Figure 1 - uploaded by Niels van Bakel
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Lower right corner shows an image taken at XCS with a beam-stop and high photon flux. The larger image shows a zoom of the right half of the beam-stop near the center of the detector. Clearly visible is the intensity drop of the signal next to the beam-stop. The white circle indicates some larger sensor pixels.
Source publication
The Timepix detector can be operated in Time-over-Threshold mode to allow for charge integration measurements as required by short (< 50 fs) x-ray pulses of the Linac Coherent Light Source (LCLS). Initial commissioning activities have started at the X-ray Correlation Spectroscopy (XCS) instrument at LCLS, where speckle patterns have been measured.
Contexts in source publication
Context 1
... LCLS beam flux was carefully increased to find the intensity where the pixels start saturating. However, Figure 1 shows that the detector signal drops to zero in the region where you expect it to saturate. Several causes have been considered but a fast discharge of the preamp storage capacitance is the most likely reason. ...
Context 2
... of the ToT values for input charges up to 400k for both holes and electrons show a drop in ToT for positive inputs above 125k holes. The maximum ToT values we measure with a hole collecting sensor according to Figure 1 correspond to an input charge of about 135k holes (60 8keV photons), and agree with the simulations. The behavior of the larger edge pixels confirms this finding: the signal drop is more pronounced in the edge region (white circle in Figure 1) because these pixels collect more charge. ...
Context 3
... maximum ToT values we measure with a hole collecting sensor according to Figure 1 correspond to an input charge of about 135k holes (60 8keV photons), and agree with the simulations. The behavior of the larger edge pixels confirms this finding: the signal drop is more pronounced in the edge region (white circle in Figure 1) because these pixels collect more charge. ...
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Citations
... The operation requires a certain number of energy thresholds. The Timepix chip [36] has a single energy threshold. The threshold can be adjusted for each pixel in order to obtain uniform performances of the whole pixel matrix. ...
... 36: Example of electronic chain for signal processing with the two shaping circuits. ...
The emergence of CdTe Photon Counting Detectors (PCD) with energy discrimination capabilities, opens up new perspectives in X-ray imaging. Medical and security applications are characterized by very high X-ray fluxes and consequently require a very fast shaper in order to limit dead time losses due to pile-up. However, if the shaper is faster than the collection of the charges in the semiconductor, there is a loss of charge called ballistic deficit. Moreover, variations of the electric field profile in the detector over time cause a change in the collection time of the charges. As a result, the conversion gain of the detector will be affected by these variations. The instability of the response is visible over time as a channel shift of the spectra, resulting in a false information of the photon energy. The aim of this work is to characterize this instability in order to understand the mechanisms behind them and to develop a method to correct its effect. We proposed a correction algorithm based on the use of two Single Delay Line (SDL) shaping amplifiers. A fast SDL is used to measure the X-ray spectra at high count rates with limited count rate losses. A slow SDL is used to measure the full collected charge in order estimate a correction factor for the compensation of the ballistic deficit fluctuations of the fast SDL. An important step is to sort the processed pulses in order to reject pile-up and other undesirable effects that may degrade the measurement of the correction factor. The proposed method was implemented in an FPGA in order to correct the ballistic deficit in real-time and to give a stable response of the detector at very high fluxes. The method was tested with a 4x4 pixels detector (CdTe) of 3 mm thickness and 800 micron pitch, which is able to measure transmitted X-ray spectra in the energy range of 20-160 kV on 256 energy bins. The developed method was initially tested at low count rate with a Co-57 and an Am-241 gamma-ray sources, then at high count rates up to ~2 Mc/s with an X-ray source. With the characterization and the validation of this innovative algorithm we prove its ability in providing a stable response of the detector over time without affecting the energy resolution (~7% at 122 keV) and the dead time (~70 ns).
... Furthermore, the processing tasks required to perform autocorrelation on a full array creates a bottleneck for XPCS experiments. These bottlenecks have been addressed in recent CMOS imaging technologies [16]- [18]. ...
We present the design, manufacture, and test results of an asynchronous event-driven address time-stamped (EDATS) pixelated array detector, operational over correlation time spans ranging from less than ${10^{ - 6}}$ to greater than ${10^4}$ s. The pixelated EDATS detector was designed to measure the equilibrium density fluctuations on a nanometer-length scale using small angle X-ray scattering in X-ray photon correlation spectroscopy (XPCS) experiments. The detector sensor chip assembly (SCA) includes a custom readout integrated circuit (ROIC), hybridized to a silicon photodiode array, optimized for 500 to 2000 eV X-ray photons. The detector is shown to be capable of handling X-ray photon event rates of 100 million X-ray events per second, with less than 85 ns timing jitter per event.
