EDAX EDS Manual

ENERGY DISPERSIVE X-RAY SPECTROMETRY--
EDS INSTRUMENTATION & SIGNAL DETECTION
1. X-Ray Detectors:
ALAN SANDBORG
EDAX INTERNATIONAL, INC.
The EDS detector is a solid state device designed to detect x-rays and convert their
energy into electrical charge. This charge becomes the signal which when processed
then identifies the x-ray energy, and hence its elemental source.
The X-ray in its interaction with solids, gives up its energy and produces electrical
charge carriers in the solid. A solid state detector can collect this charge. One of the
desirable properties of a semiconductor is that it can collect both the positive and
negative charges produced in the detector. The figure below shows the detection
process.
Figure 1.
There are two types of semiconductor material used in electron microscopy. They are
silicon (Si) and germanium (Ge). In Si, it takes 3.8 eV of x-ray energy to produce a
charge pair, and in Ge it takes only 2.96 eV of energy. The other properties of these two
types will be discussed later in this section. The predominant type of detector used is
the Si detector, so it will be favored in the discussions. With a Si detector, an O K x-ray
whose energy is 525eV will produce 525/3.8= 138 charge pairs. A Fe K x-ray will
produce 6400/3.8= 1684 charge pairs. So by collecting and measuring the charge, the
EDAX Phoenix Training Course - EDS Instrumentation & Signal Detection - page 1