Two-dimensional angular correlation

The one-dimensional angular correlation spectrometer is a relatively simple equipment (O Fig. 27.4). It is constructed from two scintillation detectors, several slits, a coincidence unit, and a storage unit. 

The scheme of a one-dimensional angular correlation equipment. In two-dimensional spectrometers, the mobile detector is moved in two perpendicular directions to map photon distribution. Sometimes the mobile detector is replaced by a position-sensitive gamma camera... 

On the other hand ARCHEOPS only scanned around a circle of constant elevation and then let the center of the circle move in right ascension as the Earth turned. This provides only a two way modulation. Since the sky itself is a two dimensional function, just about any time history of drifting baselines is consistent with some pattern on the sky. Thus ARCHEOPS is very vulnerable to striping. This can be seen in the last panel of Figure 2 of astro-ph/0310788 (Hamilton et al., 2003) which clearly shows correlated residuals aligned with the scan path. These stripes have a low enough amplitude to not interfere with measurements of the temperature-temperature angular power spectrum CjT, but they would ruin a measurement of the polarization power spectrum CfE. 

The measured angular correlation spectrum is a projection of the momentum density function of the sample. In the case of one-dimensional spectra, for example, the density function is integrated over two coordinates ... 

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. 

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