Julian Becker

The single photon sensitivity of the Adaptive Gain Integrating Pixel Detector

Abstract Single photon sensitivity is an important property of certain detection systems. This work investigated the single photon sensitivity of the Adaptive Gain Integrating Pixel Detector (AGIPD) and its dependence on possible detector noise values. Due to special requirements at the European X-ray Free Electron Laser (XFEL) the AGIPD finds the number of photons absorbed in each pixel by integrating the total signal. Photon counting is done off line on a thresholded data set. It was shown that AGIPD will be sensitive to single photons of 8xa0keV energy or more (detection efficiency ⪢ 50 % , less than 1 count due to noise per 106 pixels). Should the final noise be at the lower end of the possible range (200–400 electrons) single photon sensitivity can also be achieved at 5xa0keV beam energy. It was shown that charge summing schemes are beneficial when the noise is sufficiently low. The total detection rate of events is increased and the probability to count a single event multiple times in adjacent pixels is reduced by a factor of up to 40. The entry window of AGIPD allows 3xa0keV photons to reach the sensitive volume with approximately 70% probability. Therefore the low energy performance of AGIPD was explored, finding a maximum noise floor below 0.035 hits/pixel/frame at 3xa0keV beam energy. Depending on the noise level and selected threshold this value can be reduced by a factor of approximately 10. Even though single photon sensitivity, as defined in this work, is not given, imaging at this energy is still possible, allowing Poisson noise limited performance for signals significantly above the noise floor.

Development of LAMBDA: Large Area Medipix-Based Detector Array

The Medipix3 photon counting readout chip has a range of features — small pixel size, high readout rate and inter-pixel communication — which make it attractive for X-ray scattering and imaging at synchrotrons. DESY have produced a prototype large-area detector module that can carry a 6 by 2 array of Medipix3 chips (1536 by 512 pixels), which can be used with a single large silicon sensor (85mm by 28mm) or two ``hexa high-Z sensors. The detector head is designed to be tilable and compatible with low temperatures, and will allow high speed parallel readout of the Medipix3 chips. It consists of a ceramic board, on which the sensor assembly is mounted, and a secondary board for signal routing and voltage regulators. A prototype DAQ board using USB2 readout has also been produced. A ``quad Medipix3 sensor assembly has been mounted on the detector head, and successfully configured and read out by the DAQ board. Development has begun on a high-speed readout board, and large-area silicon assemblies are in production.

AGIPD, a high dynamic range fast detector for the European XFEL

AGIPD—(Adaptive Gain Integrating Pixel Detector) is a hybrid pixel X-ray detector developed by a collaboration between Deutsches Elektronen-Synchrotron (DESY), Paul-Scherrer-Institut (PSI), University of Hamburg and the University of Bonn. The detector is designed to comply with the requirements of the European XFEL. The radiation tolerant Application Specific Integrated Circuit (ASIC) is designed with the following highlights: high dynamic range, spanning from single photon sensitivity up to 104 12.5keV photons, achieved by the use of the dynamic gain switching technique using 3 possible gains of the charge sensitive preamplifier. In order to store the image data, the ASIC incorporates 352 analog memory cells per pixel, allowing also to store 3 voltage levels corresponding to the selected gain. It is operated in random-access mode at 4.5MHz frame rate. The data acquisition is done during the 99.4ms between the bunch trains. The AGIPD has a pixel area of 200× 200 μ m2 and a 500μ m thick silicon sensor is used. The architecture principles were proven in different experiments and the ASIC characterization was done with a series of development prototypes. The mechanical concept was developed in the close contact with the XFEL beamline scientists and is now being manufactured. A first single module system was successfully tested at APS.

Challenges for silicon pixel sensors at the European XFEL

Abstract A systematic experimental study of the main challenges for silicon-pixel sensors at the European XFEL is presented. The high instantaneous density of X-rays and the high repetition rate of the XFEL pulses result in signal distortions due to the plasma effect and in severe radiation damage. The main parameters of X-ray-radiation damage have been determined and their impact on p + n sensors is investigated. These studies form the basis of the optimized design of a pixel-sensor for experimentation at the European XFEL.

AGIPD - The adaptive gain integrating pixel detector for the European XFEL development and status

The European XFEL [1] will provide fully coherent, 100 fs X-ray pulses, with up to 1012 photons at 12 keV. The high intensity per pulse will allow recording diffraction patterns of single particles or small crystals in a single shot. Consequently 2D-detectors have to cope with a large dynamic range: detection from single photon to > 104 photons/pixel in the same image. An additional challenge is the European XFEL machine: an Electron bunch train with 10 Hz repetition rate, consisting of up to 2,700 bunches with a 220 ns spacing. Recorded images have to be stored inside the pixel during the bunch trains and readout in between. To meet these requirements, the European XFEL has launched 3 detector development projects. The AGIPD project is a collaboration between DESY, PSI and the Universities of Bonn and Hamburg. The goal is a 1024 × 1024 pixel detector, with 200 µm pixel size and a central hole for the primary beam. The ASIC operates in charge integration mode: the output of each pixels preamplifier is proportional to the charge from the sensor generated by the X-rays. The input stage of the pixel cells uses dynamically adjustable gains. The output signal is stored in an analogue memory, which has to be a compromise between noise performance and the number of images. This is operated in random access mode, providing means to overwrite bad frames for optimal use of the 352 memory cells per pixel, which have to be readout and digitized in the 99.4ms bunch gap. The detector will be built of 8 × 2 fully depleted monolithic silicon sensors with a 8 × 2 array of CMOS readout chips bump-bonded to these. Several prototypes of the readout ASIC have been produced. The results presented originate from the 16 × 16 pixel matrices AGIPD 0.2, which was bump-bonded to a pixel sensor, and AGIPD 0.3, which includes the intended control algorithm and a fast differential interface to the off-chip world.

Intensity Interferometry of Single X-Ray Pulses from a Synchrotron Storage Ring

We report on measurements of second-order intensity correlations at the high-brilliance storage ring PETRA III using a prototype of the newly developed adaptive gain integrating pixel detector. The detector records individual synchrotron radiation pulses with an x-ray photon energy of 14.4xa0keV and repetition rate of about 5xa0MHz. The second-order intensity correlation function is measured simultaneously at different spatial separations, which allows us to determine the transverse coherence length at these x-ray energies. The measured values are in a good agreement with theoretical simulations based on the Gaussian Schell model.

Performance tests of an AGIPD 0.4 assembly at the beamline P10 of PETRA III

The Adaptive Gain Integrating Pixel Detector (AGIPD) is a novel detector system, currently under development by a collaboration of DESY, the Paul Scherrer Institute in Switzerland, the University of Hamburg and the University of Bonn, and is primarily designed for use at the European XFEL. To verify key features of this detector, an AGIPD 0.4 test chip assembly was tested at the P10 beamline of the PETRA III synchrotron at DESY. The test chip successfully imaged both the direct synchrotron beam and single 7.05 keV photons at the same time, demonstrating the large dynamic range required for XFEL experiments. X-ray scattering measurements from a test sample agree with standard measurements and show the chips capability of observing dynamics at the microsecond time scale.

Towards AGIPD1.0: optimization of the dynamic range and investigation of a pixel input protection

AGIPD is a charge integrating, hybrid pixel readout ASIC, which is under development for the European XFEL [1,2]. A dynamic gain switching logic at the output of the preamplifier (preamp) is used to provide single photon resolution as well as covering a dynamic range of at least 104·12.4 keV photons [3,4]. Moreover, at each point of the dynamic range the electronics noise should be lower than the Poisson fluctuations, which is especially challenging at the points of gain switching. This paper reports on the progress of the chip design on the way to the first full-scale chip AGIPD1.0, focusing on the optimization of the dynamic range and the implementation of protection circuits at the preamplifier input to avoid pixel destruction due to high intense spots.

Study of the accumulation layer and charge losses at the Si–SiO2 interface in p+n-silicon strip sensors

Abstract Using the multi-channel transient current technique the currents induced by electron–hole pairs, produced by a focussed sub-nanosecond laser of 660xa0nm wavelength close to the Si–SiO 2 interface of p + n silicon strip sensors have been measured, and the charge-collection efficiency determined. The laser has been operated in burst mode, with bursts typically spaced by 1xa0ms, each consisting of 30 pulses separated by 50xa0ns. In a previous paper it has been reported that, depending on X-ray-radiation damage, biasing history and humidity, situations without charge losses, with hole losses, and with electron losses have been observed. In this paper we show for sensors before and after irradiation by X-rays to 1xa0MGy (SiO 2 ), how the charge losses change with the number of electron–hole pairs generated by each laser pulse, and the time interval between the laser pulses. This allows us to estimate how many additional charges in the accumulation layers at the Si–SiO 2 interface have to be trapped to significantly change the local electric field, as well as the time it takes that the accumulation layer and the electric field return to the steady-state situation. In addition, results are presented on the change of the pulse shape caused by the plasma effect for high charge densities deposited close to the Si–SiO 2 interface.

Optimization of the noise performance of the AGIPD prototype chips

The charge integrating readout electronics AGIPD (adaptive gain integrating pixel detector) is a hybrid detector system developed for the European XFEL. It features a threefold dynamic gain switching to be able to resolve single photons and to cover a dynamic range of 104·12.4 keV photons. As a result of dynamic gain switching, single photon resolution will be achieved in the high gain stage, while the maximum dynamic range will be reached in the low gain stage. The specification to resolve single photons requires a signal-over-noise ratio of at least 10 for a single incoming photon with an energy of 12.4 keV. When using a silicon sensor, that translates to an equivalent noise charge of less than 343 e-. Several AGIPD prototype chips have been designed and characterized, particularly focusing on the noise performance. During the testing phase, the dominant noise sources were identified and the corresponding circuit blocks were improved in the subsequent ASICs. This paper reports on the procedures to identify the dominating noise sources, the optimization process of the circuit blocks and discusses the effect of the optimization on the noise performance.© 2013 IOP Publishing Ltd and Sissa Medialab srl.