Aufbau eines Positronen-Zerstrahlungs-Experiments zur Vermessung von Fermikörpern

An experimental apparatus to investigate the Fermi-Sufaces (FS) of binary alloys with positron-annihilation is described. The principle of rotating specimen with the angular-correlation at constant deviation-angle ("rotating-specimen-method") is applied combined with a new "crossed-slit-geometry" using collimators which are grooves engraved in steel-plates. The resolution is vertical 0 . 88 mrad (fwhm) and horizontal 1 . 6 mrad (fwhm). The apparatus is fourfold to measure simultaneously four crystal-directions at 5$^\circ$ difference by four independent coincident-detector-pairs each using up to 18 collimator-systems (multi-collimator-geometry). In a given crystalplane the range of crystal-directions covers 100$^\circ$. With regard to symmetry this is enough to receive the cross-sections of the FS of many crystal-structures. A range-attenuation to measure the "necks" of the FS with greater accuracy is provided. Three deviation-angles of the 180$^\circ$ -emission are available : $\theta = 0^{0}, 0.5^{0}$ and $0.75^{0}$. 'i'he use of external Cu-64-positron-sources allowes investigations of all alloy-systems. At present the specimen are at room-temperature. An installation to cool them down to the temperature of liquid nitrogen is provided later. A test-measurement on pure copper-single-crystals is reported and proves the performance of the apparatus. It is shown that the present statistical error of 3% reaches 1% by reasonable efforts what seems desirable in FS-investigations. Some considerable quantitative relations of the experimental design concerning the problems of intensity and relative sensitivity are discussed. These show a remarkable advantage in relative sensitivity of the described geometry-system compared with a conventional crossed-slit-experiment of higher intensity . Herewith it is possible to investigate the variation of the neck-radii of the FS at different alloy-concentrations