass through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete; 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 immediately upon formation. Paul Villard, 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 described types of rays from radium, which included beta rays, first noted as "radioactivity" by Henri Becquerel in 1896, alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type.
In 1903, Villard's radiation was recognized as being of a type fundamentally different from 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 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 noted that gamma rays were not deflected by a magnetic field, another property making them unlike alpha and beta rays. Gamma rays were first thought to be particles like alpha and beta rays. Rutherford believed that they might be fast beta particles, but their failure to be deflected by 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, found that they were similar to X-rays, but with shorter wavelengths and higher frequency.
This was recognized as giving them more energy per photon, as soon as the latter term became accepted. A gamma decay was understood to emit a gamma photon. Natural sources of gamma rays on Earth include gamma decay from occurring radioisotopes such as potassium-40, 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 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, used for irradiating or imaging is known as a gamma source. It is called a radioactive sou