Practical Electron Microscopy and Database

The diode capacitors in photo-diode array (PDA) detectors lose charge not only through irradiation but also as a result of their thermal leakage current, which is slightly different for each element of the array. In order to obtain values which are proportional to spectral intensity, a leakage or bias spectrum must be subtracted. This bias spectrum is recorded while electrons are excluded from the array (e.g., TEM screen lowered to block the electron beam) and will remain the same provided the integration time and array temperature do not vary. To minimize the noise content of recorded data and allow longer integration times (without total discharge by thermal leakage), the photodiode array is cooled to −20 °C by a thermoelectric device.