Photodiode Array (PDA) for EELS
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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Basically, EELS (Electron Energy Loss Spectroscopy) is converted to light using a transmission phosphor, then converted to electrons using a photomultiplier for serial acquisitions or using a position-sensitive-detector for parallel acquisition, either a self-scanned photodiode array (PDA) or a charge-coupled device (CCD) [1 - 2].

In operating process of diode-array detectors, immediately prior to spectrum recording, all array elements are charged to the same potential (typically 5 V). During electron exposure, each diode is discharged by an amount which is proportional to the time-integrated electron flux at the corresponding energy loss. At the end of this integration period, the diodes are interrogated and the spectrum read out serially as a chain of pulses, the height (counts) of each pulse representing electron intensity. The signal is fed into a multichannel analyzer via multichannel scaling (MCS) circuitry.

The diode capacitors 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.

 

[1] O. L. Krivanek, C. C. Ahn & R. B. Keeney, Ultramicroscopy 22, 103 (1987).
[2] D. McMullan et al., Further development of a parallel EELS CCD detector for a VG HB501 STEM in EUREM 92 Paris (1992).


 

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