Standard Deviation of Composition Measurements in Atom Probe Analysis

Summary form only given. Atom Probe is a unique instrument to measure concentrations in metallic materials in volumes ranging from one to several ten thousands cubic nanometers. It can be used to measure the local concentration of the whole analysed volume, or to estimate the alloy composition. The list of detected atoms can be segmented into equal sized (in terms of number of atoms) sub-blocks, in which local composition is measured, leading to classical concentration profiles. Alternatively, due to the positioning capabilities of 3D atom probes, composition measurements can be focused on spatially selected areas, or concentration profiles calculated over constant sampling volumes. As each of these measurement procedure is different (as well as the information to be estimated) the variances of the obtained compositions should be estimated accordingly. This contribution is devoted to derive analytical expressions of the variance in several standard situations encountered when using atom probe. The particular aspect of the influence of the detector efficiency (which is generally not equal to unity...) on the estimates of composition measurement accuracy will be discussed