Angular correlation of annihilated radiation ACAR

Fig. 6.15. Cylindrically averaged angular correlation of annihilation radiation (ACAR) distributions for positron annihilation in the noble gases, (a) helium, (b) neon, (c) argon, (d) krypton and (e) xenon, from the work of Coleman et al. (1994). Reprinted from Journal of Physics B27, Coleman et al, Angular correlation studies of positron annihilation in the noble gases, 981-991, copyright 1994, with permission from IOP Publishing.

Figure 8.12 Cross section in the (kx, ky) plane of the calculated Fermi surface in Y 1 2 3. The experimental results from angle-resolved photoemission (ARPES), de Haas van Alphen measurements (dHvA), and angular correlation of annihilation radiations (ACAR) are indicated. From Pickett etal. [61].

Additional information on the material under study can be obtained from the measurement of the e+ -e momentum distribution as mirrored in Doppler broadening of annihilation radiation (DEAR) and the angular correlation of annihilation radiation (ACAR). DEAR can be applied to study the chemical surroundings of free-volume holes [Dlubek et al., 2000a Bamford et al., 2006b], while ACAR is able to measure the anisotropy of the hole shape, as observed for highly crystalline fibers, for example [Jean et al., 1996 Bamford et al., 2001b]. 

Positrons can be used as particle probes, suitable to detect low concentrations of defects in materials. Positron physicists generally are in need of intense positron beams for applying positron annihilation techniques such as two dimensional (2D) Angular Correlation of Annihilation Radiation (ACAR) for investigating surfaces and interfaces of materials. The 2D-ACAR technique allows high resolution measurements of the electron momentum distribution for depth, localized defects, thin layer systems, and interfaces. In addition, a submicrometer size positron beam can be created for defect depth profiling on a lateral scale smaller than a micrometer. Vacancy type defects can be mapped in a three dimensional fashion. 

Angular Correlation of Annihilation Radiation (ACAR) Method... 

ACAR angular correlation of the annihilation radiation h(k) the reduced momentum distribution (LCW function)... 

Let us mention finally, though it is obsolete, that ID angular correlation of the annihilation radiation (ID-ACAR) also provides lineshape profiles. The ID-ACAR technique consists of performing an angular correlation by resolving one of the two angles 6 and q> (see fig. 1). For example, if 0 is resolved, ID-ACAR provides the onedimensional distribution N(px) as... 

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