The Positron Annihilation Process

By measuring the annihilation rate 2, the inverse of which is the mean lifetime r (which means the survival time of the positron in the medium) it is possible to obtain directly the electron density that is encountered by the positron in the medium. Thus, a positron can serve as a test particle for the electron density of the medium. However, because of the opposite charges, a strong coulombic attraction exists between the positron and electrons of the medium, and as a result of this the electron density He is slightly enhanced from the equilibrium value in the matter medium. The measurement of positron lifetime in a given medium constitutes what is known as positron annihilation lifetime spectroscopy (PALS). 

The motion of the electron-positron pair before annihilation also causes a Doppler shift in the energy of the annihilation photons measured in the laboratory system. The frequency shift is Av/v = Vi/c, where Vi is velocity of the pair and c is the velocity of light. The difference in energies of the annihilation photons can be vwitten as  

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These models have been quite useful as a means of explaining some of the phenomena associated with the rate of positron annihilation. Other experiments, however, seemed to indicate that the "free volume" model includes far too few properties apart from the factor of density as to satisfactorily explain variations in the positron lifetimes which occur as a result of phase transitions. It would appear that in this case an important part in the positron annihilation process is played by the nature of the intermolecular Interaction and by the internal order of the structures of the molecular substance. 

Recent progress in the study of positron scattering and positron annihilation processes is reviewed in Refs. [11,170-172]. Experimentally, the positron sources from radioisotopes and from electron accelerators are quite weak and have a broad spectrum of energies as compared with electron sources, making precise measurements of positron collision processes extremely difficult. Such measurements, however, have become possible recently due to the... 

The predominant annihilation process for thermalised positrons is via the direct production of two photons (Fig. 1). If both the positron and... 

In the course of the positronium lifetime measurements, radioactive isotopes of positron decay serve as sources, e.g., sodium-22, copper-64. At the moment of the emission of the positron from this source a y-photon is also released. (In the case of, e.g., sodium-22 its energy is 1.28 MeV.) This y-photon serves as the start signal in the coincidence equipment used. The y-photon produced by the 2y-annihilation process to be studied (0.51 MeV) is the stop signal. The magnitude of the time measured between the start and stop signals (the positronium lifetime) is in the range 10 —10 s. To get a lifetime curve of adequate statistics, the apparatus repeats the time measurement about 10 — 10 times. For the details of the experimental technique see, e.g., refs. [De 53, Fe 56, Go 71a]. 

The positron has a short life and will quickly be annihilated in a reaction with an electron, producing y-photons of characteristic energy (0.51 MeV). In addition, the basic nuclear process itself is usually accompanied by the emission of y-radiation. As in the case of negatron decay a complete energy... 

Improvements in current, established technologies and the introduction of new ways to test materials, nondestmctively are expected to continue apac. One promising method is positron annihilation. The positron is the antiparticle of the electron thus apositron/electron pair is unstable and will annihilate. In this process, two gamma rays at approximately 180 to one another are emitted from the center of the mass of the pair. A very slight departure from 180° is directly proportional to the transverse component of the momentum, of the pair. The momenta of the electrons involved in such collisions can be calculated from the geometry and intensity of the gamma rays. The dynamics of the clcctron/positron system underlie the use of the technique for the study of defects in materials,... 

Akuezue, H.C. and S.K. Verma Positron Annihilation NDE at the Atomic Level, Advanced Materials Processes. 26 (Match 1992),... 

Pair production has a threshold energy of 1.022 MeV because two particles are created, one electron and one positron. Thus, some energy is stored in or used to create the mass of the pair. Notice the total electric charge is conserved because the electron charge is — le and the positron charge is +le. One of the unique features of this process is that the energy that went into the creation of the two particles will be released when the positron comes to rest and annihilates with an electron. The annihilation process is... 

The positron lifetime spectra of polyethylene and glass-filled polyethylene were resolved in four exponentials, representing different annihilation processes. The... 

The positron was subsequently discovered by Anderson (1933) in a cloud chamber study of cosmic radiation, and this was soon confirmed by Blackett and Occhialini (1933), who also observed the phenomenon of pair production. There followed some activity devoted to understanding the various annihilation modes available to a positron in the presence of electrons radiationless, single-gamma-ray and the dominant two-gamma-ray processes were considered (see section 1.2). The theory of pair production was also developed at this time (see e.g. Heitler, 1954). 

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