Sources of positrons

These four PET scans show how blood flow to different parts of the brain is affected by various activities. In this case, an oxygen isotope that is taken up by the hemoglobin in blood is used as a source of positrons. 

This decay provides a practical, usable source of positrons for experimental purposes. 

If sufficient positrons can be confined, studies of particle transport within the plasma, etc., similar to those conducted with electrons can be carried out. It may be possible to use the enhanced detection possibilities afforded since positron-electron annihilations can be detected. An ultra-cold source of positrons would also have a variety of other applications.24 For example, it has been proposed to eject trapped positrons into a plasma as a diagnostic.25 Also, positrons initially in thermal equilibrium at 4.2K within a trap would form a pulsed positron beam of high brightness when accelerated out of the trap. 

A key pioneer in the development of the Anger camera was the late William G. Myers of Ohio State University. As a graduate student at Ohio State, Bill s PhD. thesis was on the potential role of the cyclotron in biomedicine as the source of positron-emitting radiotracers. Bill had participated in the atomic bomb testing at Bikini prior to becoming an internist and nuclear physician. He traveled every summer to the Donner Laboratory to direct a course in the use of radioactive tracers in biomedicine. 

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. 

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. 

An alternative and the most generally employed source of X rays for EXAFS experiments is that obtained from synchrotron sources based on electron (or positron) storage rings. 

Cyclotrons and accelerators are sources of charged particles (i.e., protons, deuterons, a particles, etc.), and the radionuclides produced are generally proton rich and decay by positron emission and/or electron capture. A positive ion beam is eventually extracted from the cyclotron, regardless of whether positive or negative ions were accelerated. The isotope of interest is separated from the target for use in chemical syntheses. Accelerator- or cyclotron-produced radioisotopes tend to be the most expensive as only one radionuclide is produced at a time. 

Brain imaging (preferably MRI) to look for evidence of structural damage is essential after TBI. MRI, while more expensive and time-consuming, can sometimes detect small brain lesions that are missed by CT, especially in the frontal and temporal lobes that are common sources of psychiatric complications after TBI. In addition, an electroencephalogram (EEG) can detect seizure activity or other signs of abnormal brain function. Although they are not yet part of the routine post-TBI evaluation, the so-called functional brain imaging techniques such as positron emis-... 

The nuclear reaction that finally stabilizes the structure of the protostar is the fusion of two protons to form a deuterium atom, a positron, and a neutrino (1 H(p,p+v)2D). This reaction becomes important at a temperature of a few million degrees. The newly produced deuterium then bums to 3He, which in turn bums to 4He in the proton-proton chain. The proton-proton chain is the main source of nuclear energy in the Sun. With the initiation of hydrogen burning... 

Our production parameters for this generator are presented. The Xe-122/l-122 combination, a convenient source of a short-lived (3.6m) positron emitting iodine, is also discussed. Recent developments in rapid iodination procedures will broaden the potential applications of this generator. Finally, preliminary investigations of another generator derived radionuclide that may have promise is described. Tellurium-118 (6d) is the parent of the 3.5 minute positron emitter Sb-118 which may be useful for first pass angiography. 

Fredenc and Irene Joliot-Cune found in 1933 that boron, magnesium, or aluminum, when bombarded with a-particles from polonium, emit neutrons, proton, and positrons, and that when the source of bombarding particles was removed, the emission of protons and neutrons ceased, but that of positrons continued. The targets remained radioactive, and the emission of radiation fell off exponentially just as it would for a naturally occurring radioclcmcnl. The results of this work may be stated in two equations as follows ... 

The source of electromagnetic field is explicitly identified as a positron-electron pair. 

The sources of ionizing radiation were electrons and positrons with energy from 0.1 to 2.7 MeV (Ref. 142) electrons with energy from 0.1 to 20 keV (Ref. 137) and electrons with energy from 0.6 to 12 keV (Ref. 138). 

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