ANALYTIC TREATMENT OF CHARGE CLOUD OVERLAPS - AN IMPROVEMENT OF THE TOMOGRAPHIC ATOM-PROBE EFFICIENCY

Although reliable position and composition data are obtained with the Tomographic Atom Probe, the procedure of position calculation by charg e centroiding fails when the detector receives two or more ions with c lose spaced positions and the same mass-to-charge ratio. As the charge clouds of the ions overlap, they form a unique charge pattern on the multianode detector. Only one atom is represented and its position is biased. In order to estimate real positions, we have developed a corre ction method. The spatial distribution of charges inside a cloud issue d from one impact is modelled by a Gaussian law. The particular proper ties of the Gaussian enable the calculation of exact positions of the two impacts of the overlapped charge patterns and charges of correspon ding clouds. The calculation may be generalized for more than two over lapped clouds. The method was tested on a plane-by-plane analysis of a fully ordered Cu3Au alloy performed on a (100) pole.