Proton (p )

The proton [pro ː tɔn ] (plural protons [ proto nən ː ], from ancient Greek τὸ πρῶτον to Proton, the first ') is a stable, electrically positively charged hadron. In nuclear reactions, it is noted with the symbols p. The proton belongs next to the neutron and the electron to the building blocks that make up the man commonplace familiar matter.


The proton consists of two up quarks and one down quark ( formula uud). These three valence quarks surrounded by a sea of ​​gluons and quark-antiquark pairs. Only about 1% of the mass of the proton comes from the masses of the valence quarks, the rest comes from the motion and binding energy between quarks and gluons, the latter convey as power exchange particles, the strong force between quarks. The diameter of a free proton is about 1.7 × 10-15 m, ie 1.7 femtometer (fm). The proton is a baryon as the neutron.


The proton is the only stable hadron and the lightest baryon. Since decay can always only lead to lighter particles, the proton must be stable because of the Baryonenzahlerhaltung according to the standard model. Experiments at Kamiokande suggest a half-life of at least 1032 years. The Search for the proton decay is for the physics of particular importance, since it represents a possible test for theories beyond the Standard Model.

The magnetic moment is found in the quark model, the nuclear magneton referred to, calculate, which agrees well with the measured values ​​.

Protons may arise from the beta decay of neutrons:

The reverse process occurs, for example in the formation of a neutron star and is theoretically possible, but statistically extremely rare under normal conditions because the three particles would collide with precisely tuned energies simultaneously.

However, a bound in a very proton-rich nucleus Proton be transformed by beta plus decay or electron capture into a neutron.

The antimatter particles ( antiparticles ) to the proton is the antiproton, which was created artificially for the first time in 1955 by Emilio Segre and Owen Chamberlain, what the explorers brought the Nobel Prize for Physics in 1959. It has the same mass as the proton, but has a negative charge.

Protons in atomic nuclei

The nucleus of almost all nuclides consists of protons and neutrons, the nucleons. The exception is the most common isotope of hydrogen ( the hydrogen atom 1H), whose nucleus consists of only one proton. The number of protons in the nucleus, the atomic number of each element determined (via the protons determined by the number of electrons ) whose chemical properties. Atoms with the same number of protons but different numbers of neutrons are called isotopes have nearly identical chemical properties.

The protons in the nucleus contribute to the atomic mass. The strong interaction between protons and neutrons is responsible for the maintenance and stability of the atomic nucleus. While the positively charged protons know each other both attractive ( strong interactions ) and repulsive forces ( electromagnetic interaction ) occurs between neutrons and between neutrons and protons to no electrostatic force.

The di-proton, the fictional helium isotope 2He whose core consisted of only two protons, is not " teilchenstabil " because two protons may be due to the Pauli principle - in contrast to the proton and neutron in the deuteron - only in a singlet state are with antiparallel spins. Due to the strong spin dependence of the nucleon -nucleon interaction of these, however, is raised in energy and therefore not bound. Only with another neutron in the nucleus is obtained the stable 3He.

About the nuclear photoeffect protons by high-energy photons are released from the core, as in other nuclear reactions by impact faster protons, neutrons or alpha particles.

For nuclei with particularly high or particularly low number of neutrons can cause spontaneous Nukleonenemission, ie proton or neutron emission come. This is known as proton or neutron radiation. The half-lives here are always very short. In extreme proton excess (such as the iron isotope 45Fe ), the two-proton decay occur, in which even two protons are emitted at the same time ( please refer to the main article radioactivity).

Proton scattering

Scattering experiments with protons to other nucleons are performed to explore the properties of the nucleon -nucleon interactions. In the scattering of neutrons, the strong interaction is the dominant force, the magnetic interaction is completely negligible. Interspersed one proton to proton, so in addition the Coulomb force must be considered. The nuclear forces also still depend on the spin. A result of the comparison of the pp scattering with the nn scattering is that the nuclear forces are independent of the charge state of the nucleons (the proportion of Coulomb force on the cross section for pp scattering is hereby withdrawn in order to compare only the effect of nuclear forces ). With elastic or quasi-elastic scattering of electrons on protons at its form factor can be determined.

Further reactions of the proton ( Astrophysics )

  • Proton-proton reactions are one of two fusion reactions in the hydrogen burning.
  • In one of a proton rapid proton overcomes the repulsion of the Coulomb force and is a part of the core. This is called P- process at very high temperatures.

Current research

The proton is the fundamental physics research, inter alia, in systems such as the Super Proton Synchrotron (SPS ) and the Large Hadron Collider ( LHC) at CERN, the Tevatron at Fermilab or HERA.

The research with proton -antiproton collisions serves, among other looking for a physics beyond the Standard Model.

Measurements on muonic hydrogen, ie the bound system of muon and proton may also provide evidence of such extensions of the Standard Model.

Technical Applications

Accelerated protons can be used in medicine in the context of proton therapy for the treatment of tumor tissue. This is a gentler compared to conventional X-ray radiation therapy, since the protons release their energy substantially only in a narrow depth range in tissue ( the Bragg peak ) and not already on the way there.

Protons with kinetic energies in the range of about 10 to 50 MeV cyclotron serve, for example, for producing proton- rich radionuclides for medical purposes or for surface activation of machine parts for later wear measurements.

History of Research

Protons were first studied as canal rays. Wilhelm Wien presented here in 1898 found that the canal rays have the largest ratio of charge to mass at hydrogen filling of the canal -ray tube.

The proton as a building block of heavier nuclei (nitrogen) was discovered by Ernest Rutherford in 1919 and identified as the nucleus of the hydrogen atom. It is the smallest, easiest -built atom. Therefore, the particles of the proton name was (Greek: the first one ) where. Perhaps the name should at the same time also remember William Prout, who in 1815 suggested that all other chemical elements are made ​​up of hydrogen.