X-Rays and Gamma Rays interact with CdTe atoms to create an
average of one electron/hole pair for every 4.43 eV of energy lost
in the CdTe. Depending on the energy of the incoming radiation,
this energy loss is dominated by either the Photoelectric Effect or
Compton Scattering. The probability or efficiency of the detector
to “stop” the incoming radiation and create electron/hole pairs
increases with the thickness of CdTe. See Figure 2.
In order to facilitate the electron/hole collection process in
the CdTe detector, a +500 Volt potential is applied. This voltage
is too high for operation at room temperature, as it will cause excessive
leakage, and eventually a breakdown. Since the detector
in the XR-100T-CdTe is cooled, the leakage current is reduced
considerably, thus permitting the high bias voltage.
The thermoelectric cooler cools both the CdTe detector
and the input FET transistor to the charge sensitive preamplifier.
Cooling the FET reduces its leakage current and increases the
transconductance, which in turn reduce the electronic noise of
the system.
In order to further reduce the electronic noise, the feedback
capacitor and part of the current feedback network to the preamplifier
are also placed on the same substrate as the detector and
FET. This minimizes parasitic capacitance at the input.
A temperature monitoring sensor is placed on the cooled
substrate to provide a direct reading of the temperature of the internal
components, which will vary with room temperature. Once
the internal temperature gets below minus 10°C the performance
of the XR-100T-CdTe will not change with a temperature variation
of a few degrees. Hence, accurate temperature control is not
necessary when using the XR-100T-CdTe inside the laboratory