Positron annihilation transformations

Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

Numerous applications of positron annihilation spectroscopy (PAS) in investigations of the physico-chemical properties of matter require a precise understanding of the process of Ps formation. Usually it proceeds on a picosecond time scale and is strongly influenced by early (pico- and femtosecond) processes initiated by ionizations in the track of a fast positron. These early intratrack processes initiate all subsequent chemical transformations and, consequently, take a key position in radiation chemistry. 

Many spectroscopic methods, such as atomic absorption, Auger, electron spin resonance, Fourier transform infrared, laser photopyroelectrie effect, mass, Mossbauer, near infrared, nuclear magnetic resonance, positron annihilation lifetime, Raman, time-of-flight secondary mass, x-ray, x-ray photoelectron, and UV were used in various studies of PVC. The relevance and usefulness of these methods is discussed in detail elsewhere. ... 

Two series of cellulose samples, Avicel and Whatman CFll cellulose ball-milled powders with different crystallinity are studied below Tg temperature by using positron annihilation lifetime spectroscopy. A good correlation is found between ortho-positronium formation probability and crystallinity as measured by Fourier transform -infrared spectroscopy. Sub-nanometer hole distributions are found to be narrowed as a function of milling time. These are interpreted in terms of microstructural changes of cellulose. 

It should be noted that there are some experimental measurements (such as positron annihilation) which are apparently inconsistent with these valences. The difficulty arises from the fact that, although this valency scheme is useful and is consistent with many experimental facts, it is an over simplified description of the electronic nature of cerium and its alio tropes. This problem is dealt with in the discussions concerning the various models proposed to explain the a y transformation and the electronic nature of a, a and y phases (sections 5.1-5.7). 

The positron is the antiparticle of the electron. The positron has the same mass as the electron but it carries a positive charge. In the vacuum, positrons are stable elementary particles. They are generated near an atomic nucleus by y-quanta of more than 1.022 MeV energy (pair production, each y-quant produces an electron/positron pair). Nuclear transformations also yield positrons. A source of positrons is the artificial radioactive isotope Na which has a half-life of 2.6 years. Together with the positron a y-quant of 1.28 MeV is emitted. When positrons are injected into matter they react with electrons and the energy is emitted as y-quants. A typical setup for positron annihilation studies is shown in Figure 22. 

The fact that neutrinos are emitted during the transformation provides an opportunity for direct observation of the reactions taking place at the heart of the Sun. Note that antimatter is produced in this strange reaction, in the form of the positton or antielectron e+. The positrons generated immediately annihilate with electrons in the surrounding medium with subsequent emission of gamma rays. 

The sun s energy is believed to come from a series of nuclear reactions, the overall result of which is the transformation of four hydrogen atoms into one helium atom. How much energy is released in the formation of one helium atom (Hint Include the annihilation energy of the two positrons formed in the nuclear reaction with two electrons.)... 

Positron decay occurs in proton-rich nuclei. In this case, the positron (or p+ particle) is originated by conversion of a proton into a neutron, along with the emission of a neutrino to conserve the energy. Positrons are the antiparticle of electrons. In a very fast process (10 12s), emitted positrons collide with an electron of a nearby atom and both particles disappear in a process called annihilation. The necessary conservation of mass and energy accounts for the transformation of the mass of both particles into energy, which is characteristically emitted in the form of two 511-keV photons almost in opposite directions. Consequently, positron emitters are used to label radiopharmaceuticals produced with diagnostic purposes by imaging. 

Positron has the same mass and spin as the electron, but has the opposite charge. Furthermore, if a positron is surrounded by one or more electrons the positron may annihilate with one of the electrons, i.e. both particles disappear and their masses are transformed into energy which is emitted as y quanta [1-3]. 

The lifetimes given above relate to positronium in vacuum. In the medium a new possibility of destruction appears the positron bound in Ps can annihilate with one of strange electrons having appropriate (opposite) spin orientation. The process is called pick-off and leads to two quantum annihilation. If the medium is paramagnetic, another process shortening the o-Ps lifetime is possible the interaction with magnetic moments can transform o-Ps into p-Ps, which decays almost immediately (conversion process). Both e and Ps can participate in chemical reactions with molecules of the medium changing the Ps formation probability... 

When an electron and a positron, its anti particle, meet, they are annihilated and are completely transformed into energy, mostly y radiation. 

Since the positron is the antiparticle of the electron an encounter between them can lead to the subsequent annihilation of both particles. Their combined rest mass energy then appears as electromagnetic radiation. Annihilation can occur via several mechanisms direct transformation into one, two, or three photons or the formation of an intermediate, hydrogen-like bound state between the positron and the electron, called a positronium... 

With respect to the old vacuum, the new vacuum is called a dressed vacuum and the states in it are called dressed states. The new particles are called quasiparticles because they are a composite of an old particle and a series of old pairs they are dressed with a series of pairs rather than being bare particles. The new vacuum is also called a polarized vacuum because with respect to the old vacuum there is a charge polarization expressed by the presence of undressed pairs. The concept of vacuum polarization will be discussed more later. Finally, another way of looking at the change in the vacuum is that because the transformation mixes positron creation and electron annihilation operators, a normal-ordered operator in the new basis will definitely not be normal-ordered in the old basis. 

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