F. Schopper

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.

The MEGA advanced Compton telescope project

Abstract The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4–50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.

Concept study for the next generation medium energy gamma-ray astronomy mission - MEGA

A new telescope for Medium Energy Gamma-Ray Astronomy, MEGA, is being developed for the energy band 0.4 - 50 MeV as a successor to COMPTEL on CGRO. MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude with respect to past instruments and will fill a severe sensitivity gap between already scheduled hard-X-ray and high-energy gamma-ray missions. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. MEGA will have an effective area of ~100 square cm, a large field of view of ~130 degrees, angular resolution of ~2 degrees, and energy resolution of ~8% (all FWHM at ~2 MeV). Key science objectives for MEGA are the investigation of cosmic high-energy accelerators, nucleosynthesis sites with gamma-ray lines, and the mapping of large-scale structures in the Galaxy and beyond. If operated on a zenith pointing satellite MEGA will be an ideal continuous all-sky monitor for transient sources. This paper describes the development of a small scale prototype and the concept of a space mission for MEGA.

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.

MEGAlib: simulation and data analysis for low-to-medium-energy gamma-ray telescopes

The Medium-Energy Gamma-ray Astronomy library MEGAlib is an open-source object-oriented software library designed to simulate and analyze data of low-to-medium-energy gamma-ray telescopes, especially Compton telescopes. The library comprises all necessary simulation and data analysis tools including geometry construction, Monte-Carlo simulation, response creation, event reconstruction, image reconstruction, and other high-level data-analysis tools.

CsI calorimeter with 3-D position resolution

Abstract New γ-ray calorimeter have been developed for the MEGA Compton camera. They consist of arrays of small CsI(Tl) scintillator bars read out by Silicon PIN-diodes and low noise, self-triggering frontend electronics. The length of the bars (the thickness of the calorimeter) can be varied for different applications to fit the stopping power needed and the light loss tolerable. In this paper we present calibration results from 2 cm long bars with diodes on one side, and 8 cm long bars with diodes on two opposite sides. Double-sided readout gives 3-D information of interactions which will be used to overcome the limited position resolution in Anger-cameras at high energies. Simpler detection devices like Anger-cameras might finally resolve only the centre of gravity. As events from γ-rays with energies of MeV do extend over several cm, it is a prerequisite for an imaging device to resolve the interaction structure in detail. Combining CsI(Tl) scintillators, Silicon PIN-photodiodes and frontend electronics inside the housing results in a cheap rugged design. While the development in our institute is mainly done for the Compton camera prototype, many other applications are conceivable.

Development and Characterization of New 256

DEPFET detectors are silicon (Si) active pixel sensors designed and manufactured in the Max-Planck-Institut semiconductor lab. Their high spatial resolution and high energy resolution in X-rays make them attractive for particle tracking in colliders and for X-ray astronomy. This technology is foreseen for the Wide Field Imager of the International X-ray Observatory currently in study with ESA, NASA, and JAXA. New DEPFET matrixes with 256 × 256 pixels of 75-μm pitch have been produced, mounted on ceramic boards with dedicated front-end electronics and integrated in a new setup able to acquire large-format images and spectra. Excellent homogeneity has been observed. Energy resolution as low as 127 eV FWHM at 5.9 keV has been obtained including all single events of the matrix back illuminated at -45<;°C and read out at a 300-frames/s rate. This paper presents experimental methods and results.