Nuclear physics Radioactive decay
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons). It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees. A few minutes afterward, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but only in relatively small amounts. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed, because this element requires a far higher product of helium density and time than were present in the short nucleosynthesis period of the Big Bang. The Big Bang fusion process essentially shut down due to drops in temperature and density as the universe continued to expand. This first process of primordial nucleosynthesis was the first type of nucleogenesis to occur in the universe.
The subsequent nucleosynthesis of the heavier elements required heavy stars and supernova explosions. This theoretically happened as hydrogen and helium from the Big Bang condensed into the first stars 500 million years after the Big Bang. The primordial elements still present on Earth that were once created in stellar nucleosynthesis range in atomic numbers from 6 (carbon) to 94 (plutonium). Synthesis of these heavier elements occurs either by nuclear fusion (including both rapid and slow multiple neutron capture) or by nuclear fission, sometimes followed by beta decay.
By contrast, many stellar processes actually tend to destroy deuterium and isotopes of beryllium, lithium, and boron which have collected in stars after their primordial formation in the Big Bang. This effective destruction happens via the transmutation of these elements to higher atomic species. Quantities of these lighter elements in the present universe are therefore thought to have been formed mainly through billions of years of cosmic ray (mostly high-energy proton) mediated breakup of heavier elements residing in interstellar gas and dust.
In addition to the major processes of priordial nucleosynthesis in the Big Bang, stellar processes, and cosmic-ray nucleosynthesis in space, many minor natural processes continue to produce small amounts of new elements on Earth. These nuclides are naturally produced on a continuing basis via the decay of long-lived primordial radionuclides (via radiogenesis), from natural nuclear reactions in cosmic ray bombardment of elements on Earth (cosmogenic nuclides), and from other natural nuclear reactions powered by particles from radioactive decay, (producing nucleogenic nuclides).
The first ideas on nucleosynthesis were simply that the chemical elements were created at the beginnings of the universe, but no successful physical scenario for this could be identified. Hydrogen and helium were clearly far more abundant than any of the other elements (all the rest of which constituted less than 2% of the mass of the solar system, and presumably other star systems as well). At the same time it was clear that carbon was the next most common element, and also that there was a general trend toward abundance of light elements, especially those composed of whole numbers of helium-4 nuclei.
Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen to helium, but this idea was not generally accepted because it lacked nuclear mechanisms. In the years immediately before World War II Hans Bethe first provided those nuclear mechanisms by which hydrogen is fused into helium. However, neither of these early works on stellar power addressed the origin of the elements heavier than helium.
Fred Hoyle's original work on nucleosynthesis of heavier elements in stars occurred just after World War II. This work attributed production of all heavier elements formed in stars during the nuclear evolution of their compositions, starting from hydrogen. Hoyle proposed that hydrogen is continuously created in the universe from vacuum and energy, without need for universal beginning.
Hoyle's work explained how the abundances of the elements increased with time as the galaxy aged. Subsequently, Hoyle's picture was expanded during the 1960s by creative contributions by William A. Fowler, Alastair G. W. Cameron, and Donald D. Clayton, and then by many others. The creative 1957 review paper by E. M. Burbidge, G. R. Burbidge, Fowler and Hoyle (see Ref. list) is a well-known summary of the state of the field in 1957. That paper defined new processes for changing one heavy nucleus into others within individual stars, processes that could be documented by astronomers.
The Big Bang itself had been proposed in 1931, long before this period, by Georges Lemaître, a Belgian physicist and Roman Catholic priest, who suggested that the evident expansion of the Universe in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. Hoyle later gave Lemaître's model the derisive term of Big Bang, not realizing that Lemaître's model was needed to explain the existence of deuterium and nuclides between helium and carbon, as well as the fundamentally high amount of helium present not only in stars, but also in interstellar gas. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain elemental abundance in the universe.
In modern theory, there are a number of astrophysical processes which are believed to be responsible for nucleosynthesis in the universe. The majority of these occur within the hot matter inside stars. The successive nuclear fusion processes which occur inside stars are known as hydrogen burning (via the proton-proton chain or the CNO cycle), helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are able to create elements up to iron and nickel, the region of the isotopes having the highest binding energy per nucleon. Heavier elements can be assembled within stars by a neutron capture process known as the s process or in explosive environments, such as supernovae, by a number of processes. Some of the more important of these include the r process, which involves rapid neutron captures, the rp process, which involves rapid proton captures, and the p process (sometimes known as the gamma process), which involves photodisintegration of existing nuclei.
The major types of nucleosynthesis
Big Bang nucleosynthesis
Big Bang nucleosynthesis occurred within the first three minutes of the beginning of the universe and is responsible for much of the abundance of 1H (protium), 2H (D, deuterium), 3He (helium-3), and 4He (helium-4), in the universe. Although 4He continues to be produced by other mechanisms (such as stellar fusion and alpha decay) and trace amounts of 1H continue to be produced by spallation and certain types of radioactive decay (proton emission and neutron emission), most of the mass of these isotopes in the universe, and all but the insignificant traces of the 3He and deuterium in the universe produced by rare processes such as cluster decay, are thought to have been produced in the Big Bang. The nuclei of these elements, along with some 7Li, and 7Be are believed to have been formed when the universe was between 100 and 300 seconds old, after the primordial quark-gluon plasma froze out to form protons and neutrons. Because of the very short period in which Big Bang nucleosynthesis occurred before being stopped by expansion and cooling (about 20 minutes after the Big Bang), no elements heavier than beryllium (or possibly boron) could be formed. (Elements formed during this time were in the plasma state, and did not cool to the state of neutral atoms until much later).
Stellar nucleosynthesis occurs in stars during the process of stellar evolution. It is responsible for the generation of elements from carbon to iron by nuclear fusion processes. Stars are the nuclear furnaces in which H and He are fused into heavier nuclei, a process which occurs by proton-proton chain in stars cooler than the Sun, and by the CNO cycle in stars more massive than the Sun.
Of particular importance is carbon, because its formation from He is a bottleneck in the entire process. Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element used in the production of free neutrons within the stars, giving rise to the s process which involves the slow absorption of neutrons to produce elements heavier than iron and nickel (57Fe and 62Ni). Carbon and other elements formed by this process are also fundamental to life in the form that we know it.
The products of stellar nucleosynthesis are generally distributed into the universe through mass loss episodes and stellar winds in stars which are of low mass, as in the planetary nebulae phase of evolution, as well as through explosive events resulting in supernovae in the case of massive stars.
The first direct proof that nucleosynthesis occurs in stars was the detection of technetium in the atmosphere of a red giant in the early 1950s, prototypical for the class of Tc-rich stars. Because technetium is radioactive, with half life much less than the age of the star, its abundance must reflect its creation within that star during its lifetime. Less dramatic, but equally convincing evidence is of large overabundances of specific stable elements in a stellar atmosphere. A historically important case was observation of barium abundances some 20-50 times greater than in unevolved stars, which is evidence of the operation of the s process within that star. Many modern proofs appear in the isotopic composition of stardust, solid grains that condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component of cosmic dust. The measured isotopic compositions demonstrate many aspects of nucleosynthesis within the stars from which the stardust grains condensed.
This includes supernova nucleosynthesis, and produces the elements heavier than iron by an intense burst of nuclear reactions that typically last mere seconds during the explosion of the supernova core. In explosive environments of supernovae, the elements between silicon and nickel are synthesized by fast fusion. Also in supernovae further nucleosynthesis processes can occur, such as the r process, in which the most neutron-rich isotopes of elements heavier than nickel are produced by rapid absorption of free neutrons released during the explosions. It is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element.
Explosive nucleosynthesis occurs too rapidly for radioactive decay to decrease the number of neutrons, so that many abundant isotopes having equal even numbers of protons and neutrons are synthesized by the alpha process to produce nuclides which consist of whole numbers of helium nuclei, up to 16 (representing 64Ge). Such nuclides are stable up to 40Ca (made of 10 helium nuclei), but heavier nuclei with equal numbers of protons and neutrons are radioactive. However, the alpha process continues to influence production of isobars of these nuclides, including at least the radioactive nuclides 44Ti, 48Cr, 52Fe, 56Ni, 60Zn, and 64Ge, most of which (save 44Ti and 60Zn) are created in such abundance as to decay after the explosion to create the most abundant stable isotope of the corresponding element at each atomic weight. Thus, the corresponding most common (abundant) isotopes of elements produced in this way are 48Ti, 52Cr, 56Fe, and 64Zn. Many such decays are accompanied by emission of gamma-ray lines capable of identifying the isotope that has just been created in the explosion.
The most convincing proof of explosive nucleosynthesis in supernovae occurred in 1987 when gamma-ray lines were detected emerging from supernova 1987A. Gamma ray lines identifying 56Co and 57Co, whose radioactive halflives limit their age to about a year, proved that 56Fe and 57Fe were created by radioactive parents. This nuclear astronomy was predicted in 1969  as a way to confirm explosive nucleosynthesis of the elements, and that prediction played an important role in the planning for NASA's successful Compton Gamma-Ray Observatory.
Other proofs of explosive nucleosynthesis are found within the stardust grains that condensed within the interiors of supernovae as they expanded and cooled. Stardust grains are one component of cosmic dust. In particular, radioactive 44Ti was measured to be very abundant within supernova stardust grains at the time they condensed during the supernova expansion, confirming a 1975 prediction for identifying supernova stardust. Other unusual isotopic ratios within these grains reveal many specific aspects of explosive nucleosynthesis.
Cosmic ray spallation
Cosmic ray spallation produces some of the lightest elements present in the universe (though not significant deuterium). Most notably spallation is believed to be responsible for the generation of almost all of 3He and the elements lithium, beryllium and boron (some 7
Li and 7
Be are thought to have been produced in the Big Bang). The spallation process results from the impact of cosmic rays (mostly fast protons) against the interstellar medium. These impacts fragment carbon, nitrogen and oxygen nuclei present in the cosmic rays, and also these elements being struck by protons in cosmic rays. The process results in these light elements (Be, B, and Li) being present in cosmic rays at much higher proportion than they are represented in solar atmospheres, whereas H and He nuclei are represented in cosmic rays with approximately primordial abundance with regard to each other.
Beryllium and boron are not significantly produced in stellar fusion processes, because the instability of any 8Be formed from two 4He nuclei prevents simple 2-particle reaction building-up of these elements.
Theories of nucleosynthesis are tested by calculating isotope abundances and comparing with observed results. Isotope abundances are typically calculated by calculating the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions.
Minor mechanisms and processes
Amounts of certain nuclides are produced on Earth by artificial means, and this is their major source (for example, technetium). However, some nuclides are also by a number of natural means that have continued after primordial production of elements, discussed above, ceased. Often these act to produce new elements in ways that can be used to date rocks or check on the timing or source of geological processes. Although these processes are usually not major sources of nuclides, in the cases of the short-lived naturally-occurring nuclides that exhibit half-lives too short to be primordial (see list of nuclides), these processes are the entire source of the existing natural supply of the nuclide.
These mechanisms include:
- Radioactive decay leading to specific radiogenic daughter nuclides. The nuclear decay of many long-lived primordial isotopes, especially uranium-235, uranium-238, and thorium-232 produce many intermediate daughter nuclides, some of them quite short-lived, before finally decaying to isotopes of lead. The Earth's natural supply of elements like radon and polonium is via this mechanism. The atmosphere's supply of argon-40 is due mostly to the radioactive decay of potassium-40 in the time since the formation of the Earth, so most of this atmospheric argon is not primordial. In the case of alpha-decay, helium-4 is produced directly by alpha-decay, and so the helium trapped in Earth's crust is also mostly non-primordial. In other types of radioactive decay, such as cluster decay, other types of nuclei are ejected (for example, neon-20), and these eventually become newly-formed neutral atoms.
- Radioactive decay leading to spontaneous fission. This is not cluster decay, for the fission products may be split among nearly any type of atom. Uranium-235 and uranium-238 are both primordial isotopes that undergo spontaneous fission. Natural technetium and promethium are produced in this way.
- Nuclear reactions. Naturally-occurring nuclear reactions powered by radioactive decay give rise to so-called nucleogenic nuclides. This process happens when an energetic particle from a radioactive decay, often an alpha particle, reacts with a nucleus of another atom to change the nucleus into another nuclide. This process may also cause production of further subatomic particles, such as neutrons. Neutrons can also be produced in spontaneous fission and by neutron emission (a type of radioactive decay). These neutrons can then go on to produce other nuclides via neutron-induced fission, or by neutron capture. For example, some stable isotopes like neon-21 and neon-22 are produced in several routes of nucleogenic synthesis, and thus only part of their abundance is primordial.
- Nuclear reactions due to cosmic rays. By convention, these reaction-products are not termed "nucleogenic" nuclides, but rather cosmogenic nuclides. Cosmic rays continue to produce new elements on Earth by the same cosmogenic processes discussed above that produced primordial beryllium and boron. An important example is carbon-14, produced from nitrogen-14 in the atmosphere by cosmic rays. See also iodine-129 for another example.
- ^ Autobiography William A. Fowler
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- ^ D. D. Clayton and L. R. Nittler (2004). "Astrophysics with Presolar Stardust". Annual Review of Astronomy and Astrophysics 42 (1): 39–78. Bibcode 2004ARA&A..42...39C. doi:10.1146/annurev.astro.42.053102.134022.
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Nuclear processes Radioactive decay Stellar nucleosynthesis Other processesEmissionCapture
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