Thermalized positron

When an electric field was applied across the chamber some positrons annihilated prematurely, following field-induced drift to one of the electrodes. In this case the free-positron component of the lifetime spectrum was field dependent the maximum drift time, rmd, was given by the end-point of the lifetime spectrum and was due to thermalized positrons which had traversed the entire drift length l. The drift speed was then v+ = 1/rmd and the mobility could be found from... 

Tong, B.Y. (1972). Negative work function of thermal positrons in metals. 

Two-Photon Positron Annihilation. With positron annihilation, the two photons carry away the momentum of the electron as the momentum of a thermalized positron is negligible, the momentum distribution of the electron can be determined. Because the positron can be polarized, one can also get the momentum distribution for the two spin states in magnetized materials. 

The measurement of the mean lifetimes of positrons in matter has been one of the cornerstones of positron science over the past half-century. The lifetime of a positron in matter—gas, liquid or solid—will depend on the electronic environment in which it finds itself, and this in turn tells us much about the submicroscopic nature of the material. In condensed matter a positron will approach thermal energies within about lps, so that measured lifetimes are essentially those of a thermal positron in the material under study. In some gaseous environments—particularly in the noble gases—the time taken for a positron to come to thermal equilibrium with its surroundings is much longer—10°-102 ns—and this thermalisation time has to be taken into account in the analysis of time spectra. 

The spur model, proven to be valid in condensed media, proposes that Ps formation would occur through the reaction of a (nearly) thermalized positron with one of the electrons released by ionization of the medium, at the end of the e+ track, in a small region containing a number of reactive labile species (electrons, holes, excited molecules) [1],... 

Thus, one may think that the redistribution of the blob charges induced by the e+ may dominate over the repulsive interaction mentioned above, and could lead to e+ trapping within the blob. Moreover, the estimated value of the energy drop indicates that not only the thermalized positron... 

Considering the diffusion-recombination stage below, we neglect an interaction between the thermalized positron and its blob. This approximation, as we discussed above, assumes that the appearance of a positive potential in the blob, caused by outdiffusion of electrons, is nearly cancelled by the negative potential caused by e+ screening inside the blob. In this case we can apply the prescribed diffusion method to obtain the solution of Eq. (17). Let us write Cj(r,t) in the following form ... 

Mitroy and coworkers [27] have discussed the use of negative ion beams to measure positron affinities of atoms. The idea is to pass a beam of atomic anions through a swarm of thermal positrons in order to make a positron compound by the charge exchange reaction ... 

Positron Annihilation Lifetime Spectroscopy (PALS) provides a measure of free volume holes or voids, free volume, and free volume distribution, at an atomic scale. The technique exploits the fact that the positively charged positron (e" ), the antiparticle to the electron, preferentially samples regions of low positive charge density. When injected in a polymer matrix, thermalized positrons can combine with an electron to form a bound state, known as positronium (Ps). This species can only exist in a void and it rapidly annihilates on contact with the electron cloud of a molecule. For polymer studies using PALS, it is ortho-positronium (oPs, a triplet state) which is of interest. The oPs spin exchanges with electrons of opposite spin on the walls of the cavity and it is annihilated. Thus, the oPs lifetime, 13, gives a measure of the mean free volume cavity radius, whereas the relative intensity of... 

Free annihilation is only one of the possibilities for thermalized positrons to stop existing. In some substances it does not happen at all. In most materials, some or all of the originally free positrons react somehow with their surroundings. The consequence of the reaction is the formation of new positron states. Such positron states differ fi om each other and fi om free positrons in their annihilation characteristics. Moreover, although the possible states are the same for similar materials, their exact annihilation properties depend on local conditions. Consequently, these states make positrons a valuable tool in studying the structure of materials. 

It is the easiest to understand positron states in the case of metals. In a perfect, defectless single crystal, thermalized positrons are in Bloch-states similar to those of electrons (Kittel 1979 Mijnarends 1983). They are called fi ee positrons. Before their annihilation, these... 

The formation of positron states takes place in a much more complicated way in molecular solids and fluids. The structures of these materials are more complex than those of metals, and diverse radiation products are formed in the thermalization process. Consequently, any theory explaining the behavior of positrons in these materials needs the use of radiation chemistry. The so-called spur model (Mogenssen 1974) does exactly this and states that the thermalized positron reacts with the particles of its own radiation track. The spur contains ground-state (M) and excited molecules (M ), ions (M" ), free radicals (R ), and electrons (e ). These species react with the positron and, parallel, with each other. Some possible reactions are given inOEq. (27.1) ... 

The formation of positronium atoms is affected by many factors but first of all, asO Eqs. (27.1) and O (27.2) suggest, by the material itself. According to the mentioned spur model (Mogenssen 1974), thermalized positrons compete with molecules of the material and with radiation products for available electrons. Sometimes, these spedes are such effective inhibitors or scavengers that they prevent positronium formation totally. Even if the inhibition is negligible, not all of the positrons can form Ps. Positronium formation reactions given by O Eqs. (27.2b) and (27.2c) require positrons of some particular energies. 

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%. 

The general form of the electron momentum distribution p (p) sampled by thermalized positrons is given by ... 

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