G. Schaller

Femtosecond x-ray protein nanocrystallography

X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded. It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction ‘snapshots’ are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source. We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes. More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (∼200 nm to 2 μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes. This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage.

Single mimivirus particles intercepted and imaged with an X-ray laser

X-ray lasers offer new capabilities in understanding the structure of biological systems, complex materials and matter under extreme conditions. Very short and extremely bright, coherent X-ray pulses can be used to outrun key damage processes and obtain a single diffraction pattern from a large macromolecule, a virus or a cell before the sample explodes and turns into plasma. The continuous diffraction pattern of non-crystalline objects permits oversampling and direct phase retrieval. Here we show that high-quality diffraction data can be obtained with a single X-ray pulse from a non-crystalline biological sample, a single mimivirus particle, which was injected into the pulsed beam of a hard-X-ray free-electron laser, the Linac Coherent Light Source. Calculations indicate that the energy deposited into the virus by the pulse heated the particle to over 100,000 K after the pulse had left the sample. The reconstructed exit wavefront (image) yielded 32-nm full-period resolution in a single exposure and showed no measurable damage. The reconstruction indicates inhomogeneous arrangement of dense material inside the virion. We expect that significantly higher resolutions will be achieved in such experiments with shorter and brighter photon pulses focused to a smaller area. The resolution in such experiments can be further extended for samples available in multiple identical copies.

Design and technology of DEPFET pixel sensors for linear collider applications

The performance requirements of vertex detectors for future linear collider experiments is very challenging especially for the detectors innermost sensor layers. The DEPleted Field Effect Transistor (DEPFET) combining detector and amplifier operation is capable to meet these requirements. A silicon technology is presented which allows production of large sensor arrays consisting of linear DEPFET detector structures. The envisaged pixel array offers a low noise and low power operation. To ensure a high radiation length a thinning technology based on direct wafer bonding is proposed.

New results on DEPFET pixel detectors for radiation imaging and high energy particle detection

DEPFET pixel detectors are unique devices in terms of energy and spatial resolution because very low noise (ENC = 2.2e at room temperature) operation can be obtained by implementing the amplifying transistor in the pixel cell itself. Full DEPFET pixel matrices have been built and operated for autoradiographical imaging with imaging resolutions of 4.3 /spl plusmn/ 0.7 lp/mm at 22 keV. For applications in low energy X-ray astronomy the high energy resolution of DEPFET detectors is attractive. For particle physics, DEPFET pixels are interesting as low material detectors with high spatial resolution. For a Linear Collider detector the readout must be very fast. New readout chips have been designed and produced for the development of a DEPFET module for a pixel detector at the proposed TESLA collider (520 /spl times/ 4000 pixels) with 50 MHz line rate and 25 kHz frame rate. The circuitry contains current memory cells and current hit scanners for fast pedestal subtraction and sparsified readout. The imaging performance of DEPFET devices as well as present achievements towards a DEPFET vertex detector for a Linear Collider are presented.

DEPFET based focal plane instrumentation for X-ray imaging spectroscopy in space

The combined Detector-Amplifier structure DEPFET (Depleted P-channel FET) is a promising new building block for large area silicon detector devices, e.g. in X-ray astronomy and high energy physics. The DEPFET structure combines excellent energy resolution, high speed readout and low power consumption with the attractive features of random accessibility of pixels and on-demand readout. In addition, it features all advantages of a sideways depleted device in terms of fill factor and quantum efficiency. Finally, the newly introduced combination of a DEPFET structure and a silicon drift diode (SDD) like drift ring structure to form a so-called macropixel device allows for large flexibility in terms of pixel size. Presently, focal plane instrumentation for X-ray imaging spectroscopy based on DEPFET arrays is being developed for a variety of space experiments with very different requirements. The next European X-ray Observatory XEUS is going to have a wide field imager covering the full FOV, which consists of a large-area DEPFET array. The concept for the French-Italian X-ray Astronomy mission SIMBOL-X includes a focal plane array based on DEPFET macropixels, and, finally, the MIXS (Mercury Imaging X-ray Spectrometer) instrument on the European Mercury exploration mission BepiColombo also contains two DEPFET macropixel based focal plane arrays. While for XEUS and SIMBOL-X excellent energy resolution and quantum efficiency in the low energy range are mandatory, radiation hardness is imperative for MIXS. A first production of DEPFET prototype arrays showed very promising results. More sophisticated prototype devices for SIMBOL-X and XEUS with a large sensitive area as well as flight grade devices for the MIXS instrument have been produced at the MPI semiconductor laboratory in Munich/Germany. The strategies to meet the respective requirements by an appropriate design of the focal plane instrumentation are shown as well as first results of the new production.

DEPFET active pixel sensor with non-linear amplification

One of the X-ray imaging detectors in development for the European XFEL (X-ray Free Electron Laser) is the DSSC (DEPFET Sensor with Signal Compression). The DSSC sensor array will have a format of 1024 × 1024 pixels with a pixel size in the order of 200 µm. It is optimized for the detection of low-energy X-ray photons and designed to operate at frame rates up to 4.5 MHz with a dynamic range of several thousand photons per pixel and frame and single photon resolution at small photon numbers. The DSSC systems core component is an active pixel sensor based on the DEPFET (DEpleted P-channel Field Effect Transistor) structure. A DEPFET is an integrated detector-amplifier combining internal amplification, full sensitivity over the whole bulk thickness, analog data storage, readout on demand, low serial noise, and absence of reset noise. For the DSSC-adaptation of the DEPFET principle the new feature of signal compression has been added, i.e. the device has a high gain at small signals and a reduced gain at large signals. With this strongly non-linear response the DSSC-DEPFET provides the required high charge handling capacity with simultaneous single photon resolution for low-energy X-rays. This paper presents the concept of the DSSC-DEPFET. Its functionality is demonstrated by measurements of a first series of prototypes.

DEPFET Macropixel Detectors for MIXS: Integration and Qualification of the Flight Detectors

The Mercury imaging X-ray spectrometer (MIXS) on board of ESAs fifth cornerstone mission BepiColombo will be the first space instrument using DEpleted P-channel FET (DEPFET) based detectors. The MIXS spectrometer comprises two channels with identical focal plane detectors and is dedicated to energy resolved imaging of X-ray fluorescence from the mercurial surface. We report on the characterization, integration, and spectroscopic qualification of MIXS flight detectors. Detector chips were precharacterized at die level in order to select the best dies for integration and to do homogeneity and yield studies. Then, the detector chips were integrated to MIXS Detector Plane Arrays (DPAs), a complicated process due to the sophisticated mechanical structure, which allows the thermal decoupling of the detector from its readout and control chips. After integration, spectroscopic qualification measurements were done in order to analyze the detector performance and to prove the excellent spectroscopic performance of the DEPFET Macropixel detectors over a wide temperature range. The integration and spectroscopic qualification of all flight grade modules is now successfully completed.

Advancements in DEPMOSFET device developments for XEUS

DEPMOSFET based Active Pixel Sensor (APS) matrices are a new detector concept for X-ray imaging spectroscopy missions. They can cope with the challenging requirements of the XEUS Wide Field Imager and combine excellent energy resolution, high speed readout and low power consumption with the attractive feature of random accessibility of pixels. From the evaluation of first prototypes, new concepts have been developed to overcome the minor drawbacks and problems encountered for the older devices. The new devices will have a pixel size of 75 μm × 75 μm. Besides 64 × 64 pixel arrays, prototypes with a sizes of 256 × 256 pixels and 128 × 512 pixels and an active area of about 3.6 cm2 will be produced, a milestone on the way towards the fully grown XEUS WFI device. The production of these improved devices is currently on the way. At the same time, the development of the next generation of front-end electronics has been started, which will permit to operate the sensor devices with the readout speed required by XEUS. Here, a summary of the DEPFET capabilities, the concept of the sensors of the next generation and the new front-end electronics will be given. Additionally, prospects of new device developments using the DEPFET as a sensitive element are shown, e.g. so-called RNDR-pixels, which feature repetitive non-destructive readout to lower the readout noise below the 1 e- ENC limit.

A New High-Speed, Single Photon Imaging CCD for the Optical

We report on first measurements from test structures verifying a new design concept of a single photon imaging CCD for the optical. The results confirm the sensitivity of a novel avalanche diode to single electrons. Details of this structure which can be combined with a back illuminated sensor are described, measurement results include I-V curves, dark rate and temperature dependency. In addition an avalanche diode with MOSFET readout will be presented as well as an ultra low noise pnCCD which is process compatible. The successful testing of these components proves the feasibility to produce a back-illuminated single photon sensitive CCD with high frame rates and high sensitivity in a wide wavelength range.

DEPFET based X-ray detectors for the MIXS focal plane on BepiColombo

DEPFET Macropixel detectors, based on the fusion of the combined Detector-Amplifier structure DEPFET with a silicon drift chamber (SDD) like drift ring structure, combine the excellent properties of the DEPFETs with the advantages of the drift detectors. As both device concepts rely on the principle of sideways depletion, a device entrance window with excellent properties is obtained at full depletion of the detector volume. DEPFET based focal plane arrays have been proposed for the Focal Plane Detectors for the MIXS (Mercury Imaging X-ray Spectrometer) instrument on BepiColombo, ESAs fifth cornerstone mission, with destination Mercury. MIXS uses a lightweight Wolter Type 1 mirror system to focus fluorescent radiation from the Mercury surface on the FPA detector, which yields the spatially resolved relative element abundance in Mercurys crust. In combination with the reference information from the Solar Intensity X-ray Spectrometer (SIXS), the element abundance can be measured quantitatively as well. The FPA needs to have an energy resolution better than 200 eV FWHM @ 1 keV and is required to cover an energy range from 0.5 keV to 10 keV, for a pixel size of 300 x 300 μm2. Main challenges for the instrument are the increase in leakage current due to a high level of radiation damage, and the limited cooling resources due to the difficult thermal environment in the mercury orbit. By applying an advanced cooling concept, using all available cooling power for the detector itself, and very high speed readout, the energy resolution requirement can be kept during the entire mission lifetime up to an end-of-life dose of ~ 3 × 1010 10 MeV p / cm2. The production of the first batch of flight devices has been finished at the MPI semiconductor laboratory, and first prototype modules have been built. The results of the first tests will be presented here.