Positron microscope

James van House and Arthur Rich invent the positron microscope. 

A very effective materials study in the near-surface region can be performed using a positron microscope. To improve this technique, a special approach was developed in the PAS group in Munich that was based on cutting specimens at a very acute angle see Figure 4.56 or [191,192]. 

Enhanced depth resolution by using the Munich Scanning Positron Microscope... 

In the future, the application of PAS techniques to the development of new types of steels with well-defined parameters (materials for fusion reactors, etc.) or to evaluations of the effectiveness of post-radiation heat treatments can be foreseen. The utilisation of a high-precision positron microscope in RPV steel investigations would surely be a good example [156],... 

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. 

Hautojarvi, P., Rytsola, K., Tuovinen, P., Vehanen, A. and Jauho, P. (1977). Microscopic gas-liquid-like phase transition around the positron in helium gases. Phys. Rev. Lett. 38 842-844. 

Since lifetime and annihilation characteristics of a positron and the formation of the positronium atom (Ps) are determined by their microscopic chemical and phys-... 

A broad overview of traditional methods and recent developments in experimental positron spectroscopy is presented. A discussion of the generation and detection of positrons and their annihilation radiation is followed by a survey of techniques used for positron lifetime measurement, Doppler broadening spectroscopy and angular correlation of annihilation radiation, and the opportunities presented by combining these methods (e.g. in age-momentum correlation) and/or extending their capabilities by the use of monoenergetic positron beams. Novel spectroscopic and microscopic techniques using positron beams are also described. 

Furthermore, in addition to the bulk thermal properties of polymers and resists, determination of Tg of film interfaces and of ultrathin films has become an important issue in thin film imaging (bilayer, 157 nm, and EUV). Various techniques have been employed, which include ellipsometry [481,482], positron annihilation spectroscopy (PALS) [483], QCM [484], scanning viscoelasticity microscope (SVM) [485],x-ray reflectivity [486,487], and thermal probe [488]. 

Nakanishi, H., Wang S.J., and Jean, Y. C., Microscopic surface tension studied by positron annihilation, in Positron Annihilation Studies of Fluids, Sharma, S. C., Ed., World Scientific, Singapore, 1988, p. 292. 

Kanaya, T., Tsukushi, T., Kaji, K., Bartos, J., and Kristiak, J., Microscopic basis of free-volume concept as studied by quasielastic neutron scattering and positron annihilation lifetime spectroscopy, Phys. Rev. E, 60,1906-1912 (1999). 

Quantum chemistry predicts that the annihilation lifetime of a positron species is generally determined by the degree of overlapping of positron and electron wave functions, which leads e.g. to the intrinsic lifetime of the ortho-Ps of 1.4 x 10 sec. In a condensed matter it is obvious that the electron density at the position of the positron will greatly depend on the macroscopic and microscopic (mass) density and thus on parameters such as phase and temperature. This rather simple approach has led to the development of the "free volume" or "excluded volume" model (20, 28-29), whose basic feature it is that the lifetime of a positron or Ps trapped in such a material will depend on the free volume which it has available. 

We can ascribe the macroscopic insulating behavior of this compound to its extreme sensitivity to disorder on the chains, which themselves remain metallic at the microscopic level, until their ID character is strongly damaged by defects and impurities. The recent observation of metallicity in PrBa2Cu307 (Blackstead et al. 1995, Zhou Zliigang et al. 1997) is not in opposition with the conclusions drawn from the positron 2D-ACAR study. 

One of the main tasks of nuclear-reactor safety research is assessing the integrity of the reactor pressure vessel (RPV). The properties of RPV steels and the influences of thermal and neutron treatments on them are routinely investigated by macroscopic methods such as Charpy V-notch and tensile tests. It turns out that the embrittlement of steel is a very complex process that depends on many factors (thermal and radiation treatment, chemical compositions, conditions during preparation, ageing, etc.). A number of semi-empirical laws based on macroscopic data have been established, but unfortunately these laws are never completely consistent with all data and do not yield the required accuracy. Therefore, many additional test methods are needed to unravel the complex microscopic mechanisms responsible for RPV steel embrittlement. Our study is based on experimental data obtained when positron annihilation spectroscopy (PAS) and Mdssbauer spectroscopy (MS) were applied to different RPV steel specimens, which are supported by results from transmission electron microscopy (TEM) and appropriate computer simulations. 

The simple trapping model (STM) can be used to interpret the results from PL measurements [129], According to this model, for positrons implanted into a homogeneous solid with a bulk lifetime z, = 1/Ab and with N different kinds of homogeneously distributed microscopic defects with lifetimes Xi = jA and trapping rates ki, the annihilation spectrum will consist of A +1 exponentials. Through defect trapping, the bulk lifetime will be reduced to tq = l/(Ab -I- k), where... 

Using the previous equations we can derive the microscopic structure (/. e., the kinds of defects and their concentrations) from the experimentally determined lifetimes and intensities if the problem is homogeneous. This premise, however, does not hold for RPV steels completely. In inhomogeneous problems, the diffusion of positrons from the various implantation sites to the trapping centres must also be considered [125,130]. However, the mathematical difficulties associated with the corresponding diffusion-trapping model (DTM) [73] have so far prevented exact solutions from being obtained for all but the simplest problems [116,117], Thus, it is impossible to qualitatively analyse the very detailed experimental results obtained with a pulsed positron beam. 

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