Annihilation

In elementary particle physics is meant by annihilation ( Latin: annihilatio " the nullification " ) the process of pair annihilation (also: Paarzerstrahlung ), in which an elementary particle and its antiparticle are transformed together into other particles.

Initially was only the positron-electron conversion into photons, ie particles with no rest mass, known; This explains the term "extermination " or " annihilation ". With the high-energy experiments ( see below), the term expanded and now includes also cases in which, although the original particle-antiparticle pair disappears, but new pairs with rest mass, such as muons or K- mesons arise.

In any case, the energy of the original particle pair ( resting energy and kinetic energy ) is not " cancel ", but occurs in any other form again.

The annihilation of the opposite process is pair production, the formation of a particle-antiparticle pair from other power than that of a pair annihilation, eg the conversion of a photon in a heavy nucleus field into an electron and a positron.

High-energy physics research

In experiments at collider facilities allowed for research purposes - namely to produce other particles - electrons and positrons of the same and very high kinetic energy, but opposite direction of flight collide. The same is in principle also possible, for example, protons and antiprotons. Because of the favorable kinematics of such " Colliding -Beam " experiments is standing next to the rest energy and the total kinetic energy ( energy of motion ) of the two particles of conversions available.

Positron-electron annihilation in matter

Positron lower energy occur as beta radiation and as a decay product of positive muons the secondary ( ie occurring in the Earth's atmosphere ) cosmic rays. Such a positron is first slowed down by collisions with the entry into matter and may then form with existing electron there a positronium " atom". Located in parapositronium this state, so its annihilation occurs with a half -life of the order of 1 nanosecond, and yields two photons are emitted in opposite directions. However, the annihilation is also " directly " possible without the formation of a bound Positroniumzustands.

Are momentum and kinetic energy of the Positroniumatoms negligibly small, the angle between the directions of emission of the two photons exactly 180 °, and the energy of each photon is 511 keV, the rest energy of the electron or positron. However, if the system has a pulse from destruction, will be transferred to the photon, so that they are not emitted at an angle of 180 °. The difference between the actual angle to 180 ° is the angle with

The transverse component of the momentum of positronium before annihilation, me the electron mass and c is the speed of light. Since the Positroniumatom in this case also has kinetic energy, the Doppler effect occurs, so that the two photon energies with respect to 511 keV are slightly shifted. , In practice, this line 511 keV when they are observed in a gamma spectrometer always significantly widened as compared to other spectral lines.

The orthopositronium does not break in two, but three ( or rarely more ) photons. These have no discrete energies, but rather a continuous energy spectrum.

Applications of positron annihilation radiation

In solid-state physics, the annihilation radiation of 511 keV is used to determine the lifetime of positrons in solids. The life is dependent on the local electron density, and thus characteristic of certain crystal defects and therefore to the identification (eg, a space) is used. The measurement of the Doppler broadening (see above) allows identification of crystal defects and also an analysis of their chemical environment or composition.

Medical annihilation radiation is used in the imaging techniques positron emission tomography.