Rays. Positrons

The ability to measure an added radioactivity in food depends on the type of radiation (jfl-ray, positron, 7-ray), the energy of the radiation, the concentration of the radioactivity, the chemical characteristics of the radioactive element, and the chemical characteristics of the food. In other words, the measurement capability depends on the detector sensitivity for the specific radionuclides involved and on the background radioactivity in the chemically separated sample to be counted. 

Figure 16.23. Two examples of neutralino models that provide a good fit to the excess of cosmic ray positrons observed by the HEAT collaboration. The two sets of data points (open and filled squares) are derived from two different instruments flown in 1994-95 and 2000. The lines represent (i) the best expectation we have from models of cosmic ray propagation in the galaxy ( bkg. only fit ), which underestimate the data points above 7 GeV (ii) the effect of adding positrons from neutralino annihilations (lines SUSY component , SUSY+bkg. fit , and bkg. component , the latter being the resulting background component when the data are fitted to the sum of background and neutralino contributions). (Figures from Baltz, Edsjo, Freese, Gondolo(2002)).

The net result of this cycle is the formation of helium from hydrogen, with gamma rays, positrons, and neutrinos as byproducts. In addition, even heavier elements are formed ... 

Positrons are generated by the decay of certain unstable isotopes, one of the most commonly used being f Na which decays to, p Aewith the simultaneous emission of e and y-ray. The emitted positrons subsequently thermalize and annihilate with electrons of the material producing y-rays. Positrons can either annihilate directly with electrons or may capture an electron forming a meta-stable intermediate e e pair called positroniiim. 

Other emissions of interest are P-rays, positrons, and y-rays. /3-particles have been shown to be electrons, and in nuclear reactions are given the symbol e. The superscript denotes the infinitesimal mass of an electron relative to a nucleon, and the subscript refers to the electron s charge. 

This is the last chapter in Part I of the general chemistry review. In this chapter, we will discuss the different aspects of radioactivity. Radioactivity is a nuclear phenomenon. It results from natural nuclear instability or externally induced nuclear instability. We will limit our discussion of nuclear chemistry to the basic aspects of radioactivity involving radioactive emissions such as alpha emission, beta emission, gamma rays, positron emission, and electron capture. We will also review other ideas such as the half-lives of radioactive substances and the mass-energy equation. 

Cosmio-ray positrons Are there primary sources Astrophys. J. 11, 429-435 The AMS Collaboration. 

Natural titanium is reported to become very radioactive after bombardment with deuterons. The emitted radiations are mostly positrons and hard gamma rays. The metal is dimorphic. The hexagonal alpha form changes to the cubic beta form very slowly at about 88O0C. The metal combines with oxygen at red heat, and with chlorine at 550oC. 

In addition to Compton scattering, y-rays having energies above 1022 keV interact with matter by a process called pair production, in which the photon is converted into a positron and an electron. The y-ray energy in excess of the 1022 keV needed to create the pair is shared between the two new particles as kinetic energy. Each j3 -particle is then slowed down and annihilated by an electron producing two 511-keV photons. 

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

The emission of y rays follows, in the majority of cases, what is known as P decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which prompdy relaxes by giving offy rays. This is illustrated in Figure 2. 

Similar to beta decay is positron emission, where tlie parent emits a positively cliargcd electron. Positron emission is commonly called betapositive decay. Tliis decay scheme occurs when tlie neutron to proton ratio is too low and alpha emission is not energetically possible. Tlie positively charged electron, or positron, will travel at higli speeds until it interacts with an electron. Upon contact, each of tlie particles will disappear and two gamma rays will... 

Earth and the sun, and, as far as is kno wn, the stars and planets in the rest of the visible universe, are made of ordinai y matter. However, according to a theoi y fir.st proposed by Paul Dirac in 1928, for every kind of particle of ordinary matter that exists in nature, there can exist an antiparticle made of antimatter. Some antiparticles have been discovered for example, the antiparticle of the electron, called the positron, was discovered in 1932 in cosmic rays falling on earth and have also been created in experiments performed in the laboratory. Antimatter is very simi-... 

Rays of the highest energy can interact in a third way with matter, namely by pair production. In this process, which begins at about 106 ev and becomes dominant as the energy increases, the 7-ray disappears in the field of a nucleus or of an electron, and there is produced an electron-positron pair. Owing to the energy requirement, pair production is impossible with x-rays commonly used for analytical purposes. 

Electron penetration, of aluminum, 176 of x-ray target, 8, 9 Electron-positron pair production, 290 Element determinations,. bibliography, 328-331... 

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)... 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

The PET technique relies on radioactive unstable atoms that disintegrate spontaneously, giving off particles called positrons. As soon as an atom emits a positron, the positron combines with an electron. Both particles are annihilated, producing a brief flash of gamma-ray radiation that is easily detected by radiation monitors. 

Positrons cannot be observed directly because, as Figure 22-6a illustrates, when a positron encounters an electron, the two particles annihilate each other, converting their entire mass into a pair of photons. The occurrence of positron emission can be inferred from the observation of such a pair of photons. Each photon produced in this process has a specific energy Epi ton = 9.87 X lO kJ/mol. Photons with such high energy are called y rays. 

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