X-ray Correlation Spectroscopy
XCS Overview
The unprecedented brilliance and narrow pulse duration of the Linac
Name: Aymeric ROBERT
XCS group Leader
Phone: (650) 926-2747
Email: [email protected]
Coherence Light Source provides a unique opportunity to observe dynamical
changes of large groups of atoms in condensed matter systems over a wide
range of time scales using Coherent X-ray Scattering (CXS) in general and
Name: Chiara Caronna
Phone: (650) 926-3767
Email: [email protected]
X-ray Photon Correlation Spectroscopy (XPCS) in particular. The X-ray
Correlation Spectroscopy (XCS) instrument at the LCLS allows the study of
equilibrium- and non-equilibrium dynamics in disordered or modulated
The XCS instrument is located in Hutch 4 of the Far Experimental Hall of
LCLS. The XCS instrument is located about 420m away from the source.
Characteristic timescales ranging from 10 milliseconds to thousands of
seconds can be explored using a sequential mode of operation.
Ultrafast timescales ranging from 100 femtosecond to several nanoseconds
can be probed using a novel “Split and Delay” mode of operation.
Name: Sooheyong Lee
Visiting Scientist (DESY)
Phone: (650) 926-8915
Email: [email protected]
X-ray Correlation Spectroscopy
X-ray Photon Correlation Spectroscopy Technique
Coherent X-rays are particularly well suited for investigating disordered system dynamics down to nanometer
and atomic length scales, using X-ray Photon Correlation Spectroscopy [1].
When coherent light is scattered from a disordered system, the scattering pattern present a peculiar grainy
appearance also known as speckles, as illustrated in the figure (Top) [2]. These speckles originate from the
exact position of all scatterers within the system under investigation.
XPCS characterizes the temporal fluctuations in speckle patterns, produced when coherent light is scattered
by a disordered system. From these fluctuations, insight in the dynamic behavior of the system can be
XPCS, as currently performed on 3rd generation synchrotron light sources (Advanced Photon Source,
European Synchrotron Radiation Facility. PETRA III) is complementary to Dynamic Light Scattering (DLS) or
Photon Correlation Spectroscopy (PCS) with visible coherent light, techniques that probe slow dynamics
(<106 Hz) but can only access the long wavelength regime (Q < 4 ∙10-3 Å-1). Neutron-based techniques
(inelastic, quasi-elastic neutron scattering, neutron spin-echo) and Inelastic X-ray Scattering (IXS) can access
the same Q range as XPCS, but these techniques probe the dynamic properties of matter at high frequencies
(from typically 108 Hz to about 1017 Hz), as illustrated in the figure (bottom).
The XPCS areas (in transmission/diffraction and grazing incidence (GI) geometry) are defined from existing
measurements performed at the existing 3rd generation synchrotron light sources instruments.
The peak coherent flux of the LCLS radiation will be 9 orders of magnitude larger than 3rd generation
synchrotron light sources. This will allow for the first time the studies of dynamics up to about 1013 Hz at large
Q’s using the fine pulse structure of the LCLS beam. The unique capabilities of the XCS instrument at LCLS
enables to address a wide variety of scientific questions, such as coherent optical properties of hard X-rays,
phase transition dynamics, dynamics of glassy materials, surface XPCS, non- equilibrium dynamics, local
ordering in disordered materials.
[1] G. Grübel, A. Madsen, A. Robert, Soft Matter Characterization, edited by R. Borsali & R.
Pecora, pp. 954-995. Heidelberg : Springer.
[2] A. Robert, J. Appl. Cryst. 40, (2007), pp. s34-s37
X-ray Correlation Spectroscopy
Example of Scientific Programs (I)
Phase Transition Dynamics
Atomic-scale fluctuations occur at equilibrium near many phase-transitions. These fluctuations
are the basis for the new phases that form when the phase transition point is crossed by
changing relevant physical parameters, such as temperature, electric or magnetic field, pH, etc
…. XPCS is an ideal tool for observing the equilibrium dynamics of these fluctuations, and
understanding the mechanisms that control microstructure formation in materials.
For example transitions in magnetic, ferroelectric and ferroelastic materials exhibit time scales
spanning from sub-picoseconds to many seconds in the vicinity to the transition point.
Many liquids, when rapidly cooled below their freezing point, form meta-stable glasses or
amorphous phases. This applies for a wide variety of materials including metallic alloys, oxides
such as silica, polymeric materials, and many others (e.g. colloidal dispersions for which the
volume fraction is the analogous of the inverse temperature driving the phase transition). In
general, glassy materials are among the least understood at a fundamental level. Conventional
equilibrium statistical mechanics does not predict the existence of the amorphous state, whereas
it can explain most other states of matter, even quite exotic ones like superconductivity and
superfluidity. Understanding the dynamics and local ordering in glassy materials is thus an
important topic of fundamental research.
Surface and Interfaces Dynamics
Surface XPCS experiments at 3rd generation storage ring sources have been performed
successfully in the past, and surface dynamics have been investigated in a variety of systems,
such as membranes, polymer films, and liquids. Surface XPCS in combination with the X-ray
standing wave technique also allows the study of fluctuations of buried interfaces. However,
surface dynamics at the nanometer length scale is not accessible today due to insufficient
coherent photon flux. The XCS instrument at LCLS will provide exciting new possibilities in this
science area.
X-ray Correlation Spectroscopy
Example of Scientific Programs (II)
Non-equilibrium dynamics
When a disordered homogeneous material is rapidly brought to a new set of conditions,
corresponding to the coexistence of equilibrium phases, a spatial pattern of domains of each
phase develops. For example, such a change of conditions can be accomplished by a rapid
quench from high to low temperature, below the miscibility gap. This results in the creation of a
microstructure of interconnecting domains. These domains grow in order to minimize the areas of
the domain walls that separate the phases. Out-of-equilibrium behavior has been observed in
soft condensed matter glassy systems as well (i.e. attractive and repulsive colloidal glasses),
where the dynamics are slowed down as a function of time and are described as aging. The XCS
instrument at LCLS is ideal for studying these non-equilibrium systems where the dynamic
behavior needs to be probed on all length scales simultaneously.
Coherence properties of hard X-rays
Coherence is one of the most prominent features of the novel radiation produced by the LCLS. A
comprehensive understanding of the X-ray laser coherence properties is not only of fundamental
interest but a necessity for properly interpreting coherence-based experiments such as Coherent
X-ray scattering in general and XPCS in particular. The characterization of the coherence
properties of the source in terms of correlation functions and photon statistics was successfully
used for characterizing 3rd generation sources XPCS dedicated instruments. The study of first
and higher order correlation functions will allow not only determine the spatial and temporal
coherence parameters of the source (i.e. the longitudinal and transverse coherence lengths as
well as the coherent flux) but it will also provide insight into the mode structure, photon statistics
(bunching/anti-bunching) and possible non-Gaussian properties of the source.
X-ray Correlation Spectroscopy
Operation Modes (I)
Sequential Mode (timescales from 10-2 to 103 seconds)
The shortest time scale of this mode of operation is limited by the LCLS repetition rate
(i.e. up to 120Hz). Important dynamical phenomena at mesoscopic lengthscales can
occur on relatively long time scales, e.g. longer than 10-1 s .
This operation mode of operation is illustrated in the frequency-wavector graph. It
employs the very large time-averaged coherent X-ray flux from the LCLS, averaged
over 120 Hz repetition rate, to investigate dynamics by means of 2D-XPCS data
These experiments consist of collecting time-resolved sequences of speckle patterns
on an area detector as shown in figure 2 [1]. From an analysis of these sequences,
correlation times from a few inter-pulse periods up to many minutes can be obtained.
The data analysis techniques are similar to those used until now at 3rd generation
The XCS instrument at LCLS will provide higher signal rates than currently available,
thus allowing to probe dynamics at much larger wavevectors than currently possible. It
will also offer the possibility to investigate weakly scattering samples.
[1] G.B. Stephenson, G. Grübel and A. Robert, Nature Materials 8, 2010,703
Sequential Mode
X-ray Correlation Spectroscopy
Operation Modes (II)
Split-Delay Mode- (timescales from 10-12 to 10-8 seconds)
The short pulse duration of the LCLS hard X-ray radiation (< 250 fs ) allows the extension of XPCS studies to much faster
time scales than currently possible.
In order to probe time scales between 10-12 and 10-8 s (as presented in the frequency-wavevector phase space diagram),
a split-delay technique shown schematically in Figure 2 is employed and takes advantage of the unprecedented peak
brilliance of the LCLS beam.
Its concept is to split each LCLS X-ray pulse into two equal-intensity pulses separated in time, but propagating along the
same path [1]. The scattering from the two pulses will then be collected during the same exposure of an area detector.
If the sample is static (i.e. does not present any dynamics on the time scale of the time delay between the two pulses), the
contrast in the summed speckle pattern will be identical to the one of a single pulse. If on the other hand the sample
evolves on this time scale, then the summed speckle pattern will present a lower contrast [2].
By performing the contrast analysis of sets of ” single shot” summed speckle pattern as a function of time delay, the
correlation time of the system can be measured on time scales down to the pulse duration. Such contrast function (i.e.
contrast as a function of time delay) can also be linked to the intermediate scattering function (i.e. the normalized
dynamic structure factor in the time domain f(Q,t)), as usually performed in the sequential mode with the second order
intensity autocorrelation function [2].
A split and delay unit with a path length difference variable from 3x10-6 to 3m provides delay times from about 10-14 to 10-8
seconds [1].
[1] W. Roseker, H. Franz, H. Schulte-Schrepping, A. Ehnes, O. Leupold, F. Zontone, A. Robert and G. Grübel, Optics
Letters 34 (12), (2009), pp. 1768-1770.
[2] C. Gutt, L. –M. Stadler, A. Duri, T. Autenrieth, O. Leupold, Y. Chushkin and G. Grübel , Optics Express 17 (1), (2009),
pp. 55-61
Split and Delay Mode
X-ray Correlation Spectroscopy
XCS Instrument Description II
The XCS instrument is a dedicated LCLS instrument for the use of Coherent X-ray
Scattering techniques in general and X-ray photon Correlation Spectroscopy in
particular. It can operate in the hard X-ray range (5.5-25keV) on any of the harmonics
of LCLS.
Longitudinal Coherence Length [mm]
The XCS instrument operates mainly in monochromatic beam but it can also operate in
pink beam if scientifically required. By default the XCS instrument will be providing
Si(111) monochromaticity. In order to provide enhanced longitudinal coherence
lengths, XCS will also provide monochromatic beam with Si(220) and Si(511) in the
near future.
Grazing Incidence and local harmonic rejection
XCS is also providing local harmonic rejection by means of two silicon mirrors located
1.5 and 2.1m upstream the sample location. These also offer the possibility to provide
the beam with a grazing angle to the sample. Both mirrors can rotate 360°, and can
thus deflect the beam downwards of upwards depending on the required scattering
E [keV]
X-ray Correlation Spectroscopy
XCS Instrument Description II
The beam can be focused by inserting Beryllium compound refractive lenses in the
beam path. The focal length can be adjusted for a given X-ray energy by selecting an
appropriate number of individual lenses (up to 10) and stacking them. One can switch
from one stack to another (up to three) remotely. Such a unit is located 6.8 meters
upstream from the sample. XCS intend to provide later a second unit at 3 meters from
the sample, capable to provide micron size focus size. Each unit has the capability to
be translated longitudinally ±0.3m. This allow some tunability in the beam size at the
sample when not working at the focus.
The XCS instrument is providing a horizontal scattering 4-circle Huber diffractometer. It
has a local 2θ detector arm to easy crystal alignments. It can be removed from the
beam path for accommodating large sample environment that are not compatible with
the diffractometer. The top surface of the diffractometer is 300x300mm2. The distance
between the top surface of the diffractometer and its center or rotation is 270mm.
Sample Environments
The XCS instrument will try to provide a complete suite of sample of environments to
the user community. This will however happen along the operation of XCS. At the
beginning of the operation of XCS, no sample environment may be provided and the
users are expected to provide their own favorite sample environment. Any integration
issue should be discussed with the XCS instrument team.
X-ray Correlation Spectroscopy
XCS Instrument Description III
Large Angle Detector Mover
The XCS instrument has a Large Angle Detector Mover as a long sample-detector 2θ
arm. It was build by FMB/OXFORD. The LADM provides two sample detector
distances : 4 and 8m. It can rotate up to 55° scattering angle in the horizontal plane
and up to 1° in the vertical plane for Grazing Incidence scattering geometries. It
provides an evacuated fly path between the diffractometer and the detector. The
Kapton exit window can be as large as 250mm Ø. It also provides three different in
vacuum beamstops upstream the exit window.
The LADM provides Small Angle X-ray Scattering capabilities for 2θ=0°.
At the end of the LADM, two vertical and horizontal translations allow to move a 2dimensional detector to be located at a position of interest.
XCS intend to provide a dedicated detector fulfilling all requirement to perform CXS
and XPCS experiments at LCLS.
It is for now unclear whether or not that detector will be available for the first run of
operation of XCS. That detector intend to provide 100% DQE, 102 dynamic range, very
low noise ( << 1 photon) 55x55 μm2, 1k x 1k pixels, 120 fps.
At a minimum XCS will provide a standard direct illumination CCD (Princeton
Instruments, LCX) providing 50% DQE at 8keV (30% at 10keV) , 50 photon dynamic
range, very low noise ( << 1 photon) 20x20 μm2, 1.3k x 1.3k pixels, 0.3 fps.
X-ray Correlation Spectroscopy
Beam Diagnostics
Pop-in Profile Monitors
The spatial profile of the LCLS beam can be measured at various locations along the
XCS beamline using a scintillating screen and a high resolution camera-lens
combination. The screen is mounted on a translation stage to insert it into the beam.
Pop-in Intensity Monitor
The integrated intensity of the LCLS beam is measured at various locations along the
XCS beamline using a photodiode that can be inserted in the beam path.
Intensity-Position Monitor
The Compton backscattering of a thin silicon nitride foil (i.e. allowing most of the
beam to be transmitted) is used to measure the incident intensity on a shot-to-shot
basis. The back-scattering is measured using a quadrant diode located right
upstream of the foil. The integrated intensity of all the diodes provides a
measurement of the beam intensity for each. The relative signal from each tile can be
used to get the beam position.
X-ray Correlation Spectroscopy
XCS Capabilities and Parameters
X-ray Photon Correlation Spectroscopy (XPCS)
Small Angle X-ray Scattering (SAXS)
Grazing incidence Scattering (GI-SAXS, GID)
Wide Angle X-ray Scattering/Diffraction
Coherent X-ray Scattering (CXS)
Source Parameters
Photon Energy
5.5-10keV , 1st harmonic
10-25 keV , 2nd and 3rd harmonic
Source Size (e-)
~ 60 x 60 µm2 (HxV) FWHM @ 8.3 keV
Source Divergence
~ 1.1 x 1.1 µrad2 (HxV) FWHM @ 8.3 keV
Repetition Rate
120* , 60, 30, 10 Hz
Pulse Duration
Pulse Energy
Photons per Pulse
70 - 300 fs (high charge mode)
~ <10 fs (low charge mode)
~ 1-3 mJ (high charge mode)
~ 0.2 mJ (low charge mode)
~1 x 1012 (high charge mode @ 8.3 keV)
~1 x 1011 (low charge mode @ 8.3 keV)
* LCLS will operate at 120Hz in the near future. For now it routinely
operates up to 60Hz
X-ray Correlation Spectroscopy
XCS Capabilities and Parameters
Focusing Capability
A selection (up to 10) of compound refractive
lenses can be inserted in the beam to provide
focusing at a given energy. By locating the
sample at or out of focus, the beam size at the
sample can be tuned from a couple of microns
to several tens of microns.
Beam Size at Sample
(8 keV)
Up to 750 x 750 µm2 FWHM @ 8.3keV
Energy Range
Energy Resolution
5.5-10 keV (1st harmonic)
10-25 keV (2nd and 3rd harmonic)
Silicon (111) 1.4 x 10-4
Silicon (220) 6.1 x 10-5 *
Silicon (511) 1.1 x 10-5 *
Pink beam 2-3 x 10-3 *
Si(220) and (511) may not be available at start.
Si(220) and (511) may not be available at start. Operation in
pink beam is possible but is a non standard mode of operation
and therefore requires prior discussion.
Transport Tunnel
XCS Alcove
FEH Hutch 4
LCLS Beam Direction
Beam Dump
Large Angle Detector Mover
Pulse Picker
Harmonic Rejection Mirrors
Split and Delay Unit
Photon Stopper
PPS Photon Shutter
X-ray Correlation Spectroscopy
XCS Beamline Schematic

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