Inline Spectrometer as Permanent Optics at the X-ray
Correlation Spectroscopy Instrument to support
seeding operation
Amber Gray
San Jose State University
Mentor: Dr. Aymeric Robert
X-ray Correlation Spectroscopy Inst. Leader
Linac Coherent Light Source
United States Department of Energy
Office of Science, Science Undergraduate Laboratory Internship Program
SLAC National Accelerator Laboratory
Menlo Park, California
July 25th, 2012
I. Introduction
The Linac Coherence Light Source (LCLS) is the first hard X-ray Free Electron
Laser that produces ultrafast intense X-ray pulses in order to non-destructively
analyze material’s physical properties and crystalline/disordered structures. It
provides a unique opportunity to observe dynamical changes of large groups of
atoms in condensed matter systems (i.e. looking at what is happening inside a
material) over a wide range of time scales using Coherent X-ray Scattering (CXS) in
general and 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 materials [1,
2].
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 presents a peculiar grainy appearance also known as
speckles. These speckles originate from the exact position of all scatterrers within
the coherently illuminated volume. XPCS thus characterizes the temporal
fluctuations of such speckle patterns from which insights in the dynamic behavior of
the system can be revealed [1].
When lasing, due to the spikey nature (intrinsic noise) of the beam in SLAC’s
two-mile long accelerator as described by the Self Amplified Stimulated Emission
(SASE) process, monochromators and diagnostic tools are necessary means for
testing the single shot spectral properties of the beam before probing the dynamics
of materials.
II. Methods and Materials
The XCS instrument is a hard x-ray instrument that was designed to use the
LCLS FEL beam for coherent x-ray scattering techniques such as XPCS. A schematic
of the various optical components of the XCS instrument is provided on the next
page in Figure 1.
Figure 1. A schematic view of the optical components and diagnostics of the XCS
Instrument. The distances from each component to the sample are indicated in
meters. XCS can operate various monochromators: a Si(111) or (220) Large Offset
Double Crystal Monochromator (LODCM) and also an artificial Si(511) Channel Cut
Monochromator (CCM). The beam can focus to small sizes (typically 5 × 5μm2) with
Compound Refractive Lenses (CRL) available for installation at two different
locations. XCS can also operate in pink beam mode by translating most of its
components in the main LCLS beamline as indicated in red [3]
The detailed characterization of each single x-ray pulse reaching the sample
at XCS is required (i.e. intensity and spectral content). A transmissive spectrometer
is needed inline just before the sample in order to properly evaluate the spectral
content of each single pulse. The requirements of the spectrometer are to capture
the full SASE spectrum in the hard x-ray regime on a single shot basis for individual
spectral spikes. It should also cover the energy range of operation of XCS.
The design of the spectrometer includes the selection of thin silicon crystal
membranes of a given thickness for the desired transmission of hard x-rays. The
silicon crystal membranes will be bent to a specific radius of curvature in order to
provide the necessary dispersion (i.e. provide an appropriate resolution), and the
diffracted beam will then reach the detector’s scintillator screen as seen in Figure 2.
Figure 2. Dispersion geometry of the spectrometer.
As the incoming beam hits the bent crystal different parts of the beam are
diffracted at different angles satisfying Bragg’s Law given by Equation 1
𝜆 = 2𝑑 sin 𝜃𝐵 , (1)
where d is the spacing between the lattice planes of a given crystal and orientation
and 𝜃𝐵 is the Bragg angle for a particular wavelength, 𝜆.
The wavelength dispersion, Δ𝑥 on the detector is related to ΔΕ, the energy
increment and is expressed as:
𝑅 sin 𝜃
ΔΕ
Δ𝑥 = 2 tan 𝜃𝐵 ( 2 𝐵 + 𝐿′ ) Ε , (2)
where R is the radius of curvature of the crystal and 𝐿′ is the distance from the
membrane to the detector. The beam size, Η, and the radius of curvature of the
crystal are determining factors in the spectral range of the bent crystal
spectrometer [4]. Equation 3 shows this dependency
ΔΕ𝑚𝑎𝑥
Ε
H
= cot 𝜃𝐵 𝑅 sin 𝜃 .
𝐵
(3)
After understanding the dispersion geometry in order to meet the physics
requirements of the project, from an engineering standpoint it is important to start
a feasibility study in the XCS Hutch 4 to see if the space available before the sample
is capable of hosting a spectrometer with the appropriate energy range.
A preliminary study was done for a Si(111) crystal to see if the allowable
space could accommodate a He-filled plexiglass enclosure holding the crystal with
different stages to move about four different axes. It would also hold a detector
directly on top of the enclosure rotating independently of the crystal. There are two
linear and two rotational stages: x, y, theta and chi respectively. The x-stage is
necessary to move the crystal in and out of the beam path as desired in the
horizontal plane. The y-stage is to get the crystal in alignment with the beam path
direction. The y-stage specifically helps to select an appropriate curvature on the
crystal. Theta and chi stages will align the crystal and allow the diffracted X-rays to
reach the detector. The beam will be diffracted at nearly 90 degrees in the vertical
scattering plane to prevent polarization losses.
After taking measurements in the hutch it was determined there is enough
space to rotate a detector that would allow an energy range from 7.5 to 10keV. The
next step is to get approval for choosing stages with the required resolutions, then
determine the stage sizes, and finally begin a conceptual design for the XCS inline
spectrometer.
References
[1] Grübel G, Madsen A and Robert A 2008 X-ray Photon Correlation Spectroscopy in
Soft Matter Characterization, ed R Borsali and R Pecora (Heidelberg: Springer) chapter
18 pp. 954-995
[2] Stephenson G B, Grübel G and Robert A 2010 Nature Materials 8 702-703
[3] Robert A, Curtis R, Flath D, Gray A, Sikorski M, Song S, Srinivasan V, Stefanescu D
2012 To be published
[4] Zhu D et al. 2012 A Single-Shot Transmissive Spectrometer for Hard X-ray Free
Electron Lasers