M. Weaver

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.

Development of a compact fast CCD camera and resonant soft x-ray scattering endstation for time-resolved pump-probe experiments

The designs of a compact, fast CCD (cFCCD) camera, together with a resonant soft x-ray scattering endstation, are presented. The cFCCD camera consists of a highly parallel, custom, thick, high-resistivity CCD, readout by a custom 16-channel application specific integrated circuit to reach the maximum readout rate of 200 frames per second. The camera is mounted on a virtual-axis flip stage inside the RSXS chamber. When this flip stage is coupled to a differentially pumped rotary seal, the detector assembly can rotate about 100°/360° in the vertical/horizontal scattering planes. With a six-degrees-of-freedom cryogenic sample goniometer, this endstation has the capability to detect the superlattice reflections from the electronic orderings showing up in the lower hemisphere. The complete system has been tested at the Advanced Light Source, Lawrence Berkeley National Laboratory, and has been used in multiple experiments at the Linac Coherent Light Source, SLAC National Accelerator Laboratory.

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.

Performance of an LPD prototype detector at MHz frame rates under Synchrotron and FEL radiation

A MHz frame rate X-ray area detector (LPD — Large Pixel Detector) is under development by the Rutherford Appleton Laboratory for the European XFEL. The detector will have 1 million pixels and allows analogue storage of 512 images taken at 4.5 MHz in the detector front end. The LPD detector has 500 μm thick silicon sensor tiles that are bump bonded to a readout ASIC. The ASICs preamplifier provides relatively low noise at high speed which results in a high dynamic range of 105 photons over an energy range of 5–20 keV. Small scale prototypes of 32 × 256 pixels (LPD 2-Tile detector) and 256 × 256 pixels (LPD supermodule detector) are now available for X-ray tests. The performance of prototypes of the detector is reported for first tests under synchrotron radiation (PETRA III at DESY) and Free-Electron-Laser radiation (LCLS at SLAC). The initial performance of the detector in terms of signal range and noise, radiation hardness and spatial and temporal response are reported. The main result is that the 4.5 MHz sampling detection chain is reliably working, including the analogue on-chip memory concept. The detector is at least radiation hard up to 5 MGy at 12 keV. In addition the multiple gain concept has been demonstrated over a dynamic range to 104 at 12 keV with a readout noise equivalent to < 1 photon rms in its most sensitive mode.

Detector Development for the Linac Coherent Light Source

Since it began operations in 2009, the Linac Coherent Light Source (LCLS) has opened a new and dynamic frontier in terms of light sources and their associated science [1, 2]. An increase in brightness by a factor of a billion over pre-existing synchrotrons, in combination with ultra-brief pulses of coherent X-rays, is ushering in a new era in the photon sciences. Pulses with durations of 50 fs under standard conditions and below 10 fs with a reduced energy per bunch are possible. Over 1013 or 1012 X-rays per pulse can be generated at the upper and lower ends of the X-ray energy range of 285 eV to 9600 eV. One of the unique machine parameters is its strobe-like time structure, where single ultra-brief pulses are delivered at a repetition rate of 120 Hz. The above characteristics represent a singular environment in which to operate detectors and demand the development of a new class of high-frame-rate camera systems.

Femtosecond optical/hard X-ray timing diagnostics at an FEL: implementation and performance

The development of Free Electron Lasers has opened the possibility to investigate ultrafast processes using femtosecond hard x-ray pulses. In optical/x-ray light pump/probe experiments, however, the time resolution is mainly limited by the ability to synchronize both light sources over a long distance (<100 fs FWHM) rather than their pulse length (<10 fs FWHM). We have implemented a spectrally encoding x-ray to optical laser timing diagnostic into the XPP beamline at LCLS with a timing uncertainty down to 10 fs. An x-ray induced change of refractive index in a solid target is temporally probed for single pulses by a chirped white light pulse [4]. By resorting single shot data to the timestamps obtained by the diagnostics, the temporal data quality can be improved to basically pulse length limited time resolution. By interchangable targets and adjustable x-ray and laser foci, the method was successfully applied for very different x-ray parameters. These are different photon energies in the range of 6-20 keV, which at LCLS also includes application of 3rd Harmonic radiation, pulse energy, and bandwidth, when using a Si(111) monochromator.

Design of the SLAC RCE Platform: A general purpose ATCA based data acquisition system

The SLAC RCE platform is a general purpose clustered data acquisition system implemented on a custom ATCA compliant blade, called the Cluster On Board (COB). The core of the system is the Reconfigurable Cluster Element (RCE), which is a system-on-chip design based upon the Xilinx Zynq family of FPGAs, mounted on custom COB daughter-boards. The Zynq architecture couples a dual core ARM Cortex A9 based processor with a high performance 28nm FPGA. The RCE has 12 external general purpose bi-directional high speed links, each supporting serial rates of up to 12Gbps. 8 RCE nodes are included on a COB, each with a 10Gbps connection to an on-board 24-port Ethernet switch integrated circuit. The COB is designed to be used with a standard full-mesh ATCA backplane allowing multiple RCE nodes to be tightly interconnected with minimal interconnect latency. Multiple shelves can be clustered using the front panel 10-gbps connections. The COB also supports local and inter-blade timing and trigger distribution. An experiment specific Rear Transition Module adapts the 96 high speed serial links to specific experiments and allows an experiment-specific timing and busy feedback connection. This coupling of processors with a high performance FPGA fabric in a low latency, multiple node cluster allows high speed data processing that can be easily adapted to any physics experiment. RTEMS as well as Linux are ported to the module. The RCE has been used or is the baseline for several current and proposed experiments (LCLS, HPS, LSST, ATLAS-CSC, LBNE, DarkSide, ILC-SiD, etc).

2nd generation cameras for LCLS and the new challenges of high repetition rates at LCLS-II

With the experience of the first years of operation of the Linac Coherent Light Source (LCLS), SLAC developed a 2nd generation camera system with improved features and performance. The first camera to be deployed is the ePix-One, a compact camera which is a 155 mm long box with a quadratic front face of 52×52 mm2 which will feature 4 ASICs, either the ePIX100 or the ePIX10k, bump-bonded with a single sensor offering 35 × 38 mm2 active area. Combined with the ePIX100 hybrid pixel module which features 50 μm pixels and is targeted for X-ray Photon Correlation Spectroscopy and as a detector in wavelength dispersive spectrometer setups this will result in a 0.5Mpixel camera. Whereas the 100 μm pixels of ePIX10k, targeted towards protein crystallography, imaging and pump probe experiments, will provide a camera of 135kpixel. The camera uses simple Peltier/water cooling in combination with dry nitrogen purge against condensation. The compact housing and the simple interface (26pin cable & optical fiber) eases deployment and gives experimenters more flexibility in utilizing the camera where needed. The current ePix cameras support full frame readout faster than 120Hz and ROI modes which can be read at up to 1kHz rate. Next developments will target larger cameras and higher frame rates for the upcoming LCLS II.

Studies of the ePix100 low-noise x-ray camera at SLAC

A new hybrid pixel array detector, the ePix100, has been developed at SLAC for tender and hard x-ray experiments at the Linac Coherent Light Source (LCLS). It is intended for low noise and good spatial resolution applications, particularly X-ray Photon Correlation Spectroscopy (XPCS) and in combination with wavelength dispersive spectrometers. The detector has 50 μm pixel size and less than 100 e- r.m.s. noise over the range of tested operating conditions. A series of measurements to validate its performance with x-rays was carried out at the Stanford Synchrotron Radiation Lightsource (SSRL) and LCLS. Results are here reported and discussed.