P-process
Encyclopedia
The term p-process is used in two ways in the scientific literature concerning the astrophysical
origin of the elements (nucleosynthesis
). Originally it referred to a proton capture process which is the source of certain, naturally occurring, proton-rich isotope
s of the elements
from selenium
to mercury
. These nuclide
s are called p-nuclei
and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any nucleosynthesis
process supposed to be responsible for the p-nuclei.
Often, the two meanings are confused. Recent scientific literature therefore suggests to use the term p-process only for the actual proton capture process, as it is customary with other nucleosynthesis processes in astrophysics.
. Such a nuclear reaction
of type (p,γ) is called proton capture reaction. By adding a proton to a nucleus, the element
is changed because the chemical element is defined by the proton number of a nucleus. At the same time the ratio of protons to neutron
s is changed, resulting in a proton-richer isotope of the next element. This led to the original idea for the production of p-nuclei: free protons (the nuclei of hydrogen
atom
s are present in stellar plasmas
) should be captured on heavy nuclei (seed nuclei) also already present in the stellar plasma (previously produced in the s-
and/or r-process
).
Such proton captures on stable nuclides (or nearly stable), however, are not very efficient in producing p-nuclei, especially the heavier ones, because the electric charge
increases with each added proton, leading to an increased repulsion of the next proton to be added, according to Coulomb's law
. In the context of nuclear reactions this is called a Coulomb barrier
. The higher the Coulomb barrier the more kinetic energy
a proton requires to get close to a nucleus and be captured by it. The average energy of the available protons is given by the temperature
of the stellar plasma. Even if this temperature could be increased arbitrarily (which is not the case in stellar environments), protons would be removed faster from a nucleus by photodisintegration
than they could be captured at high temperature. A possible alternative would be to have a very large number of protons available to increase the effective number of proton captures per second without having to raise the temperature too much. Such conditions, however, are not found in core-collapse supernovae
which were supposed to be the site of the p-process.
Proton captures at extremely high proton densities are called rapid proton capture processes. They are distinct from the p-process not only by the required high proton density but also by the fact that very short-lived radionuclides are involved and the reaction path is located close to the proton drip line. Rapid proton capture processes are the rp-process
, the νp-process, and the pn-process.
) of a star
exploding as a type II supernova
. It was shown later that the required conditions are not found in such supernovae.
At the same time as B2FH, also Alastair Cameron independently realized the necessity to add another nucleosynthesis process to neutron capture nucleosynthesis
but simply mentioned proton captures without assigning a special name to the process. He also thought about alternatives, for example photodisintegration (called the γ-process today) or a combination of p-process and photodisintegration.
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
origin of the elements (nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . 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...
). Originally it referred to a proton capture process which is the source of certain, naturally occurring, proton-rich isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s of the elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
from selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
to mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
. These nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s are called p-nuclei
P-nuclei
p-Nuclei are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury which cannot be produced in either s- or r-process.- Definition :...
and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . 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...
process supposed to be responsible for the p-nuclei.
Often, the two meanings are confused. Recent scientific literature therefore suggests to use the term p-process only for the actual proton capture process, as it is customary with other nucleosynthesis processes in astrophysics.
The proton capture p-process
Proton-rich nuclides can be produced by sequentially adding one or more protons to an atomic nucleusAtomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. Such a nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
of type (p,γ) is called proton capture reaction. By adding a proton to a nucleus, the element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
is changed because the chemical element is defined by the proton number of a nucleus. At the same time the ratio of protons to neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s is changed, resulting in a proton-richer isotope of the next element. This led to the original idea for the production of p-nuclei: free protons (the nuclei of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s are present in stellar plasmas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
) should be captured on heavy nuclei (seed nuclei) also already present in the stellar plasma (previously produced in the s-
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
and/or r-process
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...
).
Such proton captures on stable nuclides (or nearly stable), however, are not very efficient in producing p-nuclei, especially the heavier ones, because the electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
increases with each added proton, leading to an increased repulsion of the next proton to be added, according to Coulomb's law
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
. In the context of nuclear reactions this is called a Coulomb barrier
Coulomb barrier
The Coulomb barrier, named after Coulomb's law, which is named after physicist Charles-Augustin de Coulomb , is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a nuclear reaction...
. The higher the Coulomb barrier the more kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
a proton requires to get close to a nucleus and be captured by it. The average energy of the available protons is given by the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the stellar plasma. Even if this temperature could be increased arbitrarily (which is not the case in stellar environments), protons would be removed faster from a nucleus by photodisintegration
Photodisintegration
Photodisintegration is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the...
than they could be captured at high temperature. A possible alternative would be to have a very large number of protons available to increase the effective number of proton captures per second without having to raise the temperature too much. Such conditions, however, are not found in core-collapse supernovae
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...
which were supposed to be the site of the p-process.
Proton captures at extremely high proton densities are called rapid proton capture processes. They are distinct from the p-process not only by the required high proton density but also by the fact that very short-lived radionuclides are involved and the reaction path is located close to the proton drip line. Rapid proton capture processes are the rp-process
Rp-process
The rp-process consists of consecutive proton captures onto seed nuclei to produce heavier elements. It is a nucleosynthesis process and, along with the s process and the r process, may be responsible for the generation of many of the heavy elements present in the universe...
, the νp-process, and the pn-process.
History
The term p-process was originally proposed in the famous "B2FH paper" in 1957. The authors assumed that this process was solely responsible for the p-nuclei and proposed that it occurs in the hydrogen-shell (see also stellar evolutionStellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
) of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
exploding as a type II supernova
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...
. It was shown later that the required conditions are not found in such supernovae.
At the same time as B2FH, also Alastair Cameron independently realized the necessity to add another nucleosynthesis process to neutron capture nucleosynthesis
Neutron capture nucleosynthesis
Neutron capture nucleosynthesis describes two nucleosynthesis pathways: the r-process and the s-process, for rapid and slow neutron captures, respectively. R-process describes neutron capture in a region of high neutron flux, such as during supernova nucleosynthesis after core-collapse, and yields...
but simply mentioned proton captures without assigning a special name to the process. He also thought about alternatives, for example photodisintegration (called the γ-process today) or a combination of p-process and photodisintegration.