S-process Element
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The slow neutron-capture process, or ''s''-process, is a series of reactions in
nuclear astrophysics Nuclear astrophysics is an interdisciplinary part of both nuclear physics and astrophysics, involving close collaboration among researchers in various subfields of each of these fields. This includes, notably, nuclear reactions and their rates as ...
that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (
nucleosynthesis Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...
) of approximately half the
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...
heavier than iron. In the ''s''-process, a
seed nucleus A seed nucleus is an isotope that is the starting point for any of a variety of fusion chain reactions. The mix of nuclei produced at the conclusion of the chain reaction generally depends strongly on the relative availability of the seed nucleus ...
undergoes
neutron capture 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 easily than positively charged protons, ...
to form an
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...
with one higher
atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nb ...
. If the new isotope is stable, a series of increases in mass can occur, but if it is
unstable In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...
, then
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
will occur, producing an element of the next higher
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
. The process is ''slow'' (hence the name) in the sense that there is sufficient time for this radioactive decay to occur before another neutron is captured. A series of these reactions produces stable isotopes by moving along the
valley A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ove ...
of
beta-decay stable isobars Beta-decay stable isobars are the set of nuclides which cannot undergo beta decay, that is, the transformation of a neutron to a proton or a proton to a neutron within the nucleus. A subset of these nuclides are also stable with regards to dou ...
in the
table of nuclides A table or chart of nuclides is a two-dimensional Cartesian coordinate system, graph of isotopes of the elements, in which one axis represents the number of neutrons (symbol ''N'') and the other represents the number of protons (atomic number, sy ...
. A range of elements and isotopes can be produced by the ''s''-process, because of the intervention of
alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...
steps along the reaction chain. The relative abundances of elements and isotopes produced depends on the source of the neutrons and how their flux changes over time. Each branch of the ''s''-process reaction chain eventually terminates at a cycle involving
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
, and
polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...
. The ''s''-process contrasts with the ''r''-process, in which successive neutron captures are ''rapid'': they happen more quickly than the beta decay can occur. The ''r''-process dominates in environments with higher fluxes of
free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...
s; it produces heavier elements and more neutron-rich isotopes than the ''s''-process. Together the two processes account for most of the relative
abundance of chemical elements The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by the mass-fraction (the same as weight fraction); ...
heavier than iron.


History

The ''s''-process was seen to be needed from the relative abundances of isotopes of heavy elements and from a newly published table of abundances by
Hans Suess Hans Eduard Suess (December 16, 1909 – September 20, 1993) was an Austrian born American physical chemist and nuclear physicist. He was a grandson of the Austrian geologist Eduard Suess. Career Suess earned his Ph.D. in chemistry from the U ...
and
Harold Urey Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in th ...
in 1956. Among other things, these data showed abundance peaks for strontium, barium, and
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
, which, according to
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...
and the
nuclear shell model In nuclear physics, atomic physics, and nuclear chemistry, the nuclear shell model is a model of the atomic nucleus which uses the Pauli exclusion principle to describe the structure of the nucleus in terms of energy levels. The first shell m ...
, are particularly stable nuclei, much like the
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
es are chemically inert. This implied that some abundant nuclei must be created by slow
neutron capture 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 easily than positively charged protons, ...
, and it was only a matter of determining how other nuclei could be accounted for by such a process. A table apportioning the heavy isotopes between ''s''-process and ''r''-process was published in the famous B2FH review paper in 1957. There it was also argued that the ''s''-process occurs in red giant stars. In a particularly illustrative case, the element
technetium Technetium is a chemical element with the symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous ...
, whose longest half-life is 4.2 million years, had been discovered in s-, M-, and N-type stars in 1952 by Paul W. Merrill. Since these stars were thought to be billions of years old, the presence of technetium in their outer atmospheres was taken as evidence of its recent creation there, probably unconnected with the nuclear fusion in the deep interior of the star that provides its power. A calculable model for creating the heavy isotopes from iron seed nuclei in a time-dependent manner was not provided until 1961. That work showed that the large overabundances of barium observed by astronomers in certain red-giant stars could be created from iron seed nuclei if the total neutron flux (number of neutrons per unit area) was appropriate. It also showed that no one single value for neutron flux could account for the observed ''s''-process abundances, but that a wide range is required. The numbers of iron seed nuclei that were exposed to a given flux must decrease as the flux becomes stronger. This work also showed that the curve of the product of neutron-capture cross section times abundance is not a smoothly falling curve, as B2FH had sketched, but rather has a ''ledge-precipice structure''. A series of papers in the 1970s by Donald D. Clayton utilizing an exponentially declining neutron flux as a function of the number of iron seed exposed became the standard model of the ''s''-process and remained so until the details of AGB-star nucleosynthesis became sufficiently advanced that they became a standard model for ''s''-process element formation based on stellar structure models. Important series of measurements of neutron-capture cross sections were reported from Oak Ridge National Lab in 1965 and by Karlsruhe Nuclear Physics Center in 1982 and subsequently, these placed the ''s''-process on the firm quantitative basis that it enjoys today.


The ''s''-process in stars

The ''s''-process is believed to occur mostly in asymptotic giant branch stars, seeded by iron nuclei left by a supernova during a previous generation of stars. In contrast to the ''r''-process which is believed to occur over time scales of seconds in explosive environments, the ''s''-process is believed to occur over time scales of thousands of years, passing decades between neutron captures. The extent to which the ''s''-process moves up the elements in the chart of isotopes to higher
mass number The mass number (symbol ''A'', from the German word ''Atomgewicht'' tomic weight, also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is approxima ...
s is essentially determined by the degree to which the star in question is able to produce
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s. The quantitative yield is also proportional to the amount of iron in the star's initial abundance distribution.
Iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
is the "starting material" (or seed) for this neutron capture-beta minus decay sequence of synthesizing new elements. The main
neutron source A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear p ...
reactions are: : One distinguishes the main and the weak ''s''-process component. The main component produces heavy elements beyond Sr and Y, and up to Pb in the lowest metallicity stars. The production sites of the main component are low-mass asymptotic giant branch stars. The main component relies on the 13C neutron source above. The weak component of the ''s''-process, on the other hand, synthesizes ''s''-process isotopes of elements from iron group seed nuclei to 58Fe on up to Sr and Y, and takes place at the end of
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
- and
carbon-burning The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 \beginM_\odot\end at birth) that combines carbon into other elements. It requires high temperatures (> 5&t ...
in massive stars. It employs primarily the 22Ne neutron source. These stars will become supernovae at their demise and spew those ''s''-process isotopes into interstellar gas. The ''s''-process is sometimes approximated over a small mass region using the so-called "local approximation", by which the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for nearby isotopes on the ''s''-process path. This approximation is – as the name indicates – only valid locally, meaning for isotopes of nearby mass numbers, but it is invalid at magic numbers where the ledge-precipice structure dominates. Because of the relatively low
neutron flux The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travellin ...
es expected to occur during the ''s''-process (on the order of 105 to 1011 neutrons per cm2 per second), this process does not have the ability to produce any of the heavy radioactive isotopes such as
thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...
or
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
. The cycle that terminates the ''s''-process is: captures a neutron, producing , which decays to by β decay. in turn decays to by α decay: : then captures three neutrons, producing , which decays to by β decay, restarting the cycle: : The net result of this cycle therefore is that 4
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s are converted into one
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pr ...
, two
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s, two anti-electron
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s and gamma radiation: : The process thus terminates in bismuth, the heaviest "stable" element, and polonium, the first non-primordial element after bismuth. Bismuth is actually slightly radioactive, but with a half-life so long—a billion times the present age of the universe—that it is effectively stable over the lifetime of any existing star. Polonium-210, however, decays with a half-life of to stable
lead-206 Lead (82Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the urani ...
.


The ''s''-process measured in stardust

Stardust is one component of
cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...
. Stardust is individual solid grains that condensed during mass loss from various long-dead stars. Stardust existed throughout interstellar gas before the birth of the Solar System and was trapped in meteorites when they assembled from interstellar matter contained in the planetary accretion disk in early Solar System. Today they are found in meteorites, where they have been preserved. Meteoriticists habitually refer to them as
presolar grains Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed. Presolar stardust grains formed within outflowing and cooling gases from earlier presolar stars. The stellar nucl ...
. The ''s''-process enriched grains are mostly
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...
(SiC). The origin of these grains is demonstrated by laboratory measurements of extremely unusual isotopic abundance ratios within the grain. First experimental detection of ''s''-process xenon isotopes was made in 1978, confirming earlier predictions that ''s''-process isotopes would be enriched, nearly pure, in stardust from red giant stars. These discoveries launched new insight into astrophysics and into the origin of meteorites in the Solar System. Silicon carbide (SiC) grains condense in the atmospheres of AGB stars and thus trap isotopic abundance ratios as they existed in that star. Because the AGB stars are the main site of the ''s''-process in the galaxy, the heavy elements in the SiC grains contain almost pure ''s''-process isotopes in elements heavier than iron. This fact has been demonstrated repeatedly by sputtering-ion mass spectrometer studies of these stardust
presolar grains Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed. Presolar stardust grains formed within outflowing and cooling gases from earlier presolar stars. The stellar nucl ...
. Several surprising results have shown that within them the ratio of ''s''-process and ''r''-process abundances is somewhat different from that which was previously assumed. It has also been shown with trapped isotopes of
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
and
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
that the ''s''-process abundances in the AGB-star atmospheres changed with time or from star to star, presumably with the strength of neutron flux in that star or perhaps the temperature. This is a frontier of ''s''-process studies in the 2000s.


References

{{Star Nuclear physics Neutron Astrophysics Nucleosynthesis