Lothar Strüder

The European Photon Imaging Camera on XMM-Newton: The pn-CCD camera

The European Photon Imaging Camera (EPIC) consortium has provided the focal plane instruments for the three X-ray mirror systems on XMM-Newton. Two cameras with a reflecting grating spectrometer in the optical path are equipped with MOS type CCDs as focal plane detectors (Turner 2001), the telescope with the full photon flux operates the novel pn-CCD as an imaging X-ray spectrometer. The pn-CCD camera system was developed under the leadership of the Max-Planck-Institut fur extraterrestrische Physik (MPE), Garching. The concept of the pn-CCD is described as well as the dierent operational modes of the camera system. The electrical, mechanical and thermal design of the focal plane and camera is briefly treated. The in-orbit performance is described in terms of energy resolution, quantum eciency, time resolution, long term stability and charged particle background. Special emphasis is given to the radiation hardening of the devices and the measured and expected degradation due to radiation damage of ionizing particles in the rst 9 months of in orbit operation.

Sub-pixel resolution of a pnCCD for X-ray white beam applications

A new approach to achieve sub-pixel spatial resolution in a pnCCD detector with 75 × 75 μm2 pixel size is proposed for X-ray applications in single photon counting mode. The approach considers the energy dependence of the charge cloud created by a single photon and its split probabilities between neighboring pixels of the detector based on a rectangular model for the charge cloud density. For cases where the charge of this cloud becomes distributed over three or four pixels the center position of photon impact can be reconstructed with a precision better than 2 μm. The predicted charge cloud sizes are tested at selected X-ray fluorescence lines emitting energies between 6.4 keV and 17.4 keV and forming charge clouds with size (rms) varying between 8 μm and 10 μm respectively. The 2 μm enhanced spatial resolution of the pnCCD is verified by means of an x-ray transmission experiment throughout an optical grating.

Data analysis for characterizing PNCCDS

The Max-Planck-Institute semiconductor lab develops, fabricates, tests, and qualifies pnCCDs for space and ground based applications. pnCCDs are CCDs showing high quantum efficiency up to 20 keV while delivering good spatial and energy resolution. This article describes the algorithms applied to the raw data as recorded by the data acquisition system. The main purpose of the underlying software is to qualify the individual pnCCD by measurements of monoenergetic X-ray lines, from B-K (183 eV) to Mo-Kα (17.5 keV), typically Mn-Kα (5.9 keV) under various conditions (e.g. temperature, readout speed, electrical supply voltages of the detector and electronics). Therefore characteristic parameters are determined individually for each measurement as there are read noise, gains, charge transfer efficiencies, charge splitting between neighboring pixels, energy resolution, and bad pixels while correcting for offsets, gains, charge transfer inefficiencies, non-linearities of the electronics, and while recombining the charges spread over more than one pixel. These figures are used in three ways: Firstly, operating parameters are optimized by comparing individual measurements. Secondly, the individual device is rated by combining the results of all its measurements. Thus devices can be selected for applications such as measurement setups for DESY, FLASH, or the X-ray test facility PANTER. Especially the flight modules for the X-ray astronomy mission eROSITIA will be chosen based on the key figures. Thirdly, improvements gained from detector and electronics design and production modifications are quantified closing the development loop of pnCCDs and their associated electronics.

Analysis of polycrystallinity in hen egg-white lysozyme using a pnCCD

A crystal of hen egg-white lysozyme was analyzed by means of energy-dispersive X-ray Laue diffraction with white synchrotron radiation at 2.7u2005A resolution using a pnCCD detector. From Laue spots measured in a single exposure of the arbitrarily oriented crystal, the lattice constants of the tetragonal unit cell could be extracted with an accuracy of about 2.5%. Scanning across the sample surface, Laue images with split reflections were recorded at various positions. The corresponding diffraction patterns were generated by two crystalline domains with a tilt of about 1° relative to each other. The obtained results demonstrate the potential of the pnCCD for fast X-ray screening of crystals of macromolecules or proteins prior to conventional X-ray structure analysis. The described experiment can be automatized to quantitatively characterize imperfect single crystals or polycrystals.

Characterization of eROSITA PNCCDs

The eROSITA (extended Roentgen Survey with an Imaging Telecope Array) instrument on the satellite Spektrum-Roentgen-Gamma will perform an all-sky survey as well as pointed observations in the low and medium X-ray energy regime. The seven PNCCD detectors for this instrument have been developed and manufactured at the Max Planck Institute (MPI) Semiconductor Laboratory. We summarize the characterization of this type of PNCCD regarding the spectral redistribution function in the photon energy range from 100 eV to 11 keV. We present the results of Geant4 simulations compared to our measurements. The observed spectral features and their physical origin inside the detector are explained. An overview on the different contributions to the energy resolution of the detector is given.

Application of a pnCCD for energy-dispersive Laue diffraction with ultra-hard X-rays

In this work the spectroscopic performance of a pnCCD detector in the ultra-hard energy range between 40 and 140u2005keV is tested by means of an energy-dispersive Laue diffraction experiment on a GaAs crystal. About 100 Bragg peaks were collected in a single-shot exposure of the arbitrarily oriented sample to white synchrotron radiation provided by a wiggler at BESSY II and resolved in a large reciprocal-space volume. The positions and energies of individual Laue spots could be determined with a spatial accuracy of less than one pixel and a relative energy resolution better than 1%. In this way the unit-cell parameters of GaAs were extracted with an accuracy of 0.5%, allowing for a complete indexing of the recorded Laue pattern. Despite the low quantum efficiency of the pnCCD (below 7%), experimental structure factors could be obtained from the three-dimensional data sets, taking into account photoelectric absorption as well as Compton scattering processes inside the detector. The agreement between measured and theoretical kinematical structure factors calculated from the known crystal structure is of the order of 10%. The results of this experiment demonstrate the potential of pnCCD detectors for applications in X-ray structure analysis using the complete energy spectrum of synchrotron radiation.

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser

Beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics for the installation of the new permanent CAMP end-station, a multi-purpose instrument optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments. An overview of the layout, beam transport, focusing capabilities, and experimental possibilities of this new end-station, as well as results from its commissioning and first experiments, are presented.

Direct measurement of the position accuracy for low energy X-ray photons with a pnCCD

We undertook a comparative study on optimizing the position accuracy of pnCCDs for single X-ray photon measurements. Various methods were analyzed by Monte Carlo simulations and related to experimental data obtained with a focused X-ray beam. Even with low energy photons of 1320 eV, a position accuracy much smaller than the actual pixel size of 48 μm × 48 μm can be achieved. This is possible since signal charges from a single photon interaction spread into more than one pixel, allowing a reconstruction of the original point of interaction. We found that a) making a decision on which pixels to use for the reconstruction and b) choosing a centroiding algorithm for carrying out the reconstruction were particularly crucial. For a) we introduce a new and superior method using a two step analysis with an adaptive pattern. It is compared to using a threshold or a fixed pattern. For b) we present a Center-of-Gravity method with a Gaussian correction taking into account the shape of the signal charge cloud. Both methods are also optimized for fast execution by implementing lookup tables rather than time consuming calculations. Our results show that with the appropriate analysis an uncertainty of the position measurement of better than 3.0 μm rms for 1320 eV photons is possible.

High speed simultaneous X-ray and electron imaging and spectroscopy at synchrotrons and TEMs

Silicon is a good detector material for ionizing radiation as it has sufficient stopping power for e.g. particles and X-rays. In a standard configuration with 500 µm thick Silicon X-rays up to 25 keV can be efficiently converted into signal charges as well as e.g. electrons up to 300 keV. We have developed a variety of position, energy and time resolving detectors with a sensitive thickness ranging from 50 µm to 500 µm and sensitive areas from 1 cm2 to 30 cm2. They are being used in basic research (e.g. astrophysics, high energy physics, material sciences) as well as in industrial applications (e.g. in SEM, TEM, handheld XRF spectrometers). All the devices are based on the principle of “sideward depletion”: Silicon Drift Detectors (SDDs), fully depleted pnjunction Charge Coupled Devices (pnCCDs) and the more recent Active Pixel Sensors based on the Depleted P-channel Field Effect Transistor (DePFET). They all exhibit remarkable properties for the detection of ionizing radiation, especially for photons and electrons: (a) low noise operation (2 – 5 electrons rms) close to room temperature, (b) simultaneous fast signal processing leading to high count rates, (c) sub-pixel position resolution, (d) high radiation tolerance due to the lack of active MOS structures and low energy thresholds for X-ray photons (50 eV) and electrons (500 eV) due to the back-illuminated device structures. All the above properties are simultaneously fulfilled in one device. The choice on whether a SDD, a pnCCD or a DePFET is adequate, is given by the main focus of a measurement: e.g. collecting, sensitive area, position resolution or count rate capability.