Positron annihilation spectroscopy
Encyclopedia
Positron annihilation spectroscopy (PAS) or sometimes specifically referred to as Positron annihilation lifetime spectroscopy (PALS) is a non-destructive spectroscopy
technique to study voids and defects in solids.
The technique relies on the fact that a positron that comes in the immediate vicinity of an electron will cease to exist by annihilation
. In the annihilation process of a positron
and an electron, gamma photons are set free that can be detected. If positrons are injected into a solid body, their lifetime will strongly depend on whether they end up in a region with high electron density or in a void where electrons are scarce or absent. In the latter case, the lifetime can be much longer because the probability to run into an electron is much lower.
By comparing the fraction of positrons that have a longer lifetime to those that annihilate quickly, insight can be gained into the voids or the defects of the structure. In the case of materials with a lattice structure like semiconductor
s, this can be a dislocation or some other lattice defect. In the case of amorphous polymer
s, this might be the free volume between the chains of the polymer.
The technique requires a source of positrons, and a radioactive isotope of sodium is often used for this purpose.
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
technique to study voids and defects in solids.
The technique relies on the fact that a positron that comes in the immediate vicinity of an electron will cease to exist by annihilation
Electron-positron annihilation
Electron–positron annihilation occurs when an electron and a positron collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons or, at higher energies, other particles:...
. In the annihilation process of a positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
and an electron, gamma photons are set free that can be detected. If positrons are injected into a solid body, their lifetime will strongly depend on whether they end up in a region with high electron density or in a void where electrons are scarce or absent. In the latter case, the lifetime can be much longer because the probability to run into an electron is much lower.
By comparing the fraction of positrons that have a longer lifetime to those that annihilate quickly, insight can be gained into the voids or the defects of the structure. In the case of materials with a lattice structure like semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s, this can be a dislocation or some other lattice defect. In the case of amorphous polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s, this might be the free volume between the chains of the polymer.
The technique requires a source of positrons, and a radioactive isotope of sodium is often used for this purpose.