A. Tomada

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.

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.

Design and characterization of the ePix10k prototype: A high dynamic range integrating pixel ASIC for LCLS detectors

ePix10k is a variant of a novel class of integrating pixel ASICs architectures optimized for the processing of signals in second generation LINAC Coherent Light Source (LCLS) X-Ray cameras. The ASIC is optimized for high dynamic range application requiring high spatial resolution and fast frame rates. ePix ASICs are based on a common platform composed of a random access analog matrix of pixel with global shutter, fast parallel column readout, and dedicated sigma-delta analog to digital converters per column. The ePix10k variant has 100um×100um pixels arranged in a 176×192 matrix, a resolution of 140e- r.m.s. and a signal range of 3.5pC (10k photons at 8keV). In its final version it will be able to sustain a frame rate of 2kHz. A first prototype has been fabricated and characterized. In this paper the ASIC performance in terms of noise, linearity, uniformity and cross-talk are presented, together with preliminary measurements with bump bonded sensors.

ePix: A class of front-end ASICs for second generation LCLS integrating hybrid pixel detectors

ePix is a novel class of ASICs architectures based on a common platform optimized for the processing of signals in second generation LCLS cameras. The platform architecture is composed of a random access analog matrix of pixels with a global shutter, fast parallel column readout, and dedicated sigma-delta analog to digital converters per column. It also implements a dedicated control interface and all the required support electronics to perform configuration, calibration, and readout of the matrix. Based on this platform a class of front-end ASICs and several camera modules are under development, each utilizing specific pixel architectures, to meet varying requirements. This approach reduces development time and expands the possibility of integration of detector modules in size, shape or functionality as different modules could be assembled in the same camera. The ePix platform is currently under development together with two integrating pixel architectures: ePix100 optimized for ultra-low noise applications and ePix10k optimized instead for high dynamic range applications.

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.

CSPAD upgrades at LCLS

The Cornell-SLAC Pixel Array Detector (CSPAD) is the workhorse for LCLS hard x-rays experiments. After deploying three 2.3Mpixel and many 140kpixel cameras, SLAC detector group has focused in improving the detector performances and optimizing its use in experiments. This was achieved first by improving thermo-mechanical assembly, PCB level electronics and firmware. The next step consisted in upgrading the ASIC. Continuous improvements in GUI and DAQ completed the evolution of the CSPAD systems, up to the current version V1.6, and made these cameras easy to use and optimize for a given experiment. More than 85% of the hard x-ray experiments scheduled in run 7 used one or multiple CSPAD cameras.

High-Z radiation shields for x-ray free electron laser detectors

The Linac Coherent Light Source (LCLS) produces brilliant x-ray in femtosecond pulses of high intensity. Many of the experiments performed at the LCLS use expensive pixel area detectors - the majority of which incorporate custom integrated circuit chips (ASIC). Such circuit chips are susceptible to radiation damage. To protect against this, micro-patterned tungsten foils were designed to cover the section of the circuit chip that extends beyond the sensor near the wire-bond pads. A description of the problem along with the details of how the tungsten foils were fabricated and installed will be given.

ePix100 camera: Use and applications at LCLS

The ePix100 x-ray camera is a new system designed and built at SLAC for experiments at the Linac Coherent Light Source (LCLS). The camera is the first member of a family of detectors built around a single hardware and software platform, supporting a variety of front-end chips. With a readout speed of 120u2005Hz, matching the LCLS repetition rate, a noise lower than 80 e-rms and pixels of 50u2005µm × 50u2005µm, this camera offers a viable alternative to fast readout, direct conversion, scientific CCDs in imaging mode. The detector, designed for applications such as X-ray Photon Correlation Spectroscopy (XPCS) and wavelength dispersive X-ray Emission Spectroscopy (XES) in the energy range from 2 to 10 keV and above, comprises up to 0.5 Mpixels in a very compact form factor. In this paper, we report the performance of the camera during its first use at LCLS.

Detectors in extreme conditions

Free Electron Lasers opened a new window on imaging the motion of atoms and molecules. At SLAC, FEL experiments are performed at LCLS using 120Hz pulses with 1012 -1013 photons in 10 femtoseconds (billions of times brighter than the most powerful synchrotrons). This extreme detection environment raises unique challenges, from obvious to surprising. Radiation damage is a constant threat due to accidental exposure to insufficiently attenuated beam, focused beam and formation of ice crystals reflecting the beam onto the detector. Often high power optical lasers are also used (e.g., 25TW), increasing the risk of damage or impeding data acquisition through electromagnetic pulses (EMP). The sample can contaminate the detector surface or even produce shrapnel damage. Some experiments require ultra high vacuum (UHV) with strict design, surface contamination and cooling requirements - also for detectors. The setup is often changed between or during experiments with short turnaround times, risking mechanical and ESD damage, requiring work planning, training of operators and sometimes continuous participation of the LCLS Detector Group in the experiments. The detectors used most often at LCLS are CSPAD cameras for hard x-rays and pnCCDs for soft x-rays.