Introduction

The spatial distribution of a specific element can be revealed by recording a ‘map’ of the intensity of its characteristic X-rays while the beam is scanned in a rectangular raster. A similar result can be obtained by leaving the beam position fixed while moving the specimen. Either ED or WD spectrometers can be used for X-ray mapping.

In the pre-digital era the normal form of X-ray image was the ‘dot map’, in which each recorded photon produced a bright dot on the display at a point corresponding to the position of the beam on the sample. The density of dots then showed variations in the concentration of the selected element. However, this approach has been superseded by digital mapping, which is described in the following section.

Digital mapping

With computer-controlled instruments X-ray maps are produced by recording the number of X-ray photons for a fixed time at each point in the scanned area and storing the data in the computer memory. A visible image is generated by converting this number into brightness on the screen (see Fig. 6.1). In its raw form the data consist of the number of X-ray counts recorded for each pixel. This can be converted into a standard type of image format in which the intensities are converted to ‘grey levels’ (typically 256). However, in this process information regarding absolute X-ray intensities, and thus concentrations, is lost.