3-DIMENSIONAL CHEMICAL CHARACTERIZATION OF COMPLEX SILVER-HALIDE MICROCRYSTALS BY SCANNING ION MICROPROBE MASS ANALYSIS

A new procedure based on scanning ion microprobe mass analysis (SIMS) is developed to characterize populations of silver halide microcrystal s present in a photographic emulsion with emphasis on the thickness di stribution of the surface layers usually found on these microcrystals in the 5-30 mn range, Using a Cameca IMS-4f instrument one is able to image the lateral and in-depth distributions of the halides in individ ual microcrystals. A digital image processing system interfaced with t he SIMS instrument permits the acquisition of spatially resolved mass- selected data (ion images) for a number of single microcrystals, Small regions (similar to 300 nm in diameter) within each crystal are selec ted a posteriori for local area ''retro'' depth profiling by computer reconstruction. By averaging the gray values measured in suitable part s of the crystal, the thickness of the surface layers can be determine d, allowing one to make a three-dimensional reconstruction of the chem ical composition of the microcrystals, Using this methodology, the cha racteristics (thickness and chemical composition) of surface and subsu rface layers of different samples can be compared and subclasses in on e population can be detected, With the aid of home-developed hardware and software, the image acquisition procedure was automated, making it possible to automatically measure a sufficiently large number of micr ocrystals, In this way the analysis time was decreased and the statist ical relevance of the data improved. Two examples are given which illu strate the power and practical use of the developed methodology.