G. Blaj

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.

Negative Pressures and Spallation in Water Drops Subjected to Nanosecond Shock Waves

Most experimental studies of cavitation in liquid water at negative pressures reported cavitation at tensions significantly smaller than those expected for homogeneous nucleation, suggesting that achievable tensions are limited by heterogeneous cavitation. We generated tension pulses with nanosecond rise times in water by reflecting cylindrical shock waves, produced by X-ray laser pulses, at the internal surface of drops of water. Depending on the X-ray pulse energy, a range of cavitation phenomena occurred, including the rupture and detachment, or spallation, of thin liquid layers at the surface of the drop. When spallation occurred, we evaluated that negative pressures below -100 MPa were reached in the drops. We model the negative pressures from shock reflection experiments using a nucleation-and-growth model that explains how rapid decompression could outrun heterogeneous cavitation in water, and enable the study of stretched water close to homogeneous cavitation pressures.

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.

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.

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.

Performance of silicon drift detectors at LCLS

Silicon drift detectors (SDDs) are a well-established technology that has revolutionized spectroscopy in fields as diverse as geology and dentistry. At a first glance it would seem that detectors with such a slow response would not be suitable for the new ultra-fast x-ray free-electron lasers (FEL) coming online. However, for a subset of experiments at FELs, SDDs can make substantial contributions. Many measurements involve only several distinct photon energies known a priori, allowing pile-up deconvolution and accurate spectroscopic photon counting. Often the unknown spectrum is interesting, carrying science data, or the background measurement is useful to identify unexpected signals. We investigated the performance of SDDs at x-ray FELs, in particular the ability to deconvolve the spectrum that results from various combinations of a few wavelengths and the possibility of separately recording photons that are absorbed at different radii (thus having varying drift times). The analytic approach presented here permits isolating individual photon energies and interaction radii from pile-up events of 0 to 5 photons sampled from 6 monochromatic lines in a single SDD with accurate pile-up deconvolution, timing extraction and clipping correction. The usefulness of SDDs will continue into the x-ray FEL era of science. Their successors, the ePixS hybrid pixel detectors, already offer hundreds of pixels with similar performance in a compact, robust and affordable package.

Design and characterization of the ePix100a: A low noise integrating pixel ASIC for LCLS detectors

ePix100 is the first 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. 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 ePix100 variant is optimized for low noise application requiring high spatial resolution and fast frame rates. The ASIC has pixels of 50×50 μm2 size arranged in a 352×384 array, a resolution of 50e- r.m.s., and a signal range of 35fC (100 photons at 8keV). In its final version it will be able to sustain a frame rate of 1 kHz. Currently a full-size analog version, ePix100a, has been fabricated in TSMC CMOS 0.25 μm technology. The ePix100a has been fully characterized and results are here reported.

Optimal Pulse Processing, Pile-Up Decomposition, and Applications of Silicon Drift Detectors at LCLS

Silicon drift detectors (SDDs) revolutionized spectroscopy in fields as diverse as geology and dentistry. For a subset of experiments at ultrafast, X-ray free-electron lasers (FELs), SDDs can make substantial contributions. Often the unknown spectrum is interesting, carrying science data, or the background measurement is useful to identify unexpected signals. Many measurements involve only several discrete photon energies known a priori, allowing single-event decomposition of pile-up and spectroscopic photon counting. We designed a pulse function and demonstrated that the signal amplitude (i.e., proportional to the detected energy and obtained from fitting with the pulse function), rise time, and pulse height are interrelated, and at short peaking times, the pulse height and pulse area are not optimal estimators for detected energy; instead, the signal amplitude and rise time are obtained for each pulse by fitting, thus removing the need for pulse shaping. By avoiding pulse shaping, rise times of tens of nanoseconds resulted in reduced pulse pile-up and allowed decomposition of remaining pulse pile-up at photon separation times down to hundreds of nanoseconds while yielding time-of-arrival information with the precision of 10 ns. Waveform fitting yields simultaneously high energy resolution and high counting rates (two orders of magnitude higher than current digital pulse processors). At pulsed sources or high photon rates, photon pile-up still occurs. We showed that pile-up spectrum fitting is relatively simple and preferable to pile-up spectrum deconvolution. We developed a photon pile-up statistical model for constant intensity sources, extended it to variable intensity sources (typical for FELs), and used it to fit a complex pile-up spectrum. We subsequently developed a Bayesian pile-up decomposition method that allows decomposing pile-up of single events with up to six photons from six monochromatic lines with 99% accuracy. The usefulness of SDDs will continue into the X-ray FEL era of science. Their successors, the ePixS hybrid pixel detectors, already offer hundreds of pixels, each with a similar performance to an SDD, in a compact, robust and affordable package.