Positron annihilation spectroscopy

A Feynman diagram

Positron Annihilation Spectroscopy (PAS) is a non-destructive spectroscopy technique used in materials science for studying the microstructural features of materials. PAS exploits the phenomenon of positron annihilation, which occurs when a positron (the antiparticle of the electron) interacts with an electron, resulting in the annihilation of both particles and the production of two or more gamma rays. This process is sensitive to the electronic environment in the material, making PAS a powerful tool for investigating defects, porosity, and chemical composition at the atomic and sub-atomic levels.

Principles of Positron Annihilation

When a positron is injected into a material, it will thermalize and eventually annihilate with an electron, emitting two 511 keV gamma rays in opposite directions. The lifetime of the positron before annihilation and the energy and angular distribution of the emitted gamma rays depend on the local electronic environment. By measuring these parameters, information about the material's microstructure can be obtained.

Lifetime Spectroscopy

In Positron Annihilation Lifetime Spectroscopy (PALS), the focus is on measuring the time between the injection of the positron into the material and its annihilation. The lifetime of the positron in the material is sensitive to the presence of defects, such as vacancies and dislocations, because these defects can trap positrons, thereby extending their lifetimes.

Doppler Broadening

Doppler Broadening Spectroscopy is another technique used in PAS, which measures the broadening of the 511 keV gamma ray line. This broadening occurs due to the Doppler effect, as the annihilation process involves electrons that are in motion relative to the positron. The extent of the broadening can provide information about the momentum distribution of the electrons, which is related to the type and concentration of defects in the material.

Applications

PAS has been applied in various fields of materials science, including the study of:

- Semiconductors, for defect identification and characterization. - Metals and alloys, to study vacancy formation and migration. - Polymers and biomaterials, for porosity measurement and characterization. - Nanostructured materials, to investigate surface and interface effects.

Advantages and Limitations

The main advantage of PAS is its sensitivity to defects and changes in the electronic environment at the atomic level, which are often not detectable by other techniques. It is also non-destructive and can be applied to a wide range of materials. However, the technique requires access to positron sources, which can be a limitation. Additionally, the interpretation of PAS data can be complex and requires a good understanding of the material's microstructure and the underlying physics of positron annihilation.

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