G. Haller

High-resolution protein structure determination by serial femtosecond crystallography

Size Matters Less X-ray crystallography is a central research tool for uncovering the structures of proteins and other macromolecules. However, its applicability typically requires growth of large crystals, in part because a sufficient number of molecules must be present in the lattice for the sample to withstand x-ray—induced damage. Boutet et al. (p. 362, published online 31 May) now demonstrate that the intense x-ray pulses emitted by a free-electron laser source can yield data in few enough exposures to uncover the high-resolution structure of microcrystals. A powerful x-ray laser source can probe proteins in detail using much smaller crystals than previously required. Structure determination of proteins and other macromolecules has historically required the growth of high-quality crystals sufficiently large to diffract x-rays efficiently while withstanding radiation damage. We applied serial femtosecond crystallography (SFX) using an x-ray free-electron laser (XFEL) to obtain high-resolution structural information from microcrystals (less than 1 micrometer by 1 micrometer by 3 micrometers) of the well-characterized model protein lysozyme. The agreement with synchrotron data demonstrates the immediate relevance of SFX for analyzing the structure of the large group of difficult-to-crystallize molecules.

DESIGN AND PERFORMANCE OF THE SLD VERTEX DETECTOR : A 307 MPIXEL TRACKING SYSTEM

This paper describes the design, construction, and initial operation of SLDs upgraded vertex detector which comprises 96 two-dimensional charge-coupled devices (CCDs) with a total of 307 Mpixel. Each pixel functions as an independent particle detecting element, providing space point measurements of charged particle tracks with a typical precision of 4 μm in each co-ordinate. The CCDs are arranged in three concentric cylinders just outside the beam-pipe which surrounds the e+e− collision point of the SLAC Linear Collider (SLC). The detector is a powerful tool for distinguishing displaced vertex tracks, produced by decay in flight of heavy flavour hadrons or tau leptons, from tracks produced at the primary event vertex. The requirements for this detector include a very low mass structure (to minimize multiple scattering) both for mechanical support and to provide signal paths for the CCDs; operation at low temperature with a high degree of mechanical stability; and high speed CCD readout, signal processing, and data sparsification. The lessons learned in achieving these goals should be useful for the construction of large arrays of CCDs or active pixel devices in the future in a number of areas of science and technology.

The CSPAD megapixel x-ray camera at LCLS

The Linear Coherent Light Source (LCLS), a free electron laser operating from 250eV to10keV at 120Hz, is opening windows on new science in biology, chemistry, and solid state, atomic, and plasma physics1,2. The FEL provides coherent x-rays in femtosecond pulses of unprecedented intensity. This allows the study of materials on up to 3 orders of magnitude shorter time scales than previously possible. Many experiments at the LCLS require a detector that can image scattered x-rays on a per-shot basis with high efficiency and excellent spatial resolution over a large solid angle and both good S/N (for single-photon counting) and large dynamic range (required for the new coherent x-ray diffractive imaging technique3). The Cornell-SLAC Pixel Array Detector (CSPAD) has been developed to meet these requirements. SLAC has built, characterized, and installed three full camera systems at the CXI and XPP hutches at LCLS. This paper describes the camera system and its characterization and performance.

X‐ray detectors at the Linac Coherent Light Source

This paper offers an overview of area detectors developed for use at the Linac Coherent Light Source (LCLS) with particular emphasis on their impact on science. The experimental needs leading to the development of second-generation cameras for LCLS are discussed and the new detector prototypes are presented.

A 700-MHz switched-capacitor analog waveform sampling circuit

Analog switched-capacitor memory circuits are suitable for use in a wide range of applications where analog waveforms must be captured or delayed, such as the recording of pulse echo events and pulse shapes. Analog sampling systems based on switched-capacitor techniques offer performance superior to that of flash A/D converters and charge-coupled devices with respect to cost, density, dynamic range, sampling speed, and power consumption. This paper proposes an architecture with which sampling frequencies of several hundred megahertz can be achieved using conventional CMOS technology. Issues concerning the design and implementation of an analog memory circuit based on the proposed architecture are presented. An experimental two-channel memory with 32 sampling cells in each channel has been integrated in a 2-/spl mu/m CMOS technology with poly-to-poly capacitors. The measured nonlinearity of this prototype is 0.03% for a 2.5 V input range, and the memory cell gain matching is 0.01% rms. The dynamic range of the memory exceeds 12 b for a sampling frequency of 700 MHz. The power dissipation for one channel operated from a single +5 V supply is 2 mW. >

Performance Report for Stanford/SLAC Multi-Channel Sample-and-Hold Device

Test results on a newly developed Multi-Channel Sample- And-Hold Calorimeter Data Unit (CDU) are presented. The device is organized as 32 input channels, each consisting of four storage cells to take samples of the 32 analog signals at four separate times. The design goals for the development were wide dynamic range and long hold times. Therefore, each storage cell is laid out in a fully differential way and consists of a sampling stage for the signal and another identical stage for a reference voltage. Results on the performance of the device are described.

The Cornell-SLAC pixel array detector at LCLS

The Cornell-SLAC pixel array detector (CSpad) is a general-purpose integrating hybrid pixel x-ray camera developed for use at the Linear Coherent Light Source (LCLS) x-ray free electron laser at the SLAC National Accelerator Laboratory (SLAC). The detector has a full well capacity of about 2.Sk photons in low-gain mode and a SIN of about 6 in high-gain mode. Its 2.3M pixels are read out at 120 Hz. The detector comprises 32 500μm silicon sensors bump-bonded to 64 185×194-pixel ASICs. The pixel size is 110μm. The water-cooled detector quadrants can be radially moved in-situ to vary the beam aperture. SLAC has built, calibrated, and optimized three complete camera systems based on a sensor and ASIC designed by Cornell. The camera is read out by a DAQ system which provides extensive online monitoring and prompt analysis capabilities. We have also built a dozen smaller cameras in a portable form-factor for use in confined spaces and for ease of development, testing, and deployment. Through 2012 user experiments have taken almost a petabyte of data with these detectors in a variety of applications. We have extensively tested the detector at synchrotrons and with an x-ray tube, in addition to commissioning tests at the LCLS, investigating linearity, cross-talk, homogeneity, and radiation hardness. The SLAC detector group is deploying improved support infrastructure and an updated ASIC and electronics based on this experience. This paper describes the instrument, its calibration and performance, and presents preliminary results from the updated camera.

Waveform Sampler CAMAC Module

A Waveform Sampler Module (WSM) for the measurement of signal shapes coming from the multi-hit drift chambers of the SLAC SLD detector is described. The module uses a high speed, high resolution analog storage device (AMU) developed in collaboration between SLAC and Stanford University. The AMU devices together with high speed TTL clocking circuitry are packaged in a hybrid which is also suitable for mounting on the detector. The module is in CAMAC format and provides eight signal channels, each recording signal amplitude versus time in 512 cells at a sampling rate of up to 360 MHz. Data are digitized by a 12-bit ADC with a 1 ¿s conversion time and stored in an on-board memory accessible through CAMAC.

An analog memory integrated circuit for waveform sampling up to 900 MHz

The design and implementation of a switched-capacitor memory suitable for capturing high-speed analog waveforms is described. Highlights of the presented circuit are a 900 MHz sampling frequency (generated on chip), input signal independent cell pedestals and sampling instances, and cell gains that are insensitive to component sizes. A two-channel version of the memory with 32 cells for each channel has been integrated in a 2-/spl mu/m complementary metal oxide semiconductor (CMOS) process with polysilicon-to-polysilicon capacitors. The measured rms cell response variation in a channel after cell pedestal subtraction is less than 0.3 mV across the full input signal range. The cell-to-cell gain matching is better than 0.01% rms, and the nonlinearity is less than 0.03% for a 2.5-V input range. The dynamic range of the memory exceeds 13 bits, and the peak signal-to-(noise+distortion) ratio for a 21.4 MHz sine wave sampled at 900 MHz is 59 dB. >

Analog floating-point BiCMOS sampling chip and architecture of the BaBar CsI calorimeter front-end electronics system at the SLAC B-factory

The design and implementation of an analog floating-point sampling integrated circuit for the BaBar detector at the SLAC B-Factory is described. The CARE (Custom Auto-Range Encoding) circuit is part of an 18-bit dynamic range sampling system with a 4-MHz waveform digitization rate for the CsI calorimeter. The architecture and methodology of the system are described. The CARE integrated circuit receives dual-range (gain of 1 and 32) 13-bit signals from the 18-bit range preamplifiers mounted directly on the CsI crystals and converts the input at a rate of 4 MHz to an auto-range floating-point format with a 10-bit analog mantissa and 2 digital range bits (for 4 ranges). Additional functions integrated on the chip are averaging and selection circuitry for signals originating from two independent diodes per crystal and range-selection overwrite circuitry. The circuit will be mounted within the detector structure and thus low power dissipation is essential. The circuit has been fabricated in a 1.2-/spl mu/m BiCMOS process with polysilicon-to-polysilicon capacitors and polysilicon resistors. Measurement results are presented. One complete CARE channel dissipates 25 mW.