Coherent X-ray Imaging Instrument

The Coherent X-Ray Imaging (CXI) instrument will make use of the unique brilliant hard X-ray pulses from LCLS to image single sub-micron particles. The full transverse coherence of the LCLS laser will allow single particles to be imaged at high resolution while the short pulse duration will limit radiation damage during the measurement. The instrument will allow imaging of biological samples beyond the damage limit that cannot be overcome with synchrotron sources. Samples can be introduced to the x-ray beam either fixed on targets or using a particle injector that will deliver free-standing particles to the beam. Two high quality focusing optics will generate two foci (1000 and 100 nm) and this will allow imaging of single nanoparticles of various sizes, pushing the limit down to single biomolecules.

Far Experimental Hall, Hutch 5 » complete instrument map

X-ray scattering 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 could be focused onto a single molecule, which would 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 will offer the possibility of determining structures at resolution beyond the damage limit for samples which do not form crystals, including important classes of biological macromolecules.

Imaging of reproducible biomolecules
Only a two-dimensional diffraction pattern will 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 plane. Three-dimensional structural information about highly-reproducible molecules such as viruses, large proteins or molecular complexes could be derived if a series of the molecules were delivered into the LCLS beam one after the other. Each molecule would have a different orientation and a full 3D diffraction data set could be obtained from a large number of identical copies of the molecule, complex or virus. Atomic or near-atomic resolution structures could be obtained for difficult to crystallize biomolecules.

Nanocrystallography of proteins
It 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 will allow these to be used for structure determination. Assuming all the nanocrystals possess the same crystal symmetry, a series of nanocrystals could be illuminated by LCLS X-ray pulses and the diffraction patterns recorded. The variations in alignment of the crystal axes from sample to sample will be relatively straightforward to determine from the Bragg peaks in the diffraction patterns. A full 3D set similar to conventional protein crystallography could be built up and standard crystallography phasing methods could yield the protein structure. It will be possible to deliver these nanocrystals directly into the beam without a substrate.

Imaging of nanoparticles
LCLS will make it possible to obtain two-dimensional projections of any non-reproducible nanoparticle and three-dimensional images of reproducible objects. Furthermore, the LCLS beam could be attenuated to a level slightly below the damage threshold of an inorganic nanoparticle and a full 3D reconstruction could be obtained from a single particle using multi-image tomographic techniques. The transverse coherence length of the LCLS will allow small particles to be imaged in 3D at near-atomic resolution.

Imaging of hydrated living cells
Living cells are all unique at the atomic level. It will therefore not be feasible to obtain a full three-dimensional image of a cell at atomic resolution at LCLS since damage would occur to the single cell with a single exposure. However, LCLS will offer the capability to study fully hydrated cells beyond the damage limit in two dimensions. The cells will be injected sequentially into the X-ray beam and a 2D diffraction pattern will be collected from each LCLS shot. The rapid injection of the cell into vacuum would prevent them from drying out and they would remain fully hydrated during the measurement. The high peak flux of LCLS will make this measurement possible.

X-ray-matter interactions
The LCLS source will produce hard X-ray fields of unprecedented high intensity. It will allow for the first time tests of radiation damage models under such extreme conditions. 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.

Pump-probe imaging
It would be possible, with the use of an optical pump laser, to image photo-induced changes in non-crystalline samples with sub-picosecond time resolution. Currently, no pump laser is provided in the CXI instrument. However, these experiments are possible with a user-supplied laser system.