Preparation of LaserPolarized Xenon at High Xe Densities and High Resonant Laser Powers Provided by

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Preparation of Laser-Polarized Xenon at High
XeDensities and High Resonant Laser Powers
Provided by Volume Holographic Grating-Narrowed
LDAs

• Boyd M. Goodson1, Nicholas Whiting1, Panayiotis
  Nikolaou1, Neil Eschmann1, Michael J. Barlow2
• 1Department of Chemistry and Biochemistry
• Southern Illinois University, Carbondale
• 2Sir Peter Mansfield Magnetic Resonance Centre
• University of Nottingham, UK

DAMOP 2009 U. Virginia
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OP Limitations Xecell and Laser Output

• Highest PXe requires low Xecell (lt50 torr)
• Xe / Rb collisions ? reduced PRb, capping PXe
• Due to interplay of PXe, Xecell, overall NMR
  signal reaches steady-state (increases in
  Xecell compensated by PXe losses).
• Hard to achieve simultaneously high PXe and
  Xecell ? limiting some applications.

e.g., Meersmann _at_ co-workers JCP 2003.

• Typical LDAs high powers, low costs (but) broad,
  uneven lineshapes.
• Inefficient use of laser output.
• Collision broadening (high p, T)
• Frequency-narrowed LDAs.
• External Cavities
• Volume Holographic Gratings

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Volume Holographic Grating (VHG)-Narrowed LDAs

• Bulk slabs of photosensitive glass with Bragg
  planes of varying (ni)
• Retro-reflects narrow emission band into laser
  elements, forcing lasing at injected l.

• Narrows LDA output with high efficiency,
  tolerance
• High power and narrow Dl (at low cost, w/ high
  ease of use)
• ? more efficient absorption under milder OP
  conditions
• ? higher resonant laser fluxes

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Tunability of a Standard VHG-LDA

• VL1
• (Spectra-Physics Comet)
• 26 W (50 A), FWHM0.27 nm
• VL1 l-offset can be tuned by varying driving
  current.
• VL2
• (Spectra-Physics Integra)
• 55 W (96 A), FWHM0.49 nm

VL1
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OP Apparatus

• Var. gas density/composition available for
  loading via custom manifold.
• Nominal conditions 300 torr Xe, backfilled with
  N2 to 2,000 torr total.
• T 70-120 oC, t 5-15 min.

• PXe measured via NMR at 9.4 T

Saha, Nikolaou, Whiting, Goodson, Chem. Phys.
Lett., 428, 268 (2006).
Cell Rosen et al., Rev. Sci. Instrum., 70, 1546
(1999).
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Initial Studies of Temperature and l-offset on PXe

• 3-fold (W for W) improvement in PXe when
  switching from Standard (non-narrowed) LDA to
  VL1.
• Temperature curve for two lasers similar mild OP
  conditions modest fraction of light absorbed ?
  laser power limited.

• Increased current ? higher flux, closer to Rb D1
  ? lower PXe!
• Benefit from slight offset by allowing for better
  illumination of OP cell.

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Measuring Rb Electron Spin Polarization, PRb

• Laser absorbed quasi-homogenously
• Monitor small changes in amount of light
  transmitted when Bo is cycled.
• Efficient depletion pumping of
• ground-state m sublevels.
• Provides in situ estimate of PRb

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Tracking PXe, PRb, vs. Cell Illumination

• High transmittance
• ? PXe tracks PRb.
• PRb increases as laser l approaches Rb D1, while
  PXe and transmittance both fall.
• Although PRb (along z) is optimal near Rb D1,
  poor transmittance indicates inferior cell
  illumination.

? PXe is greatest at an intermediate
offset--where both PRb and transmittance are
high.
Whiting et al., JMR, 197, 249 (2009).
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Effect of Laser Flux and Xecell on PXe

• Expected smooth, monotonic decrease in PXe with
  rising Xecell.
• Instead PXe increases with Xecell, peaks (at
  300 torr), then decreases but remains
  uncharacteristically high at elevated Xecell.
• Potentially useful for situations where
  simultaneously high PXe and Xe are desired.

VL2

• Effect not due to cell contamination, collection
  efficiency, or laser energy-dependent mechanisms
• Remain laser-power limited, as PXe rises linearly
  with flux (except at low Xe).

Whiting et al., JMR, 197, 249 (2009).
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Low-Field NMR and Laser Retro-reflection

• Measure Xe NMR in situ (Magritek Aurora, w/
  home-made pulse/detect coil and cell mounts
  (PTFE) and Bucking coil (100-fold reduction in
  noise)
• 2 mirror retro-reflects laser light ? 30
  free increase in PXe (under nominal conditions)

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Effects of Cell Temperature, Xecell on PXe

• Optimal cell temperature (Topt) is strongly
  dependent on Xecell.
• Lower Xecell ? higher Topt (and vice-versa)
• Increase in Xecell gives higher NMR signal ?
  even at 1400 torr.
• Independent of Xecell, Topt is poorly sensitive
  to both N2 and total cell pressure.

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Effects of Xe and Topt on PXe Build-up

• PXe build-up rate increases with OP cell exhaust
  temperature
• (gSE increases with Rb).
• Dividing initial build-up slope by Xe estimate
  of OP efficiency

• Complex dependence on both Tcell and Xecell
• At fixed (lowish) Tcell, slope follows our PXe
  trend OP at Topt gives expected trend
• Although gSE increases with temperature, PRb and
  PXe may decrease due to poor cell illumination.
• gSE also depends on SE pathway (binary vs. vdW)
  
• ? dependent on OP gas composition.

90 C
Xe
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Recent High-Field PXe Values

• Low Xe favors high T, vice-versa
• Among highest PXe values achieved at such high
  Xecell
• Enhancements 60,000 at 50 torr Xe, and gt12,000
  at 2000 torr Xe
• Origin of interplay of temperature, Xe
  concentration?

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Tunable VHG-LDA with On-Chip Grating

• 80 W (_at_fiber, 60-70 W _at_cell)
• 0.3 nm Dl
• 1.5 nm tunability

Barlow et al., ENC Conf. (2009).
QPC
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PXe , T vs. Xecell, Tcell

• When PXe maxed (at TOPT), l for peak absorbance
  matches l for peak PXe
• As Tcell goes up, peak absorbance deepens,
  red-shifts, and broadens
• Clear benefit to OP at each Xe's TOPT (esp.
  lower Tcell for increased Xe)
• Rb absorbance dependent on Xe shift much
  greater than expected
• (0.2 vs. 0.04 nm at 2000 torr Xe).

2000 torr Xe
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• In presence of Xe, Rb D lines
• Broaden
• Shift
• Grow an increasingly large (red-side) shoulder!
• Xe-dependent Rb lineshape may be contributing to
  observed effects.

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Summary

• VHG-narrowed LDAs high laser flux / narrow
  linewidths
• ? high Rb absorption efficiency ? up to 3-fold
  improvement in PXe
• Slight offset from Rb D1 demonstrated to enhance
  PXe
• PRb monitored via changes in laser transmission
  while cycling Bo.
• Anomalous dependence of PXe on Xecell from
  interplay of Topt, Xecell
• Exploiting this effect Further optimization ?
  high PXe (including 55, 32, 23, and 11 at
  50, 300, 500, 2000 torr Xe).
• On-chip grating gives narrowed (lt0.3 nm),
  tunable, 80 W LDA
• Origin of Topt / Xecell not (yet) understood
  Xecell-dep Rb D spectrum could be contributing
  factor.
• To achieve best PXe at given Xecell, all OP
  parameters should be optimized
• Results could have impact on other SE OP designs,
  Alkali metals (Cs, K), and noble gases (He, Kr),
  as well as applications

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Acknowledgements

• OP Team
• Nick Whiting
• Panayiotis Nikolaou
• Neil A. Eschmann
• Dr. Michael J. Barlow
• (University of Nottingham, UK)
• Other Group Members
• Kassie Chaffee, Ping He
• Indra Saha, Jennifer Shapiro
• Laura Walkup, Laura Buck
• Kyle Power
• Shavonne Montgomery

Support NSF (CAREER REU) Research
Corporation School of Medical and Surgical
Sciences-University of Nottingham, UK GE
Healthcare-Amersham