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Coherent X-ray Imaging Instrument
The Coherent X-Ray Imaging (CXI) instrument (Boutet et al New Journal of Physics (2010)) makes use of the unique brilliant hard X-ray pulses from LCLS to perform a wide variety of experiments utilizing various techniques. The primary capability of CXI is to make use of the high peak power of the focused x-ray beam using the “diffraction-before-destruction” method. This technique prevents damage to a sample during the measurement by performing the measurement faster than the damage or destruction process with ultrashort pulses. This is particular advantageous for biological samples that suffer from electronic and structural damage during long continuous exposures to x-rays.
While designed originally to image single sub-micron particles using Coherent Diffractive Imaging (CXI) techniques, the CXI instrument consists of a highly flexible instrumentation suite to make use of hard x-rays primarily in a vacuum sample environment. It is available for Serial Femtosecond Crystallography (SFX) measurements capable of determining the structure of biomolecules using nanocrystals. It is also suitable for any forward scattering experiment requiring or benefiting from a vacuum sample environment. A variety of tools and devices have been developed that allow CXI to make use of other techniques such as X-ray Emission Spectroscopy, back-scattering, small and wide angle scattering, ion and electron time of flight spectroscopy. A flexible pump laser system is available for time-resolved experiments in the femtosecond time scale. CXI is available for any scientific field requiring use of the LCLS beam, including structural biology, material science, materials in extreme conditions, atomic molecular and optical physics, chemistry, soft condensed matter and high field x-ray science. Samples can be introduced to the x-ray beam either fixed on targets or using a particle injector that can deliver free-standing particles or samples in a liquid jet to the beam. Experiments at atmospheric pressure are possible under certain limited circumstances. High quality focusing optics are available to generate three foci (10, 1 and 0.1 micron). The CXI instrument operates primarily in the 5-11 keV range with capabilities for operation under reduced performance above 11 keV with use of the harmonics of the beam.
X-ray diffraction has long been used to determine atomic structures of
biomolecules. The X-ray dose needed to achieve a given resolution for a
particular sample can be calculated. It can be shown that the dose required to
image a single biological molecule is much larger than the dose required to
completely destroy the molecule through radiation damage processes. X-ray
crystallographers mitigate this problem by spreading the damage over billions
of molecules in a single crystal, greatly enhancing the diffraction signal.
Since the molecules are all identical and precisely aligned in the crystal, the
X-ray scattering information is preserved and the structure can be determined.
LCLS offers another way around the damage problem. Since the FEL X-ray pulse
is very intense and very short, it is possible in principle to deliver the
required dose to a nano-scale sample and record the scattered X-ray information
before the damage processes have time to destroy the sample. In other words, an
LCLS X-ray pulse can be focused onto a single molecule, which gets be destroyed
– but not before the scattered X-rays are already on their way to the detector
carrying the information needed to deduce the image. The Coherent X-ray Imaging
(CXI) Instrument offers the possibility of determining structures at resolution
beyond the damage limit for samples which do not form large crystals, including
important classes of biological macromolecules.
Coherent Diffractive Imaging of reproducible biomolecules
a two-dimensional diffraction pattern can be collected from a single
biomolecule before it is destroyed by the LCLS beam. Such a two-dimensional
pattern encodes information about a projection image of the object onto a plane
parallel to the detector. Three-dimensional structural information about
highly-reproducible molecules such as viruses, large proteins or molecular
complexes can be derived if a series of the molecules is delivered into the
LCLS beam one after the other. Each molecule would have a different orientation
and a full 3D diffraction data set can be obtained from a large number of
identical copies of the sample. In theory, it could be possible to obtain high
resolution structures for difficult to crystallize biomolecules. Research and development
is required to demonstrate this capability.
Serial Femtosecond Nanocrystallography
is often the case that large crystals of a certain protein cannot be grown but
a large number of very small crystals can readily be obtained. These sub-micron
crystals do not scatter enough X-rays to yield an atomic structure using
conventional protein crystallography techniques. The high flux of LCLS could
allow these to be used for structure determination. This high flux also has the
advantage of allowing in principle damage-free higher resolution of
radiation-sensitive samples such as metalloenzymes for example. Assuming all
the nanocrystals possess the same crystal symmetry, a series of nanocrystals
can be illuminated by LCLS X-ray pulses and the diffraction patterns recorded.
The variations in alignment of the crystal axes from sample to sample can be
determined from indexing the Bragg peaks in the diffraction patterns. A full 3D
set similar to conventional protein crystallography can be assembled from many
small crystals and yield the protein structure. The nanocrystals can be
delivered into the CXI beam using a liquid jet (Weierstall et all, Rev. Sci. Instrum. (2010)),
LCP jet (Weirstall et al, Nature Communications (2014)),
an electrospinning jet (Sierra et al, Acta Cryst D (2012))
or mounted to a suitable substrate in vacuum (Frank et al, IUCrJ (2014)).
The liquid jet options provides high data rate while the substrate mounting and LCP jet can
help minimize sample consumption.
Imaging of nanoparticles
makes it possible to obtain two-dimensional projections of any non-reproducible
nanoparticle and three-dimensional images of reproducible objects using the
diffract-the-destroy technique. Furthermore, the CXI beam can be attenuated to
a level slightly below the damage threshold of an inorganic nanoparticle and a full
3D reconstruction can in principle be obtained from a single particle using
multi-image tomographic techniques. The transverse coherence length of the CXI
beam can allow small particles to be imaged in 3D at high resolution.
LCLS source produces hard X-ray fields of unprecedented high intensity. It
allows for the first time tests of radiation damage models under such extreme
conditions as well as tests of fundamental x-ray physics under extreme fields.
These models are directly relevant to atomic-resolution imaging since the
damage suffered during a pulse must be limited or the image reconstruction will
suffer. The interaction of the powerful LCLS pulses with solid matter can be
studied using the CXI instrument under unique extreme conditions using
high-quality focusing optics.
is possible, with the use of an optical pump laser, to obtain dynamic
information of photo-induced changes in non-crystalline and crystalline samples
with sub-picosecond time resolution. A flexible pump laser system (nanosecond
and femtosecond lasers) is provided at the CXI instrument to allow a wide range
of dynamics studies.
in Extreme Conditions
ultrashort pulses of LCLS open new possibilities to study short lived states of
matter in extreme conditions. These extreme or transient states can be created
with a pump laser and then probed with high time resolution with LCLS pulses
using diffraction (Milathianaki et al, Science (2013)),
scattering or in principle emission techniques.
Molecular and Optical Science
CXI instrument is primarily an in-vacuum high peak power instrument using hard
x-rays. This makes CXI the ideal home for AMO science that requires the use of
x-rays above 4-5 keV. Time-of-flight spectrometers and photon detectors can be
added to the CXI sample chambers to allow AMO studies.
Using a Lipidic Cubic Phase Jet
Liu et al
Femtosecond Crystallography of G Protein–Coupled Receptors" Science
342 1521 (2013)
Using a Gas Dynamic Virtual Nozzle
Using an Electrostatic Jet
Emmission Spectroscopy Simultaneously with SFX
Experimental Phasing of SFX Data
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy