Transcript Bragg, P. Siddons

Bragg Spectrographs for LCLS
Diagnostics and Science
D. Peter Siddons
Zhong Zhong
NSLS
Brookhaven National Laboratory
Upton, NY 11973
Outline
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Basic idea
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Problems
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The SPPS prototype
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Resolution limits
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Reflection-transmission design
Basic idea
Source
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Find a geometry which
presents a range of Bragg
angles to an x-ray beam in
a correlated manner.
Xtal
Detect spatially-dispersed
spectrum using a rapidreadout integrating 1-D
position-sensing detector.
Detector
Problems
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Not very efficient:
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Each element of crystal surface
only reflects one energy.
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Each element of crystal surface
only sees one 'ray' from incident
beam.
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Source
Xtal
but LCLS is very bright!
LCLS beam is collimated and
small:
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Usual schemes need big beams
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No beam-divergence-driven
Bragg angle variations
Detector
Problems
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Not very efficient:
–
Each element of crystal surface
only reflects one energy.
–
Each element of crystal surface
only sees one 'ray' from incident
beam.
●
●
Source
Xtal
but LCLS is very bright!
LCLS beam is collimated and
small:
–
Usual schemes need big beams
–
No beam-divergence-driven
Bragg angle variations
Detector
The SPPS prototype spectrograph
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Described in SRI2003 paper
Uses convex bend to
generate Bragg angle range
Uses highly asymmetric cut
to spatially expand small
beam
Detector is close to
spectrograph
–
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compact arrangement, can
fit anywhere.
Some possibilities for
chirped pulse manipulation
Details
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Si (4 2 2) reflection at Ni or
Cu K-edges
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Different asymmetries
needed for different energies
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cut for 4 degrees incidence
angle at edge energy
~ 0.3eV resolution
Results
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Spectra measured
at NSLS for Ni Kedge
One spectrum from
SPPS at Cu edge
Would work well in
the far
experimental hall at
LCLS (beam size
~1mm).
Resolution limits
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Resolution limit comes from intrinsic Bragg
width of reflection order chosen
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highest order from silicon at 8keV is (4 4 4)
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intrinsic resolution in symmetric case is 39 meV
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asymmetry makes it 100 meV
Can use asymmetry to help
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but it is in the wrong sense for the designs shown
earlier.
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In the right sense, the bending radius would need to
be very small, strains could remove any gain, and the
spatial dispersion would be absent.
Transmission-Reflection design
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Ideal spectrograph would use all of the beam at all
energies to generate the spectrum.
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This is what Zhong was describing in his talk on Laue designs
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Laue case offers no opportunity for spatial dispersion within
the optic.
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The T-R design does so in a piecewise manner.
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Can seperate spatial dispersion and asymmetry.
Many consecutive thin Bragg elements at progressively
different angles
Each diffracting element operates in the thin-Bragg
regime.
Details
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Many thin lamellae (~10um)
microfabricated on curved
substrate
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Si (4 2 2) or diamond (3 3 1) both
provide < 40meV
Each lamella picks an energy from
same part (in fact, all) of incident
beam
What about absorption?
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Silicon @ 8keV has mu = 14/mm, so
max elements is ~10
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Diamond is 10 x better, mu=1.4/mm,
so 100 elements could work.
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They would both work better at
higher energies! e.g. 3rd order @
24keV -> 1meV for diamond (9 3 3),
mu=0.4/mm
How to make it?
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Reactive Ion Etching can deliver high aspect-ratio
structures in silicon, so this structure should be
straightforward.
People have deposited single-crystal diamond on
silicon using CVD techniques.
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Can orientation be controlled?
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Can it be RIE'd?
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how good crystals are they?
How serious are strain issues for resolution?
Detector
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All spectrographs need a 1D position-sensitive
detector.
Should have readout time << inter-bunch time
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120Hz for LCLS
Should have noise level << signal
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10^12/pulse -> 10^8 per 1% pixel
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Poisson noise 10^-4
50nC of charge produced in a Si diode: electronic noise
should not be an issue.
Sumary
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A simple flash spectrograph has been built and
(minimally) tested at SPPS.
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< 0.4eV resolution
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~ 150eV coverage
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Suitable for XANES experiments and diagnostics.
A suggestion for a microstructured device which
could answer some of the 'problems' with the
simple device.
At 24keV, a Laue version of this could provide
optimised resolution/coverage.