Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 267
The X-ray detector (see fig. (7.1)) consists of a reverse-biased silicon p-i-n junction
coated with gold. X-rays strike the detector, resulting in excitation of electrons into the
conduction band, leaving positive holes. The number of electron-hole pairs generated is
proportional to the energy of the X-ray detected [5]. As the detector is reverse-biased, a
current will flow each time an X-ray strikes the detector. In pure silicon this current
would be negligible compared to the applied voltage. Therefore, the resistivity of the
detector is increased by doping the silicon with lithium, and cooling the detector with
liquid nitrogen to around 77 K. Now, when an X-ray enters the detector, the current is
much larger relative to the applied voltage, making detection, amplification, and
recording easier [5]. The detector is protected by a beryllium window (7-8 µm thick) to
prevent condensation of contaminants onto the cold detector. However, this means X-rays
from elements lighter than Be (Z ≤ 4) cannot be detected [5].
The current generated by the X-rays lasts < 1 µs, and is known as a pulse. The pulse is
amplified and transmitted to a multi-channel analyser (MCA). The MCA consists of
thousands of channels corresponding to different X-ray energies [5]. The MCA collects
the energies of the X-rays detected, and plots a histogram of energy against number of
counts. A suitable spectra can be obtained in 1 to 2 minutes, however if the counts per
second (cps) exceed 1000, some of the pulses will be rejected by the MCA [5]. This
information is displayed on the PC linked to the SEM. The software can then assign
elements to the peaks, and quantify the amount of each element present by integrating the
peaks.
There are a number of limitations of the EDX system. Elements lighter than beryllium
cannot be detected due to the presence of the Be-window. Fluorescence from samples