Positron activation, thermal

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Mills Jr., A.P. (1979). Thermal activation measurement of positron binding... 

One of the earlier and still one of the most interesting applications of coincidence counting in activation analysis is the isotopic determination of 6Li by Coleman 84>. In this method aqueous solutions of LiOH are irradiated with thermal neutrons to produce tritons by means of the 6Li(w,f)4He reaction. The tritons then react with the oxygen in the aqueous solution by the 160(f, )18F reaction. The 18F is a positron... 

The use of 14 MeV neutron activation principally for major elements, Ge(Li) detectors for trace elements following thermal neutron irradiations, and gamma-gamma coincidence techniques for positron or cascade gamma-ray emitters as discussed in the previous sections, provide the analyst with powerful tools for devising schemes for non-destructive analysis. A few additional activation techniques which may be useful in special applications are discussed briefly below. In most of these cases rather sophisticated instrumentation is required. It is unlikely, therefore, that these techniques will be in routine use in a facility devoted principally to analytical applications. In some cases, however, arrangements may be made for part time use of a more extensive nuclear facility for a specific analytical problem. 

SS, 3 X 2nun). A Nal detector continuously measures the passing activity through the sample loop. A GC run is remotely started at the moment that the sample loop is filled with [ NJ-NHs. The GC is equipped with a Haysep P column (SS, mesh 80-100, O.D. 1/8). The GC run is started at 90°C, and after 1 minute, the temperature is increased with 10°C/min to 130°C. The products are analysed by a thermal conductivity detector (TCD). A Nal detector, directed at the TCD, monitors labelled products. Behind the TCD detector, a heated electrical 3-way valve selects a small part from the [ NJ-NHg peak. Depending on the experiment, a pulse time of 2-10 s is used to inject the labelled gaseous ammonia into the reactant stream. The required specific activity of the radioactive ammonia pulse is 0.1 MBq/ml minimum to meet the statistical requirements of the positron emission profiling experiments. 

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. 

Thermally activated detrapping of positrons trapped in a dislocation core (an initial shallow trap) may occur at elevated temperatures, which makes vd temperature dependent [102],... 

Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. Positron annihilation lifetime spectroscopy (PALS) is an efficient technique used for the analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, namely, molecular and free-volume theories. According to the free-volume theory, the diffusion is not a thermally activated process as in the molecular model, but it is assumed to be the result of random redistributions of free-volume voids within a polymer matrix. Cohen and Turnbull developed the free-volume models that describe the diffusion process when a molecule moves into a void larger than a critical size, Vc- Voids are formed during the statistical redistribution of free volume within the polymer. It is found that the relative fractional free volume of unfilled polymer decreases on the addition of layered silicates. The decrease is attributed to the interaction between layered silicate and polymer because of the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free-volume concentration or relative fractional free volume [49]. 

Fig. 6. Thermal activation of positronium from a Cu(lll) surface bombarded with 30 eV positrons (Ref 21).

Second, the target is heated so that the positrons trapped in the surface state are thermally activated to form thermal positronium in the ground state[21j. Fig. 6 shows the thermal activation of Ps from a Cu(lll) surface. The process is analogous to thermionic emission of electrons or to thermal desorption of hydrogen, and can be described by the usual statistical mechanics arguments[28]. 

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