M. Porro

Calibration of the non-linear system response of a prototype set-up of the DSSC detector for the European XFEL

The DSSC (DEPFET Sensor with Signal Compression) is a new instrument with non-linear compression of the input signal in the sensor and with parallel signal processing (filtering, linear amplification, and digitization) for all pixels. The DSSC will serve as 2d imaging detector at the European X-ray Free Electron Laser (XFEL.EU) currently under construction in Hamburg, Germany. The DSSC design goal is to achieve at the same time single photon detection and high dynamic range of about 104 photons, both for photon energies down to 0.5 keY and read-out speeds up to 4.5 MHz. Realization of this goal requires an accurate calibration of the non-linear system response (NLSR) over the full dynamic range of the detector. We present our strategy for calibrating the NLSR, for each of the 1024 × 1024 DSSC pixels, in the laboratory. The feasibility of our calibration strategy is demonstrated experimentally by calibrating the NLSR of a DSSC prototype set-up consisting of a prototype DEPFET sensor with non-linear signal compression connected to a prototype read-out ASIC.

Study of PMOS front-end solution with signal compression for XFEL MiniSDD X-ray detectors

In this work we present the study and the experimental results on two different front-end stages for the MiniSDD pixel sensors of the DSSC detector for photon science applications at the European XFEL GmbH in Hamburg. The detector must be able to cope with an image frame rate up to 4.5 MHz and must achieve a dynamic range up to 104 photons/pixel/pulse with a photon energy of 1 keV. In order to achieve this high dynamic range and single photon sensitivity at the same time, the front-end must provide a non-linear amplification. The non-linear response is obtained with a simple circuit that pushes the input PMOSFET into triode region as the input signal increases. Since the readout ASIC has more than 4000 channels operating in parallel, particular care was devoted to the homogeneity and the robustness of the implemented solution, especially with respect to power supply rejection ratio and the cross talk among channels.

Validation of proton tests in air for detector calibration over a wide range of charge injection levels

We successfully evaluated the possibility of using a pulsed monoenergetic proton beam as a diagnostic tool for semiconductor detectors response mapping at high charge densities. In order to ease the setup of the detector under test we explored the opportunity of performing tests with protons in air. We qualified a polyimide film window (Upilex-S, 7.5 μm nominal thickness) as proton extraction window and the energy loss in air as a function of distance. The tests have been carried out in vacuum first, in order to evaluate the energy loss due to the window only, followed by in-air tests aimed at the investigation of the total energy degradation of the extracted proton beam.

Calibration of the non-linear system characteristic of the DSSC detector for the European XFEL

The DSSC (DEPFET Sensor with Signal Compression) is a new instrument with non-linear compression of the input signal and with parallel signal processing (filtering, linear amplification, and 8/9-bit digitization) for all pixels. The DSSC will serve as ultra-fast 2d megapixel imaging detector at the European XFEL (X-ray Free Electron Laser) currently under construction in Hamburg, Germany. The DSSC design goal is to achieve at the same time single photon detection and high dynamic range of about 104 photons, both for photon energies down to 0.5 keV and read-out speeds up to 4.5 MHz. Realization of this goal requires an accurate calibration of the non-linear system characteristic (NLSC) over the full dynamic range of the detector. The NLSC describes the relation between the signal charge collected in a sensor pixel and the respective digital output of the DSSC system, and must be determined for each of the 1024 × 1024 DSSC pixels. We report results from a set of measurements aimed at calibrating the NLSC of a single DSSC prototype pixel for photon energies of about 0.5 and 1 keV using 55Fe as X-ray line source. In addition, a 109Cd source was used for independent calibration and verification. For further independent verification, we compared the DSSC prototype results to measurements performed with an external test bench providing 14-bit resolution.

Methods for calibrating the gain and offset of the DSSC detector for the European XFEL

The DEPFET Sensor with Signal Compression (DSSC) will be a 2d 1Mpx imaging detector for the European X-ray Free Electron Laser facility (XFEL.EU), that is currently under construction in Hamburg. The DSSC is foreseen as a photon counting detector for soft X-ray radiation from 0.5 keV up to 6 keV. Driven by its scientific requirements, the design goals of the detector system are foremost low noise, a high dynamic range and a high frame rate of up to 4.5 MHz. Signal compression, amplification and digitization will be performed in the focal plane. Utilizing an in-pixel active filtering stage and an 8/9-bit ADC, the detector will provide parallel readout of all pixels. A critical step of calibrating the detector is the determination of the system gain and offset based on peak energies of X-ray calibration line sources such as 55Fe. This is demanding due to the intrinsically low spectral resolution of the DSSC. The results of studies on the stability and performance of automated procedures for peak fitting in single pixel spectra with a low energy resolution were presented on a poster.

First functionality tests of a 64 × 64 pixel DSSC sensor module connected to the complete ladder readout

The European X-ray Free Electron Laser (XFEL.EU) will provide every 0.1 s a train of 2700 spatially coherent ultrashort X-ray pulses at 4.5 MHz repetition rate. The Small Quantum Systems (SQS) instrument and the Spectroscopy and Coherent Scattering instrument (SCS) operate with soft X-rays between 0.5 keV–6 keV. The DEPFET Sensor with Signal Compression (DSSC) detector is being developed to meet the requirements set by these two XFEL.EU instruments. The DSSC imager is a 1 mega-pixel camera able to store up to 800 single-pulse images per train. The so-called ladder is the basic unit of the DSSC detector. It is the single unit out of sixteen identical-units composing the DSSC-megapixel camera, containing all representative electronic components of the full-size system and allows testing the full electronic chain. Each DSSC ladder has a focal plane sensor with 128× 512 pixels. The read-out ASIC provides full-parallel readout of the sensor pixels. Every read-out channel contains an amplifier and an analog filter, an up-to 9 bit ADC and the digital memory. The ASIC amplifier have a double front-end to allow one to use either DEPFET sensors or Mini-SDD sensors. In the first case, the signal compression is a characteristic intrinsic of the sensor; in the second case, the compression is implemented at the first amplification stage. The goal of signal compression is to meet the requirement of single-photon detection capability and wide dynamic range. We present the first results of measurements obtained using a 64× 64 pixel DEPFET sensor attached to the full final electronic and data-acquisition chain.

A Bulk Control Circuit for Open-Loop Front-Ends for X-Ray Pixel Detectors

In this paper, we present a bulk control circuit to correct the chip-to-chip process variations of an open-loop nonlinear front-end (FE) for X-ray pixel detectors. Our study was carried out in the framework of the Depfet sensor with signal compression detector development for the European X-ray free electron laser. The presented circuit is capable to stabilize the FE response in presence of threshold voltage variations, acting on the bulk voltages of the FE’s transistors and exploiting the body effect. The control circuit does not affect the noise performances of the FE. The working principle of the proposed control circuit and the first experimental results obtained with a first prototype realized in the 130-nm IBM technology are presented in this work.

First operation of a DSSC hybrid 2D soft X-ray imager with 4.5 MHz frame rate

The DSSC (DEPFET Sensor with Signal Compression) collaboration develops a hybrid pixelated X-Ray photon detector with 4.5 MHz frame rate and immediate amplitude digitization for experiments at the European XFEL. We present the first full format 14.9×14 mm2 F1 pixel readout ASIC for the DSSC detector. The readout architecture is specially adapted to the burst structure of the XFEL (bursts of 2880 pulses spaced by down to 220 ns at a rate of 10 Hz) by in-pixel digitization and digital hit data storage and data transfer during the burst gaps. The readout ASIC contains 64×64 pixels of 229×204 μm2size and includes per pixel two low noise front-end versions for DEPFET and silicon drift detectors (SDD), a single-slope 8-bit ADC and local memory. Measurements using the F1 ASIC and a matching mini-SDD sensor matrix are shown.

Calibration sources and techniques for large format X-ray imagers at XFEL

In this work we compare different calibration sources and focus on their combination to optimize the calibration of large-format X-ray imagers over a wide range. This activity is carried out in the framework of the DSSC project and the primary aim is the calibration of the DSSC camera. We considered pulsed IR laser, radioactive sources, X-ray tube, electrical injection devices, X-ray synchrotron beam, low-energy protons, LED sources. The relevant features (deposited energy and pulse width) are critically summarized and calibration strategies will be discussed.

Safety-interlock system of the DSSC X-ray imager

In this paper, the DSSC safety-interlock system is introduced. It is designed to keep the DSSC mega-pixel camera in a safe-state. The system is composed of four inter-communicating sub-systems referred here as a master SIB (safety-interlock board) and three partner SIBs. Each SIB monitors and processes 75 temperature sensors mainly located in the focal plane and also distributed inside the camera-head electronics. It monitors the signals from two pressure sensors, one humidity sensor as well as the connectivity to sixteen low-voltage and two high-voltage cables. The master SIB supervises the signals to and from external equipments like the cooling system, pressure gauge, power crates and status information from experimental environment. SIB is an 8-layer printed-circuit board (PCB) with a micro-controller sitting on top as its central processing unit. A decision-matrix software reads the sensor status continuously and determines whether the camera is in a safe state or not. All error and warnings flags are sent to the detector control software framework and the end user directly. These flags are also stored every 100 ms together with the image data. The full SIB sensor data is sent to the back end for offline analysis. A shutdown sequence is initiated by SIB in case of a critical failure. The safety-interlock philosophy, hardware design, and software architecture along with test results will be presented.