GAS ELECTRON-DIFFRACTION APPARATUS AT THE UNIVERSITY-OF-ANTWERPEN

A description is given of the Antwerpen gas electron diffraction unit. Its electron optical system maintains a stability of the electron wav elength of about 0.01% over several hours and of about 0.3% over at le ast one year. Differences with respect to the well-known Balzers unit are the following. The electron beam is shielded from external magneti c fields by a mu metal cylinder, and the machine background is reduced by a movable aluminum disk in addition to graphite coating of the dif fraction chamber's interior. Furthermore, features enhancing the servi ceability are: (i) built-in means to accurately measure the nottie-to- photographic plate distances, (ii) a processor controlled vacuum syste m, (iii) means for easy adjustment of beam centring and estimation of exposure time, (iv) semi-automatic recording of the diffraction patter n, and marking of the diffraction centre on the photographic plate, an d (v) built-in safety measures in case of a vacuum breakdown. Two gas- inlet systems are available with active temperature ranges between -19 0 and +30 degrees C, and between +20 and +200 degrees C, respectively. (C) 1997 Elsevier Science B.V.