In the hybrid photon counting (HPC) technology, each pixel functions as a sensor and read out at the same time due to bump bonding the sensor material to the read-out chip at each pixel position. When a photon is absorbed, it is converted into an electron-hole pair and a proportional charge, which is further captured in an electrical field. This minimizes the spread of the signal over the whole detector resulting in a point spread function no wider than a pixel and maximizes spacial resolution. With the HPC technology, it becomes possible to eliminate detector noise and dark current entirely, effectively removing the detector as a source of noise. Additionally, the implementation of dual energy discrimination enables a substantial reduction in fluorescence effects using a lower threshold, while also significantly diminishing cosmics and radioactive decay signals using an upper threshold. As a result, this combination achieves an exceptional signal-to-noise ratio. By combining the direct photon to charge conversion and the quick read out electronics, the detectors offer maximal scanning speed, enabling in-situ investigations of rapid dynamic processes.
Energy dependent quantum efficiency behavior for CdTe and Si sensors in an EIGER2 detector. Plot curtesy of DECTRIS.
Recently not only silicon is used as detector sensor material, but also cadmium telluride. Choosing between those two materials mainly depends on the employed wavelength(s). CdTe sensors have a higher quantum efficiency (QE) for shorter, harder wavelengths, which is especially striking when using a silver anode. In this case the QE rises from around 27% (Si-sensor) to > 90% (CdTe sensor) when measured for an EIGER2 detector.
However not only the sensor material influences the detection outcome, but also the sensor thickness. The silicon sensors are available with a thickness of 450 µm and 1000 µm for MYTHEN2 and PILATUS3 and in 450 µm for EIGER2 and tests clearly show that the QE rises when a thicker sensor is used for shorter wavelengths. The effect is inverse, yet not quite as striking for shorter wavelengths, while Cu radiation has an excellent QE of ≥ 96% in all cases.
|Energy [keV]||QE at 450 µm||QE at 1000 µm|
|5.4 (Cr)||94 %||> 80 %|
|8.0 (Cu)||98 %||96 %|
|12.4 (1 Angstrom)||84 %||97 %|
|17.5 (Mo)||47 %||76 %|
|22.2 (Ag)||27 %||50 %|
A further criterion separating the PILATUS3 and EIGER2 detectors is their respective pixel size of 172 x 172 µm2 and 75 x 75 µm2, respectively. The pixel size determines the resolution as indicated by the experimental data to the left.
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