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The
Microanalysis System
How the
EDS Detector Works
The EDS detector converts the
energy of each individual X-ray into a voltage signal of proportional size.
This is achieved through a three stage process. Firstly the X-ray is
converted into a charge by the ionization of atoms in the semiconductor
crystal. Secondly this charge is converted into the voltage signal by the
FET preamplifier. Finally the voltage signal is input into the pulse
processor for measurement. The output from the preamplifier is a voltage
‘ramp’ where each X-ray appears as a voltage step on the ramp.
EDS detectors are designed to
convert the X-ray energy into the voltage signal as accurately as possible.
At the same time electronic noise must be minimized to allow detection of
the lowest X-ray energies.
How the
crystal converts X-ray energy into charge
When an incident X-ray strikes
the detector crystal its energy is absorbed by a series of ionizations
within the semiconductor to create a number of electron-hole pairs. The
electrons are raised into the conduction band of the semiconductor and are
free to move within the crystal lattice. When an electron is raised into the
conduction band it leaves behind a ‘hole’, which behaves like a free
positive charge within the crystal. A high bias voltage, applied between
electrical contacts on the front face and back of the crystal, then sweeps
the electrons and holes to these opposite electrodes, producing a charge
signal, the size of which is directly proportional to the energy of the
incident X-ray.
The role of
the FET
The charge is converted to a
voltage signal by the FET preamplifier. During operation, charge is built up
on the feedback capacitor. There are two sources of this charge, current
leakage from the crystal caused by the bias voltage applied between its
faces, and the X-ray induced charge that is to be measured. The output from
the FET caused by this charge build-up is a steadily increasing voltage
‘ramp’ due to leakage current, onto which is superimposed sharp steps due to
the charge created by each X-ray event. This accumulating charge has to be
periodically restored to prevent saturation of the preamplifier. Therefore
at a pre-determined charge level the capacitor is discharged, a process
called restoration. Restoration can be achieved either by pulsed optical
restore where light from an LED is shone onto the FET, or by using direct
injection of charge into a specially designed FET.
The noise is
strongly influenced by the FET, and noise determines the resolution of a
detector particularly at low energies. Low noise is also required to
distinguish low energy X-rays such as beryllium from noise fluctuations
(Fig. 5) . Direct charge restoration via the FET introduces less noise than
optical restore. At high count rates, the restoration periods limit the
maximum output rate and any after-effects of the restoration (Fig. 4) will
affect pulse measurement. Direct charge restoration via the FET is
considerably faster and avoids the after-effects associated with optical
restore so that noise and resolution are less likely to degrade with
increasing count rate.
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