Positron source thermalization

A very good review paper pertaining to tissue imaging was written by Stephen Joffe [171]. He cites 34 published articles on topics such as oxygen measurements, but the strength of the article is in the detailed comparison of NIR with other methods like X-ray, MRI, ultrasound, positron emission, thermal emission, electrical impedance, and more. He discusses the various types of equipment used, detectors, and light sources. He also gives one of the best short descriptions of time-resolved spectroscopy seen in any review article. 

Fig. 6.10. Mean positron annihilation rate (denoted here as Af) at various gas densities for N2 and Ar gases at different temperatures. Key A, N2 at 130 K , Ar at 160 K A, N2 at 297 K , Ar at 297 K. The original sources for these measurements are given by Heyland et al. (1986). The broken line indicates the linear rise expected, equation (6.3), for a constant (Zeff) of 27. Reprinted from Physics Letters A119, Heyland et al., On the annihilation rate of thermalized free positrons in gases, 289-292, copyright 1986, with permission from Elsevier Science.

Mills Jr., A.P. and Pfeiffer, L. (1979). Desorption of surface positrons a source of free positronium at thermal velocities. Phys. Rev. Lett. 43 1961-1964. 

A hot fluid model would be highly desirable for applications in astrophysics. As we have already mentioned, the formation of RES in the primordial plasma could be an important source of large-scale nonuniformities in density and temperature, which seeded the formation of galaxies and clusters of galaxies [4], In particular, it is conjectured that in the early universe matter was present in the form of a mixture of electrons, positrons and photons in thermal equilibrium at a temperature above me2. It is evident that the propagation of relativistic EM waves in such peculiar environment should be addressed in the framework of a hot-plasma model. 

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. 

Positrons emitted for a radioactive source (such as 22Na) into a polymeric matrix become thermalized and may annihilate with electrons or form positronium (Ps) (a bound state of an electron and positron). The detailed mechanism and models for the formation of positronium in molecular media can be found in Chapters 4 and 5 of this book. The para-positronium (p-Ps), where the positron and electron have opposite spin, decays quickly via self-annihilation. The long-lived ortho positronium (o-Ps), where the positron and electron have parallel spin, undergo so called pick-off annihilation during collisions with molecules. The o-Ps formed in the matrix is localized in the free volume holes within the polymer. Evidence for the localization of o-Ps in the free volume holes has been found from temperature, pressure, and crystallinity-dependent properties [12-14]. In a vacuum o-Ps has a lifetime of 142.1 ns. In the polymer matrix this lifetime is reduced to between 2 - 4 ns by the so-called pick-off annihilation with electrons from the surrounding molecule. The observed lifetime of the o-Ps (zj) depends on the reciprocal of the integral of the positron (p+(rj) and electron (p.(r)) densities at the region where the annihilation takes place ... 

Activation method — a target is placed at or near the reactor core, or the activated source is moved in and out of the reactor. Positrons can be created by activation of a source material (e g., copper) by thermal neutrons. The positrons can then be extracted after their emission or by moving the source material out of the activation zone to the experimental set up (as a solid source type or loop type). 

To convert the primary activity to a beam of slow positrons, a moderator is placed close to the radioactive source. In most cases, this moderator is made of a metal with a negative positron work function, like W or Ni. In a metal, a particle slows down to thermal energy mthin a few psec and may then diffuse a large distance ( 1000 A in defect-firee metals) before it aimihilates. If the positron reaches the moderator surface, there is a good chance that it will be emitted into the vacuum. Such positrons come off preferentially normal to the surface, with an energy equal to added to their thermal energy in the moderator. The energy spread AE is therefore typically 0.1eV. 

Various source-moderator geometries have been employed. Figure 3.2 shows two basic forms. In the reflection geometry, a bulk moderator is used, and the positrons enter and exit the same surface. In the transmission geometry, a thin moderator foil is applied, and those positrons that are thermalized and transmitted are extracted to create a beam. A figure of merit, which is often quoted in connection with a source-moderator combination, is the efficiency e. It is defined as the number of extracted, slow... 

Moderators do not have to be made of metals. Recently, for example. Mills and Gullikson [3.6] reported a record efficiency of 0.7% obtained with solid Ne combined with a cup -shaped source-moderator geometry. Also, it has been suggested that the diffusion to the surface of thermalized positrons may be enhanced if an electric field is created in a semiconductor moderator (Lynn and McKee [3.7]). A theoretical study (Beling et al. [3.8]) indicates that such field-assisted moderators should make possible efficiencies as large as 10%. 

When a positron is emitted from a source and then penetrates into a solid, it quickly loses kinetic energy until it reaches the thermal level (Figure 4.26). This thermalised positron moves around in the solid by diffusion and finally annihilates with an electron. 

When a positron is emitted from a source, and penetrates into a solid, it quickly loses its kinetic energy to thermal energy. The thermalised positron moves around in the solid by diffusion and finally annihilates with one of the electrons in its surroundings. All of the energy from the electron-positron annihilation is converted into two annihilation y-rays, which can be detected. The annihilation rate of a positron is determined by the local electron density in the locale of the positron. Thus, positrons... 

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