Internal conversion is a radioactive decay process wherein an excited nucleus interacts electromagnetically with one of the orbital electrons of the atom. This causes the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but not from the nucleus. For this reason, the high-speed electrons resulting from internal conversion are not called beta particles, since the latter come from beta decay, where they are newly created in the nuclear decay process. Internal conversion is possible whenever gamma decay is possible, except in the case where the atom is ionised. During internal conversion, the atomic number does not change, thus no transmutation of one element to another takes place. Since an electron is lost from the atom, a hole appears in an electron shell, subsequently filled by other electrons that descend to that empty, lower energy level, in the process emit characteristic X-ray, Auger electron, or both.
The atom thus emits high-energy electrons and X-ray photons, none of which originate in that nucleus. The atom supplied the energy needed to eject the electron, which in turn caused the latter events and the other emissions. Since primary electrons from internal conversion carry a fixed part of the characteristic decay energy, they have a discrete energy spectrum, rather than the spread spectrum characteristic of beta particles. Whereas the energy spectrum of beta particles plots as a broad hump, the energy spectrum of internally converted electrons plots as a single sharp peak. In the quantum mechanical model of the electron, there is a non-zero probability of finding the electron within the nucleus. During the internal conversion process, the wavefunction of an inner shell electron is said to penetrate the volume of the atomic nucleus; when this happens, the electron may couple to an excited energy state of the nucleus and take the energy of the nuclear transition directly, without an intermediate gamma ray being first produced.
The kinetic energy of the emitted electron is equal to the transition energy in the nucleus, minus the binding energy of the electron to the atom. Most internal conversion electrons come from the K shell, as these two electrons have the highest probability of being within the nucleus. However, the s states in the L, M, N shells are able to couple to the nuclear fields and cause IC electron ejections from those shells. Ratios of K-shell to other L, M, or N shell internal conversion probabilities for various nuclides have been prepared. An amount of energy exceeding the atomic binding energy of the s electron must be supplied to that electron in order to eject it from the atom to result in IC. There are a few radionuclides in which the decay energy is not sufficient to convert a 1s electron, these nuclides, to decay by internal conversion, must decay by ejecting electrons from the L or M or N shells as these binding energies are lower. Although s electrons are more for IC processes due to their superior nuclear penetration compared to electrons with orbital angular momentum, spectral studies show that p electrons are ejected in the IC process.
After the IC electron has been emitted, the atom is left with a vacancy in one of its electron shells an inner one. This hole will be filled with an electron from one of the higher shells, which causes another outer electron to fill its place in turn, causing a cascade. One or more characteristic X-rays or Auger electrons will be emitted as the remaining electrons in the atom cascade down to fill the vacancies; the decay scheme on the left shows that 203Hg produces a continuous beta spectrum with maximum energy 214 keV, that leads to an excited state of the daughter nucleus 203Tl. This state decays quickly to the ground state of 203Tl, emitting a gamma quantum of 279 keV; the figure on the right shows the electron spectrum of 203Hg, measured by means of a magnetic spectrometer. It includes the continuous beta spectrum and K-, L-, M-lines due to internal conversion. Since the binding energy of the K electrons in 203Tl amounts to 85 keV, the K line has an energy of 279 - 85 = 194 keV; because of lesser binding energies, the L- and M-lines have higher energies.
Because of the finite energy resolution of the spectrometer, the "lines" have a Gaussian shape of finite width. Internal conversion is favoured whenever the energy available for a gamma transition is small, it is the primary mode of de-excitation for 0+→0+ transitions; the 0+→0+ transitions occur where an excited nucleus has zero-spin and positive parity, decays to a ground state which has zero-spin and positive parity. In such cases, de-excitation cannot take place by way of emission of a gamma ray, since this would violate conservation of angular momentum, hence other mechanisms like IC predominate; this shows that internal conversion is not a two-step process where a gamma ray would be first emitted and converted. The competition between internal conversion and gamma decay is quantified in the form of the internal conversion coefficient, defined as α = e / γ where e is the rate of conve
A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as "nucleons". One or more protons are present in the nucleus of every atom; the number of protons in the nucleus is the defining property of an element, is referred to as the atomic number. Since each element has a unique number of protons, each element has its own unique atomic number; the word proton is Greek for "first", this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental particle, hence a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, protons are hadrons, like neutrons, the other nucleon, are composed of three quarks.
Although protons were considered fundamental or elementary particles, they are now known to be composed of three valence quarks: two up quarks of charge +2/3e and one down quark of charge –1/3e. The rest masses of quarks contribute only about 1% of a proton's mass, however; the remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. Because protons are not fundamental particles, they possess a physical size, though not a definite one. At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom; the result is a protonated atom, a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, chemically a free radical.
Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules, which are the most common molecular component of molecular clouds in interstellar space. Protons are composed of three valence quarks, making them baryons; the two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons. A modern perspective has a proton composed of the valence quarks, the gluons, transitory pairs of sea quarks. Protons have a positive charge distribution which decays exponentially, with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms, based on a simplistic interpretation of early values of atomic weights, disproved when more accurate values were measured. In 1886, Eugen Goldstein discovered canal rays and showed that they were positively charged particles produced from gases. However, since particles from different gases had different values of charge-to-mass ratio, they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson. Wilhelm Wien in 1898 identified the hydrogen ion as particle with highest charge-to-mass ratio in ionized gases. Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table is equal to its nuclear charge; this was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra.
In 1917, Rutherford proved that the hydrogen nucleus is present in other nuclei, a result described as the discovery of protons. Rutherford had earlier learned to produce hydrogen nuclei as a type of radiation produced as a product of the impact of alpha particles on nitrogen gas, recognize them by their unique penetration signature in air and their appearance in scintillation detectors; these experiments were begun when Rutherford had noticed that, when alpha particles were shot into air, his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. After experimentation Rutherford traced the reaction to the nitrogen in air, found that when alphas were produced into pure nitrogen gas, the effect was larger. Rutherford determined that this hydrogen could have come only from the nitrogen, therefore nitrogen must contain hydrogen nuclei. One hydrogen nucleus was being knocked off by the impact of the alpha particle, producing oxygen-17 in the process; this was 14N + α → 17O + p.
(This reaction wo
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more than positively charged protons, which are repelled electrostatically. Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways: as a slow process. Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions, but can be formed by neutron capture. Neutron capture on protons yields a line at 2.223 MeV predicted and observed in solar flares. At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold is irradiated by neutrons, the isotope 198Au is formed in a excited state, decays to the ground state of 198Au by the emission of γ rays. In this process, the mass number increases by one; this is written in short form 197Au198Au. If thermal neutrons are used, the process is called thermal capture.
The isotope 198Au is a beta emitter. In this process the atomic number rises by one; the r-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission in between neutron captures. The mass number therefore rises by a large amount. Only afterwards, the unstable nuclei decay via many β− decays to stable or unstable nuclei of high atomic number; the absorption neutron cross-section of an isotope of a chemical element is the effective cross sectional area that an atom of that isotope presents to absorption, is a measure of the probability of neutron capture. It is measured in barns. Absorption cross section is highly dependent on neutron energy; as a generality, the likelihood of absorption is proportional to the time the neutron is in the vicinity of the nucleus. The time spent in the vicinity of the nucleus is inversely proportional to the relative velocity between the neutron and nucleus. Other more specific issues modify this general principle.
Two of the most specified measures are the cross-section for thermal neutron absorption, resonance integral which considers the contribution of absorption peaks at certain neutron energies specific to a particular nuclide above the thermal range, but encountered as neutron moderation slows the neutron down from an original high energy. The thermal energy of the nucleus has an effect. In particular, the increase in uranium-238's ability to absorb neutrons at higher temperatures is a negative feedback mechanism that helps keep nuclear reactors under control. Neutron capture is involved in the formation of isotopes of chemical elements; as a consequence of this fact the energy of neutron capture intervenes in the standard enthalpy of formation of isotopes. Neutron activation analysis can be used to remotely detect the chemical composition of materials; this is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral security.
The most important neutron absorber is 10B as 10B4C in control rods, or boric acid as a coolant water additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are xenon, hafnium, cobalt, titanium, erbium, europium and ytterbium; these occur in combinations such as Mo2B5, hafnium diboride, titanium diboride, dysprosium titanate and gadolinium titanate. Hafnium, one of the last stable elements to be discovered, presents an interesting case. Though hafnium is a heavier element, its electron configuration makes it identical with the element zirconium, they are always found in the same ores. However, their nuclear properties are different in a profound way. Hafnium absorbs neutrons avidly, it can be used in reactor control rods, whereas natural zirconium is transparent to neutrons. So, zirconium is a desirable construction material for reactor internal parts, including the metallic cladding of the fuel rods which contain either uranium, plutonium, or mixed oxides of the two elements.
Hence, it is quite important to be able to separate the zirconium from the hafnium in their occurring alloy. This can only be done inexpensively by using modern chemical ion-exchange resins. Similar resins are used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, sometimes thorium. Beta decay Induced radioactivity List of particles Neutron emission Radioactive decay Rays: α — β — γ — δ — ε p-process Thermal Neutron Capture Data Thermal Neutron Cross Sections at the International Atomic Energy Agency
The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave within the nucleus, each has a mass of one atomic mass unit, they are both referred to as nucleons, their properties and interactions are described by nuclear physics. The chemical and nuclear properties of the nucleus are determined by the number of protons, called the atomic number, the number of neutrons, called the neutron number; the atomic mass number is the total number of nucleons. For example, carbon has atomic number 6, its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons; some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes. Within the nucleus and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom.
Neutrons are produced copiously in nuclear fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission and neutron capture processes; the neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc. in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor and the first nuclear weapon. Free neutrons, while not directly ionizing atoms, cause ionizing radiation; as such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.
Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. An atomic nucleus is formed by a number of protons, Z, a number of neutrons, N, bound together by the nuclear force; the atomic number defines the chemical properties of the atom, the neutron number determines the isotope or nuclide. The terms isotope and nuclide are used synonymously, but they refer to chemical and nuclear properties, respectively. Speaking, isotopes are two or more nuclides with the same number of protons; the atomic mass number, symbol A, equals Z+N. Nuclides with the same atomic mass number are called isobars; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The most common nuclide of the common chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Though it is not a chemical element, the neutron is included in this table; the free neutron has 1.674927471 × 10 − 27 kg, or 1.00866491588 u. The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields; the neutron has a magnetic moment, however. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. A free neutron is unstable, decaying to a proton and antineutrino with a mean lifetime of just under 15 minutes; this radioactive decay, known as beta decay, is possible because the mass of the neutron is greater than the proton. The free proton is stable. Neutrons or protons bound in a nucleus can be stable or unstable, depending on the nuclide.
Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Protons and neutrons behave identically under the influence of the nuclear force within the nucleus; the concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions therefore have an energy density, more than ten million times that of chemical reactions; because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. The ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible.
In nuclear fission, the absorption of a neutron by a heavy nuclide causes the nuclide to become unstable and break into light nuclides and additional neu
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 rapid neutron-capture process, or so-called r-process, is a set of nuclear reactions that in nuclear astrophysics is responsible for the creation of half the abundances of the atomic nuclei heavier than iron synthesizing the entire abundance of the two most neutron-rich stable isotopes of each heavy element. Chemical elements heavier than iron are enabled by the force between nucleons to be capable of six to ten stable isotopic forms having the same nuclear charge Z but differing in neutron number N, each of whose natural abundances contribute to the natural abundance of the chemical element; each isotope is characterized by the number of neutrons. The r-process synthesizes new nuclei of the heaviest four isotopes of any heavy element, being responsible for the abundances of its two heaviest isotopes, which are referred to as r-only nuclei; the most abundant of these contribute to the r-process abundance peaks near atomic weights A = 82, A = 130 and A = 196. The r-process entails a succession of rapid neutron captures by one or more heavy seed nuclei beginning with nuclei in the abundance peak centered on 56Fe.
The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay before another neutron arrives to be captured, a sequence, halted only when the neutron-rich nuclei cannot physically retain another neutron. The r-process therefore must occur in locations. Early studies reasoned that 1024 free neutrons per cm3 would be required if the temperature were about one billion degrees in order that the waiting points, at which no more neutrons can be captured, be at the atomic numbers of the abundance peaks for r-process nuclei; this amounts to a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova or decompression of neutron-star matter thrown off by a binary neutron star merger; the relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research. A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions.
These led to the discovery of the elements fermium in nuclear weapon fallout. The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, nucleosynthesis by means of slow captures of neutrons; the s-process occurs within ordinary stars AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons; the r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for the entire abundance of chemical elements heavier than iron; the historical challenge has been to locate physical settings appropriate for their time scales. The need for a physical setting providing rapid capture of neutrons was seen from the relative abundances of isotopes of heavy chemical elements given in a table of abundances by Hans Suess and Harold Urey in 1956.
Not only did their abundance table reveal larger than average abundances of natural isotopes containing magic numbers of neutrons but abundance peaks about 10 amu lighter than those containing magic numbers of neutrons in their structure. They realized that captures of free neutrons should be part of any explanation because of the lack of electric repulsion between nuclei and chargeless neutrons; this phenomenology suggested that these lighter subsidiary abundance peaks could result from radioactive nuclei having the magic neutron numbers but ten fewer protons. To achieve this would require radioactive neutron-rich isotopes to capture another neutron faster than they can undergo beta decay in order to create abundance peaks that will decay subsequently to germanium and platinum, elements prominent in the r-process abundance peaks. According to the nuclear shell model, radioactive nuclei that would decay into isotopes of these elements must have closed neutron shells near the neutron drip line, where more neutrons cannot be added.
The neutron-capture flow must therefore wait for beta decay at those so-called waiting points, which therefore grow larger in abundance similar to water in a dammed up river. For those hitherto unexplained abundance peaks, which are 10 u lighter than the s-process abundance peaks, to be created by rapid neutron capture implied that other neutron-rich nuclei would be synthesized by the same process; that process, rapid neutron capture by neutron-rich isotopes, is called the r-process. A table apportioning the heavy isotopes phenomenologically between s-process and r-process isotopes was published in 1957 in the famous B2FH review paper which named the r-process and outlined the physics that guides it. Alastair G. W. Cameron published a smaller study about the r-process in the same year; the stationary r-process as described by the B2FH paper was first demonstrated in a time-dependent calculation at Caltech by Phillip A. Seeger, William A. Fowler and Donald D. Clayton, who found that no single temporal snapshot matched the solar r-process abundances, but
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