Positron process

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. 

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. 

Nuclei that have too many protons relative to their number of neutrons correct this situation in either of two ways. They either capture one of their Is electrons or they emit a positron (a positively charged particle with the same mass as an electron). Either process effectively changes a proton to a neutron within the nucleus. 

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. 

Positron emission tomography (PET) is an imaging technique that relies on the emission of positrons from radionucleotides tagged to an injectable compound of interest. Each positron emitted by the radioisotope collides with an electron to emit two photons at 180° from each other. The photons are detected and the data processed so that the source of the photons can be identified and an image generated showing the anatomical localization of the compound of interest. 

A PET scan requires a substance called a tracer. A suitable tracer must accumulate in the target organ, and it must be modified to contain unstable radioactive atoms that emit positrons. Glucose is used for brain imaging, because the brain processes glucose as the fuel for mental and neural activities. A common tracer for PET brain scans is glucose modified to contain radioactive fluorine atoms. Our molecular inset shows a simplified model of this modified glucose molecule. 

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. 

Nuclides that are unstable because they have odd-odd composition can be converted into stable nuclides with even -even composition through any of three processes electron emission, positron emission, or electron capture. Each... 

C22-0047. Identify the product of each of the following decay processes (a) Fe emits a positron and a y 55 59... 

PALS is based on the injection of positrons into investigated sample and measurement of their lifetimes before annihilation with the electrons in the sample. After entering the sample, positron thermalizes in very short time, approx. 10"12 s, and in process of diffusion it can either directly annihilate with an electron in the sample or form positronium (para-positronium, p-Ps or orto-positronium, o-Ps, with vacuum lifetimes of 125 ps and 142 ns, respectively) if available space permits. In the porous materials, such as zeolites or their gel precursors, ort/zo-positronium can be localized in the pore and have interactions with the electrons on the pore surface leading to annihilation in two gamma rays in pick-off process, with the lifetime which depends on the pore size. In the simple quantum mechanical model of spherical holes, developed by Tao and Eldrup [18,19], these pick-off lifetimes, up to approx. 10 ns, can be connected with the hole size by the relation ... 

Beta (+) or positron emission ( J>+) This type of decay occurs when a nucleus has a greater number of protons than neutrons. In this process, a proton is converted into a neutron by emitting a positive particle known as a (11 particle or positron. The positron is a particle having the mass of an electron but carrying a positive charge. It is sometimes called the antielectron and shown as e+. The reaction can be shown as... 

Electron capture accomplishes the same end result as positron emission, but because the nuclear charge is low, positron emission is the expected decay mode in this case. Generally, electron capture is not a competing process unless Z 30 or so. 

The first reaction is a fusion of two protons to produce a 2H nucleus, a positron (e+) and a neutrino (ve). The second reaction is a proton capture with the formation of 3He and a y-ray. In the third reaction two 3He nuclei fuse to give 4He and two protons. The total energy released in one cycle is 26.8 MeV or 4.30 x 10-12 J. An important product of this process is the neutrino and it should provide a neutrino flux from the Sun that is measurable at the surface of the Earth. However, the measured flux is not as big as calculated for the Sun - the so-called neutrino deficit... 

A low n p ratio in a nucleus gives a situation which may be stabilized by the conversion of a proton into a neutron. One process which may effect this is positron emission,... 

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... 

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