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A figurative depiction of the helium-4 atom. In the nucleus, the two protons are shown in red and neutrons blue. This depiction shows the particles as separate, whereas in an actual helium atom, the protons are superimposed in space and most likely found at the very center of the nucleus, and the same is true of the two neutrons. Thus all four particles are most likely found in exactly the same space. Classical images of separate particles thus fail to model known charge distributions in very small nuclei

The nucleus is the very dense region consisting of nucleons (protons and neutrons) at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons. 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 diameter of the nucleus is in the range of 1.6 fm (1.6×10−15 m) for hydrogen (the diameter of a single proton) to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electronic cloud), by a factor of about 23,000 (uranium) to about 145,000 (hydrogen).

The branch of physics concerned with studying and understanding the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics.

Contents

Introduction

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Etymology

The term nucleus is from Latin nucleus ("kernel"), derived from nux ("nut"). In 1844, Michael Faraday used the term to refer to the "central point of an atom". The modern atomic meaning was proposed by Ernest Rutherford in 1912.[1] The adoption of the term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule], that "the atom is composed of the kernel and an outer atom or shell".[2]

Nuclear makeup

The nucleus of an atom consists of protons and neutrons (two types of baryons) bound by the nuclear force (also known as the residual strong force). These baryons are further composed of subatomic fundamental particles known as quarks bound by the strong interaction. Which chemical element an atom represents is determined by the number of protons in the nucleus. Each proton carries a single positive charge, and the total electrical charge of the nucleus is spread fairly uniformly throughout its body, with a fall-off at the edge.

Major exceptions to this rule are the light elements hydrogen and helium, where the charge is concentrated most highly at the single central point (without a volume of uniform charge), as would be expected for fermions (in this case, protons) in 1s states without orbital angular momentum.[3]

As each proton carries a unit of charge, the charge distribution is indicative of the proton distribution. The neutron distribution probably is similar.[3]

Protons and neutrons

Protons and neutrons are fermions, with different values of the isospin quantum number, so two protons and two neutrons can share the same space wave function since they are not identical quantum entities. They sometimes are viewed as two different quantum states of the same particle, the nucleon.[4][5] Two fermions, such as two protons, or two neutrons, or a proton + neutron (the deuteron) can exhibit bosonic behavior when they become loosely bound in pairs.

In the rare case of a hypernucleus, a third baryon called a hyperon, with a different value of the strangeness quantum number can also share the wave function. However, the latter type of nuclei are extremely unstable and are not found on Earth except in high energy physics experiments.

The neutron has a positively charged core of radius ≈ 0.3 fm surrounded by a compensating negative charge of radius between 0.3 fm and 2 fm. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.[6]

Forces

Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms internally together (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus).

The nuclear force is highly attractive at very small distances, and this overwhelms the repulsion between protons which is due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential), only nuclei smaller than a certain size can be completely stable. The largest known completely stable (e.g., stable to alpha, beta, and gamma decay) nucleus is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximal size of 208 particles are unstable and (as a trend) become increasingly short-lived with larger size, as the number of neutrons and protons which compose them increases beyond this number. However, bismuth-209 is also stable to beta decay and has the longest half-live to alpha decay of any known isotope, estimated at longer than the age of the universe.

The residual strong force is effective over a very short range (usually only a few fermis; roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons (i.e., between protons and neutrons to form [NP] deuteron, and also between protons and protons, and neutrons and neutrons). It also is effective for the stability of one 3-body nucleon system [PNP], helium-3, while the triton [NPN] is unstable and decays to helium-3.

Halo nuclei and strong force range limits

The effective absolute limit of the range of the strong force is represented by halo nuclei such as lithium-11 or boron-14, in which dineutrons, or other collections of neutrons, orbit at distances of about ten fermis (roughly similar to the 8 fermi radius of the nucleus of uranium-238). These nuclei are not maximally dense. Halo nuclei form at the extreme edges of the chart of the nuclides—the neutron drip line and proton drip line—and are all unstable with short half-lives, measured in milliseconds; for example, lithium-11 has a half-life of less than 8.6 milliseconds.

Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons [NN, NNN] or protons [PP, PPP]. Examples: Nuclei which have a single neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments, never two, and are called Borromean because of this behavior (referring to a system of three interlocked rings in which breaking any ring frees both of the others). 8He and 14Be both exhibit a four-neutron halo. Nuclei which have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive electromagnetic forces of the excess proton(s).

Nuclear models

There are many different historical models of the atomic nucleus, to this day none of which completely alone explain experimental data on nuclear structure. A useful review of 37 known models of the atomic nucleus is provided by Cook.[7]

The nuclear radius (R) is considered to be one of the basic things that any model must explain. For stable nuclei (not halo nuclei or other unstable distorted nuclei) the nuclear radius is roughly proportional to the cube root of the mass number (A) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations:

The stable nucleus has approximately a constant density and therefore the nuclear radius R can be approximated by the following formula,

R = r_0 A^{1/3} \,

where A = Atomic mass number (the number of protons, Z, plus the number of neutrons, N) and r0 = 1.25 fm = 1.25 × 10−15 m. In this equation, the constant r0 varies by 0.2 fm, depending on the nucleus in question, but this is less than 20% change from a constant.[8]

In other words, packing protons and neutrons in the nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight spherical or semi-spherical bag (some stable nuclei are not quite spherical, but are known to be prolate).

Liquid drop models

Early models of the nucleus viewed the nucleus as a rotating liquid drop. In this model, the trade-off of long-range electromagnetic forces and relatively short-range nuclear forces, together cause behavior which resembled surface tension forces in liquid drops of different sizes. This formula is successful at explaining many important phenomena of nuclei, such as their changing amounts of binding energy as their size and composition changes (see semi-empirical mass formula), but it does not explain the special stability which occurs when nuclei have special "magic numbers" of protons or neutrons.

Shell models and other quantum models

A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory. These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells.

In these models, the nucleons may occupy orbitals in pairs, due to being fermions, but the exact nature and capacity of nuclear shells differs from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nuclei) is quite different from the central electromagnetic potential well which binds electrons in atoms. Some resemblance to atomic orbital models may be seen in a small atomic nucleus like that of helium-4, in which the two protons and two neutrons separately occupy 1s orbitals analogous to the 1s orbitals for the two electrons in the helium atom, and achieve unusual stability for the same reason. Nuclei with 5 nucleons are all extremely unstable and short-lived, yet, helium-3, with 3 nucleons, is very stable even with lack of a closed 1s orbital shell. Another nuclei with 3 nucleons, the triton hydrogen-3 is unstable and will decay into helium-3 when isolated. Weak nuclear stability with 2 nucleons {NP} in the 1s orbital is found in the deuteron hydrogen-2, with only one nucleon in each of the proton and neutron potential wells. While each nucleon is a fermion, the {NP} deuteron is a boson and thus does not follow Pauli Exclusion for close packing within shells. Lithium-6 with 6 nucleons is highly stable without a closed second 1p shell orbital. For light nuclei with total nucleon numbers 1 to 6 only those with 5 do not show some evidence of stability. Observations of beta-stability of light nuclei outside closed shells indicate that nuclear stability is much more complex than simple closure of shell orbitals with magic numbers of protons and neutrons.

For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict the magic numbers of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly-inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element. Similarly, the distance from shell-closure explains the unusual instability of isotopes which have far from stable numbers of these particles, such as the radioactive elements 43 (technetium) and 61 (promethium), each of which is preceded and followed by 17 or more stable elements.

There are however problems with the shell model when attempt is made to account for nuclear properties well away from closed shells. This as led to complex post hoc distortions of the shape of the potential-well to fit experimental data, but the question remains, do these mathematical manipulations actually correspond to the spatial deformations in real nuclei—it remains an open question. Problems with the shell model have lead some to propose realistic two-and three-body nuclear force effects involving nucleon cluster, and then build the nucleus on this basis. Two such cluster models are the Close-Packed Spheron Model of Linus Pauling and the 2D Lsing Model of MacGregor.[7]

Consistency between models

As with the case of superfluid liquid helium, atomic nuclei are an example of a state in which both (1) "ordinary" particle physical rules for volume and (2) non-intuitive quantum mechanical rules for a wave-like nature apply. In superfluid helium, the helium atoms have volume, and essentially "touch" each other, yet at the same time exhibit strange bulk properties, consistent with a Bose-Einstein condensation. The latter reveals that they also have a wave-like nature and do not exhibit standard fluid properties, such as friction. For nuclei made of hadrons which are fermions, the same type of condensation does not occur, yet nevertheless, many nuclear properties can only be explained similarly by a combination of properties of particles with volume, in addition to the frictionless motion characteristic of the wave-like behavior of objects trapped in Schrödinger quantum orbitals.

See also

Notes

  1. ^ D. Harper. "Nucleus". Online Etymology Dictionary. http://www.etymonline.com/index.php?search=Nucleus&searchmode=none. Retrieved 2010-03-06. 
  2. ^ G.N. Lewis (1916). "The Atom and the Molecule". Journal of the American Chemical Society 38: 4. http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/bond/papers/corr216.3-lewispub-19160400.html. 
  3. ^ a b J.-L. Basdevant, J. Rich, M. Spiro (2005). Fundamentals in Nuclear Physics. Springer. p. 13, Fig. 1.1. ISBN 0387016724. http://books.google.com/books?id=OFx7P9mgC9oC&pg=PA375&dq=helium+%22nuclear+structure%22&lr=&as_brr=0#PPA13,M1. 
  4. ^ A.G. Sitenko, V.K. Tartakovskiĭ (1997). Theory of Nucleus: Nuclear Structure and Nuclear Interaction. Kluwer Academic. p. 3. ISBN 0792344235. http://books.google.com/books?id=swb9QpqOqtAC&pg=PA464&dq=isbn=0792344235#PPA3,M1. 
  5. ^ M.A. Srednicki (2007). Quantum Field Theory. Cambridge University Press. pp. 522–523. ISBN 9780521864497. 
  6. ^ J.-L. Basdevant, J. Rich, M. Spiro (2005). Fundamentals in Nuclear Physics. Springer. p. 155. ISBN 0387016724. http://books.google.com/books?id=OFx7P9mgC9oC&pg=PA375&dq=helium+%22nuclear+structure%22&lr=&as_brr=0#PPA155,M1. 
  7. ^ a b N.D. Cook (2006). Models of the Atomic Nucleus. Springer. p. 56. ISBN 3540285695. http://books.google.com/books?id=faBp90f5X_4C&pg=PA56&lpg=PA56&dq=collective+models+of+atomic+nucleus&source=bl&ots=l1AzfHl8Nk&sig=6M6psrlubfFt9NwOvtZDW8BW5kg&hl=en&ei=KNNbSrP1MsexlAeX6ZHwDA&sa=X&oi=book_result&ct=result&resnum=3. 
  8. ^ K.S. Krane (1987). Introductory Nuclear Physics. Wiley-VCH. ISBN 0-471-80553-X. 

References

  • N.D. Cook (2006). Models of the Atomic Nucleus. Springer. ISBN 3540285695. 

External links


Simple English

The nucleus of an atom is the very small dense part of an atom, in its center made up of nucleons (protons and neutrons). The size (diameter) of the nucleus is between 1.6 fm (10-15 m) (for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium). These sizes are much smaller than the size of the atom itself by a factor of about 23,000 (uranium) to about 145,000 (hydrogen). Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus with a very small contribution from the orbiting electrons. The word nucleus is from 1704, meaning “kernel of a nut”. In 1844, Michael Faraday used nucleus to describe the “central point of an atom”. The modern atomic meaning was proposed by Ernest Rutherford in 1912.[1] The use of the word nucleus in atomic theory, however, did not happen immediately. In 1916, for example, Gilbert N. Lewis wrote, in his famous article The Atom and the Molecule, that “the atom is composed of the kernel and an outer atom or shell”.

File:Helium atom
A drawing of the helium atom. In the nucleus, the protons are in red and neutrons are in blue.

Contents

Introduction

Nuclear makeup

The nucleus of an atom is made up of protons and neutrons (two types of baryons) joined by the nuclear force. These baryons are further made up of sub-atomic fundamental particles known as quarks joined by the strong interaction.

Isotopes and nuclides

The isotope of an atom is based on the number of neutrons in the nucleus. Different isotopes of the same element have very similar chemical properties. Different isotopes in a sample of a chemical can be separated by using a centrifuge or by using a mass spectrometer. The first method is used in producing enriched uranium from regular uranium, and the second is used in carbon dating.

The number of protons and neutrons together determine the nuclide (type of nucleus). Protons and neutrons have nearly equal masses, and their combined number, the mass number, is about equal to the atomic mass of an atom. The combined mass of the electrons is very small when compared to the mass of the nucleus; protons and neutrons weigh about 2000 times more than electrons.

History

The discovery of the electron by J. J. Thomson was the first sign that the atom had internal structure. At the turn of the 20th century the accepted model of the atom was J. J. Thomson's "plum pudding" model in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it. By the turn of the century physicists had also discovered three types of radiation coming from atoms, which they named alpha, beta, and gamma radiation. Experiments in 1911 by Lise Meitner and Otto Hahn, and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete. That is, electrons were ejected from the atom with a range of energies, rather than the discrete amounts of energies that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it indicated that energy was not conserved in these decays. The problem would later lead to the discovery of the neutrino (see below).

In 1906 Ernest Rutherford published "Radiation of the α Particle from Radium in passing through Matter"[2]. Geiger expanded on this work in a communication to the Royal Society[3] with experiments he and Rutherford had done passing α particles through air, aluminum foil and gold foil. More work was published in 1909 by Geiger and Marsden[4] and further greatly expanded work was published in 1910 by Geiger, [5] In 1911-2 Rutherford went before the Royal Society to explain the experiments and propound the new theory of the atomic nucleus as we now understand it.

Around the same time that this was happening (1909) Ernest Rutherford performed a remarkable experiment in which Hans Geiger and Ernest Marsden under his supervision fired alpha particles (helium nuclei) at a thin film of gold foil. The plum pudding model predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. He was shocked to discover that a few particles were scattered through large angles, even completely backwards in some cases. The discovery, beginning with Rutherford's analysis of the data in 1911, eventually led to the Rutherford model of the atom, in which the atom has a very small, very dense nucleus consisting of heavy positively charged particles with embedded electrons in order to balance out the charge. As an example, in this model nitrogen-14 consisted of a nucleus with 14 protons and 7 electrons, and the nucleus was surrounded by 7 more orbiting electrons.

The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in 1929. By 1925 it was known that protons and electrons had a spin of 1/2, and in the Rutherford model of nitrogen-14 the 14 protons and six of the electrons should have paired up to cancel each others spin, and the final electron should have left the nucleus with a spin of 1/2. Rasetti discovered, however, that nitrogen-14 has a spin of one.

In 1930 Wolfgang Pauli was unable to attend a meeting in Tübingen, and instead sent a famous letter with the classic introduction "Dear Radioactive Ladies and Gentlemen". In his letter Pauli suggested that perhaps there was a third particle in the nucleus which he named the "neutron". He suggested that it was very light (lighter than an electron), had no charge, and that it did not readily interact with matter (which is why it had not yet been detected). This desperate way out solved both the problem of energy conservation and the spin of nitrogen-14, the first because Pauli's "neutron" was carrying away the extra energy and the second because an extra "neutron" paired off with the electron in the nitrogen-14 nucleus giving it spin one. Pauli's "neutron" was renamed the neutrino (Italian for little neutral one) by Enrico Fermi in 1931, and after about thirty years it was finally demonstrated that a neutrino really is emitted during beta decay.

In 1932 Chadwick realized that radiation that had been observed by Walther Bothe, Herbert L. Becker, Irène and Frédéric Joliot-Curie was actually due to a massive particle that he called the neutron. In the same year Dmitri Ivanenko suggested that neutrons were in fact spin 1/2 particles and that the nucleus contained neutrons and that there were no electrons in it, and Francis Perrin suggested that neutrinos were not nuclear particles but were created during beta decay. To cap the year off, Fermi submitted a theory of the neutrino to Nature (which the editors rejected for being "too remote from reality"). Fermi continued working on his theory and published a paper in 1934 which placed the neutrino on solid theoretical footing. In the same year Hideki Yukawa proposed the first significant theory of the strong force to explain how the nucleus holds together.

With Fermi and Yukawa's papers the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (or positron). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high energy photons (gamma decay).

The study of the strong and weak nuclear forces led physicists to collide nuclei and electrons at ever higher energies. This research became the science of particle physics, the most important of which is the standard model of particle physics which unifies the strong, weak, and electromagnetic forces.

Modern nuclear physics

A light nucleus can contain hundreds of nucleons which means that with some approximation it can be treated as a classical system, rather than a quantum-mechanical one. In the resulting liquid-drop model, the nucleus has an energy which arises partly from surface tension and partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend of binding energy with respect to mass number, as well as the phenomenon of nuclear fission.

Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the nuclear shell model, developed in large part by Maria Goeppert-Mayer. Nuclei with certain numbers of neutrons and protons (the magic numbers 2, 8, 20, 50, 82, 126, ...) are particularly stable, because their shells are filled.

Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as high spin and excitation energy. Nuclei may also have extreme shapes (similar to that of American footballs) or extreme neutron-to-proton ratios. Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from an accelerator. Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced a phase transition from normal nuclear matter to a new state, the quark-gluon plasma, in which the quarks mingle with one another, rather than being segregated in triplets as they are in neutrons and protons.

Modern topics in nuclear physics

Spontaneous changes from one nuclide to another: nuclear decay

If a nucleus has too few or too many neutrons it may be unstable, and will decay after some period of time. For example, nitrogen-16 atoms (7 protons, 9 neutrons) beta decay to oxygen-16 atoms (8 protons, 8 neutrons) within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is turned into a proton and an electron by the weak nuclear force. The element of the atom changes because while it previously had seven protons (which makes it nitrogen) it now has eight (which makes it oxygen). Many elements have multiple isotopes which are stable for weeks, years, or even billions of years.

Nuclear fusion

When two light nuclei come into very close contact with each other it is possible for the strong force to fuse the two together. It takes a great deal of energy to push the nuclei close enough together for the strong force to have an effect, so the process of nuclear fusion can only take place at very high temperatures or high densities. Once the nuclei are close enough together the strong force overcomes their electromagnetic repulsion and squishes them into a new nucleus. A very large amount of energy is released when light nuclei fuse together because the binding energy per nucleon increases with mass number up until nickel-62. Stars like our sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos. The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. Research to find an economically viable method of using energy from a controlled fusion reaction is currently being undertaken by various research establishments (see JET and ITER).

Nuclear fission

For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones. This splitting of atoms is known as nuclear fission.

The process of alpha decay may be thought of as a special type of spontaneous nuclear fission. This process produces a highly asymmetrical fission because the four particles which make up the alpha particle are especially tightly bound to each other, making production of this nucleus in fission particularly likely.

For certain of the heaviest nuclei which produce neutrons on fission, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a so-called chain reaction. [Chain reactions were known in chemistry before physics, and in fact many familiar processes like fires and chemical explosions are chemical chain reactions]. The fission or "nuclear" chain-reaction, using fission-produced neutrons, is the source of energy for nuclear power plants and fission type nuclear bombs such as the two that the United States used against Hiroshima and Nagasaki at the end of World War II. Heavy nuclei such as uranium and thorium may undergo spontaneous fission, but they are much more likely to undergo decay by alpha decay.

For a neutron-initiated chain-reaction to occur, there must be a critical mass of the element present in a certain space under certain conditions (these conditions slow and conserve neutrons for the reactions). There is one known example of a natural nuclear fission reactor, which was active in two regions of Oklo, Gabon, Africa, over 1.5 billion years ago. Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the earth's core results from radioactive decay. However, it is not known if any of this results from fission chain-reactions.

Production of heavy elements

As the Universe cooled after the big bang it eventually became possible for particles as we know them to exist. The most common particles created in the big bang which are still easily observable to us today were protons (hydrogen) and electrons (in equal numbers). Some heavier elements were created as the protons collided with each other, but most of the heavy elements we see today were created inside of stars during a series of fusion stages, such as the proton-proton chain, the CNO cycle and the triple-alpha process. Progressively heavier elements are made during the evolution of a star. Since the binding energy per nucleon peaks around iron, energy is only released in fusion processes occurring below this point. Since the creation of heavier nuclei by fusion costs energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a slow neutron capture process (the so-called s process) or by the rapid, or r process. The s process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The r process is thought to occur in supernova explosions because the conditions of high temperature, high neutron flux and ejected matter are present. These stellar conditions make the successive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells (magic numbers). The r process duration is typically in the range of a few seconds.

Other pages

References

  1. Nucleus – Online Etymology Dictionary
  2. Philosophical Magazine (12, p 134-46)
  3. Proc. Roy. Soc. July 17, 1908
  4. Proc. Roy. Soc. A82 p 495-500
  5. Proc. Roy. Soc. Feb. 1, 1910

Other websites

Particles in Physics
Elementary: Fermions: Quarks: up - down - strange - charm - bottom - top
Leptons: electron - muon - tau - neutrinos
Bosons: Gauge bosons: photon - W and Z bosons - gluons
Composite: Hadrons: Baryons: proton - neutron - hyperon
Mesons: pion - kaon - J/ψ
Atomic nuclei - Atoms - Molecules
Hypothetical: Higgs boson - Graviton - Tachyon

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