Electron-Positron Annihilation Radiation

A gamma-ray line at 0.511 MeV results from the mutual annihilation of an electron and a positron, a particle-antiparticle pair. A number of radioactive decay chains (see Table I) result in the emission of a positron as a decay product, which will annihilate upon first encounter with an electron. Also of astrophysical importance is the production of electrons and positrons via the photon-photon pair-creation process. Such pair plasmas are found in the vicinity of compact objects, such as neutron stars and black holes, that are associated with heated accretion disks and relativistic flows and jets, within which particle acceleration is known to occur. Thus, relatively narrow lines of 0.511-MeV annihilation radiation are expected to arise in the interstellar medium through the decay of dispersed, nucleosynthetic radionuclides, while broadened, Doppler-shifted, and possibly time-variable lines may occur in the high-energy and dense environments associated with compact objects. 

Direct annihilation of an electron-positron pair leads to the emission of two photons. If the particles are at 

Prior to the launch of the CGRO, from the 1970s on, a number of balloon and early satelHte instruments detected the presence of apparently variable annihilation radiation from the direction of the Galactic Center. This led 

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In the case of two-photon annihilation, the energy conservation law has one more serious consequence on photons. Both should have an energy of 0.511 MeV. This energy is so uniquely characteristic to electron-positron annihilation that Anderson (1932) proved the existence of positrons by the detection of this annihilation radiation. Although the above rule is rather strict, it is altered, again, a little by the momentum of the electrons. 

In the case y-ray lines in the region 0.1 < E < 1 MeV a similar situation arose. A single first-principles measurement using positron annihilation radiation (rather broad and illshaped) as a calculable marker normalized the convenient 411 keV line from the Hg daughter of Au [34]. This excellent and difficult measurement used 3-ray spectroscopy to measure the small difference between photo electrons promoted by 511 keV radiation from the K shell of uranium and photo electrons promoted from its... 

Annihilation (Positron-Electron)—An interaction between a positive and a negative electron in which they both disappear their rest mass, being converted into electromagnetic radiation (called annihilation radiation) with two 0.51 MeV gamma photons emitted at an angle of 180° to each other. 

The fifth type of radioactive emission, gamma radiation, does not result in a change in the properties of the atoms. As a result, they are usually omitted from nuclear equations. Gamma emissions often accompany other alpha or beta reactions—any decay that has an excess of energy that is released. For example, when a positron collides with an electron, two gamma rays are emitted, a phenomenon usually referred to as annihilation radiation. 

The first anti-particle discovered was the anti-electron, the so-called positron, in 1933 by Anderson [3] in the cloud chamber due to cosmic radiation. The existence of the anti-electron (positron) was described by Dirac s hole theory in 1930 [4], The result of positron—electron annihilation was detected in the form of electromagnetic radiation [5]. The number and event of radiation photons is governed by the electrodynamics [6, 7]. The most common annihilation is via two- and three-photon annihilation, which do not require a third body to initiate the process. These are two of the commonly detected types of radiation from positron annihilation in condensed matter. The cross section of three-photon annihilation is much smaller than that of two-photon annihilation, by a factor on the order of the fine structure constant, a [8], The annihilation cross section for two and three photons is greater for the lower energy of the positron—electron pair it varies with the reciprocal of their relative velocity (v). In condensed matter, the positron—electron pair lives for only the order of a few tenths to a few nanoseconds against the annihilation process. 

Measurements of the Doppler broadening of the annihilation radiation produced by various molecules has been related to annihilation at specific sites within molecules by Iwata, et al. [15]. Prom the observed 7-ray spectra, the line width of the dominate peak, which comes from valence electrons, was extracted. Thus, for each molecule there is a single measured quantity, the fine width. For a series of hydrocarbons, the observed fine widths were found to be linear in the fraction of electrons in C-C (or C-H) bonds. Each type of bond was assumed to contain two electrons. Prom a linear fit of this data, fine widths for the C-C and C-H bonds were extracted and found to be 2.06 and 2.42 keV, respectively. These agree reasonably with an old theoretical estimate in which the positron density was assumed to be constant over the molecule [16]. ... 

It should be noted that the S parameters of both o-Ps pick-off and free-positron annihilation are lower than that of the Si substrate, because positrons predominantly annihilate with electrons of oxygen in the Si02 network. Only p-Ps self-annihilation has a higher S value than that of Si. The S parameter observed in conventional Doppler- broadening-of-annihilation radiation is the average of p-Ps, o-Ps, and free-positron annihilation. Therefore, if the Ps fraction decreases due to the presence of defects, impurities, etc., the intensity of the narrow momentum component due to p-Ps self-annihilation decreases, and as a result the averaged S parameter decreases. 

For DBES data three main factors contribute to the S parameter in polymers (1) free-volume content, (2) free-volume size, and (3) chemical composition. First, larger free-volume content contributes to a larger S value. DBES measures radiation near 511 keV where a major contribution comes from p-Ps. This p-Ps contribution is only 1/3 the o-Ps intensity as that in I3 of PAL data. Second, when p-Ps is localized in a defect with a dimension fix, the momentum Ap has a dispersion according to the Heisenburg uncertainty principle AxAp > h/4n. The S parameter from DBES spectra is a direct measure of the quantity of momentum dispersion. In a larger size hole where Ps is localized, there will be a larger S parameter due to smaller momentum uncertainty. Therefore, in a system with defects or voids, such as polymers, the S parameter is a qualitative measure of the defect size and defect concentration. The value of the S parameter also depends on the momentum of the valence electrons, which annihilate with the positrons. The absolute value of the S parameter therefore, may differ from polymer to polymer. Third, the S parameter depends on the electron momentum of the elements. As the atomic number of the elements increases, the electron momentum increases, and thus the S parameter decreases. Fortunately, in chemicals of... 

As discussed previously, when an expl is irradiated with fast neutrons a N nucleus captures the incident fast neutron and ejects two slow neutrons. The resulting nucleus, N, is excited (radioactive) and decays with a io min half-life to stable C. In this last transition, a positron, 3<-, is emitted. Because of its opposite charge, the J3+ is strongly attracted by a nearby electron in the resulting collision, both the positron and electron are annihilated and in the process of annihilation, the masses of the colliding particles are converted into two 0.511 MeV quanta of electromagnetic radiation. These 7rays are what are detected to indicate the possible presence of an expl... 

In the case of absorption of P radiation, emission of y-ray photons is observed positrons are the antiparticles of electrons (section 3.2). After having given off their energy by the interactions (a) to (c), they react with electrons by annihilation and emission of predominantly two y-ray photons with an energy of 0.51 MeV each in opposite directions (conservation of momentum). The energy of 2 x 0.51 — 1.02 MeV is equivalent to 2me, the sum of the masses of the electron and the positron. This annihilation radiation allows identification and measurement of P radiation. 

Curie and Joliot obtained positron-electron pairs from heavy metals bombarded with high energy (5 MeV) y-rays derived from beryllium mixed with polonium. The average life of the positron is about 10 sec. On colliding with an electron both are annihilated and y-radiation—the annihilation radiation—is emitted. The formation and annihilation of a positron-electron pair is thus represented. 

When an electron and a positron meet, they are replaced by two gamma rays, called the annihilation radiation. Calculate the energies of these radiations, assuming that the kinetic energies of the incoming particles are 0. 

Similar to beta decay is positron emission, where the parent emits a positively charged electron. Positron emission is commonly called beta-positive decay. This decay scheme occurs when the neutron-to-proton ratio is too low and alpha emission is not energetically possible. The positively charged electron, or positron, will travel at high speeds until it interacts with an electron. Upon contact, each of the particles will disappear and two gamma rays will result. When two gamma rays are formed in this manner, it is called annihilation radiation. 

The experimental set-up consists of a positron source ( Na), a scintillation counter, to detect the y radiation from the positronium decay, and electronic peripheral equipment to analyse the time spectrum of the positron annihilation. 

The emitted positrons react within a short range with electrons of the surrounding matter and produces two y-quants, leaving the object in opposite direction with an angle of nearly 180° (annihilation radiation). 

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