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.
Notes
[1] Geoff Brumfiel (July 7, 2010). "The proton shrinks in size". Nature. doi:10.1038/news.2010.337.
[2] D. Harper. "Nucleus" (http://www.etymonline.com/index.php?search=Nucleus&searchmode=none). Online Etymology Dictionary. . Retrieved 2010-03-06.
[3] G.N. Lewis (1916). "The Atom and the Molecule" (http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/bond/papers/ corr216.3-lewispub-19160400.html). Journal of the American Chemical Society 38 (4): 4. doi:10.1021/ja02261a002. .
[4] J.-L. Basdevant, J. Rich, M. Spiro (2005). Fundamentals in Nuclear Physics (http://books.google.com/?id=OFx7P9mgC9oC& pg=PA375&dq=helium+"nuclear+structure"). Springer. p. 13, Fig. 1.1. ISBN 0387016724. .
[5] A.G. Sitenko, V.K. Tartakovskiĭ (1997). Theory of Nucleus: Nuclear Structure and Nuclear Interaction (http://books.google.com/ ?id=swb9QpqOqtAC&pg=PA464&dq=isbn=0792344235#PPA3,M1). Kluwer Academic. p. 3. ISBN 0792344235. .
[6] M.A. Srednicki (2007). Quantum Field Theory. Cambridge University Press. pp. 522–523. ISBN 9780521864497.
[7] J.-L. Basdevant, J. Rich, M. Spiro (2005). Fundamentals in Nuclear Physics (http://books.google.com/?id=OFx7P9mgC9oC& pg=PA375&dq=helium+"nuclear+structure"). Springer. p. 155. ISBN 0387016724. .
[8] N.D. Cook (2010). Models of the Atomic Nucleus (2nd ed.). Springer. p. 57 ff.. ISBN 978-3-642-14736-4. [9] K.S. Krane (1987). Introductory Nuclear Physics. Wiley-VCH. ISBN 0-471-80553-X.
References
• N.D. Cook (2010). Models of the Atomic Nucleus (2nd ed.). Springer. ISBN 978-3-642-14736-4.
External links
• The Nucleus - a chapter from an online textbook (http://www.lightandmatter.com/html_books/4em/ch02/ ch02.html)
• The LIVEChart of Nuclides - IAEA (http://www-nds.iaea.org/livechart) in Java (http://www-nds.iaea.org/ livechart) or HTML (http://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html)
• Article on the "nuclear shell model," giving nuclear shell filling for the various elements (http://www. halexandria.org/dward472.htm). Accessed Sept. 16, 2009.
Proton
Proton
The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)
Classification Baryon
Composition 2 up quarks, 1 down quark
Statistics Fermionic
Interactions Gravity, Electromagnetic, Weak, Strong
Symbol p, p+, N+
Antiparticle Antiproton
Theorized William Prout (1815)
Discovered Ernest Rutherford (1919)
Mass 1.672621777(74) × 10−27 kg[1]
938.272046(21) MeV/c2[1] 1.007276466812(90) u[1]
Mean lifetime >2.1 × 1029 yr (stable)
Electric charge 1 e
1.602176565(35) × 10−19 C[1]
Charge radius 0.8775(51) fm[1]
Electric dipole moment <5.4 × 10−24 e·cm Electric polarizability 1.20(6) × 10−3 fm3 Magnetic moment 1.410606743(33) × 10−26 J·T−1[1] 1.521032210(12) × 10−3 μB[1] 2.792847356(23) μN[1] Magnetic polarizability 1.9(5) × 10−4 fm3 Spin 1⁄2 Isospin 1⁄2 Parity +1 Condensed I(JP) = 1⁄2(1⁄2+)
The proton is a subatomic particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.
In the standard model of particle physics, the proton is a hadron, composed of quarks. Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle. A proton is composed of two up quarks and one down quark, and is about 1.6–1.7 fm in diameter.[2]
The free proton is stable and is found naturally in a number of situations. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay, and also result from the decay of free neutrons, which are unstable. In all such cases, protons must lose sufficient velocity and (kinetic energy) to allow them to become associated with electrons, since this is a relatively low-energy interaction. However, in such an association, the character of the bound proton is not changed, and it remains a proton.
The attraction of low-energy protons to electrons, either free electrons or electrons as present in normal matter, causes such protons to soon form chemical bonds with atoms. This happens at sufficiently "cold" temperatures (comparable to temperatures at the surface of the Sun). In interaction with normal (non plasma) matter, low-velocity free protons are attracted to electrons in any atom or molecule with which they come in contact, causing them to combine. In vacuum, a sufficiently slow proton may pick up a free electron, becoming a neutral hydrogen atom, which then will then react chemically with other atoms if they are available and sufficiently cold.
Description
Protons are spin-½ fermions and are composed of three quarks,[3] making them baryons (a sub-type of hadrons). The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.[2] The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.[4]
Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol "H") 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 atoms are composed of two or more protons and various numbers of neutrons.
Stability
The spontaneous decay of free protons has never been observed, and the proton is therefore considered a stable particle. However, some grand unified theories of particle physics predict that proton decay should take place with lifetimes of the order of × 1036 yr, and experimental searches have established lower bounds on the mean lifetime of the proton for various assumed decay products.
Experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of 6.6 × 1033 yr for decay to an antimuon and a neutral pion, and 8.2 × 1033 yr for decay to a positron and a neutral pion.[5] Another experiment at the Sudbury Neutrino Observatory in Canada searched for gamma rays resulting from residual nuclei resulting from the decay of a proton from oxygen-16. This experiment was designed to detect decay to any product, and established a lower limit to the proton lifetime of 2.1 × 1029 yr.[6]
However, protons are known to transform into neutrons through the process of electron capture (also called inverse beta decay). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is:
The process is reversible; neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way, with a mean lifetime of about 15 minutes.