Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 262
All SEMs have facilities for detecting SEs and backscattered electrons (BSEs); either can
be used for imaging on the CRT, but each provides different information about the
sample. X-rays are generally used for chemical analysis, and will be discussed later. The
different types of radiation generated are a result of inelastic scattering. Only radiation
generated which escapes the sample will be detected. X-rays are not absorbed easily,
therefore most will escape from the sample and the volume of the material contributing to
X-rays (the sampling volume) is almost equivalent to the interaction volume [5].
Electrons which are backscattered will not escape the sample if they are generated at
depths greater than around 1 µm, resulting in a smaller region that the backscattered
signal is gathered from. The actual sampling volume is around 0.1 µm into the sample,
for materials of medium atomic weight, as low energy BSEs originate too far from the
incident beam to be detected [5]. A small number of SEs are generated from escaping
BSEs, but most are from electrons in the incident beam which enter the sample. Because
of this, the SE signal originates from a region slightly larger than the diameter of the
incident beam [5].
Electron detection
SEs are detected using an Everhart-Thornley detector, which is based on a scintillator-
photomultiplier system. For flat samples, this type of detectors is highly efficient,
collecting virtually all the SEs [5]. It should be noted, BSEs travelling at the appropriate
angle, will strike the Everhart-Thornley detector, although the number of BSEs is
negligible. The two most common types of BSE detectors are scintillator-photomultiplier