Polarized Beams Using He-3 at the NCNR Triple-Axis Spectrometers

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Polarized Beams Using He-3 at the NCNR
Triple-Axis Spectrometers

• Ross Erwin
• Tom Gentile
• Wangchun Chen
• Sarah McKenney

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Outline

• NCNR triple-axis
• BT7 specs
• He-3 pros and cons
• Setup and components
• Transport tests
• Performance
• Monitoring tool

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NCNR Triple-Axis Spectrometers

• 1 cold neutron (SPINS). Can use SM transmission
  polarizers.
• 2 thermal in confinement (BT7 and BT9).
• BT2 recently decommissioned was only thermal
  polarized beam using Heusler.
• BT9 (current BNL secondary) is scheduled for
  upgrade similar to BT7.

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BT7 specifications

• Contact Ying Chen (301-975-6442)
  Ying.Chen_at_nist.gov or Jeff Lynn (301-975-6246)
  Jeff.Lynn_at_nist.gov
• Monochromators doubly focusing (DFM)
  interchangeable 20x20cm2 PG002 and Cu220
• Analyzer-Detector air-pad system with horizontal
  focusing or flat crystal array and multi-detector
  or PSD.
• Flux double-focusing PG002 at 40meV (open-50)
  flux1.8x108 n/cm2/s
• Background 1 ct/min at 14.7 elastic PG filter
  (open-50)
• See www.ncnr.nist.gov/instruments/bt7_new/

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DFM Contact Stephen Smee (410-516-7097)
Instrument Development Group at Johns Hopkins
Smee_at_pha.jhu.edu
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BT7 with the BNL magnet
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BT7 3-axis Spectrometer with He-3 Polarized Beam
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Polarized Beam Setup

• He-3 cells are polarized offline and transferred
  to the BT7 solenoids at the start of the
  experiment.
• If the cell has been already characterized, just
  check the transmission.
• Setup spin transport components flippers and
  guide fields.
• Setup is comparable to Heusler monochromator
  change.

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Why He-3?

• Not a monochromator.
• Use with any monochromator.
• Wide wavelength range.
• Wide angular acceptance.
• Transmission and neutron polarization competitive
  with crystal polarizers.
• Future performance improvements and online
  pumping

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He-3 problems

• Transmission wavelength dependent. Different
  cells for different wavelength ranges.
• Requires highly homogeneous magnetic field
  environment and thus takes up beam line space.
• Related high magnetic field experimental
  challenges.
• Non-pumped has time dependent transmission.
• Requires transport from longitudinal to
  transverse neutron polarization.

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He-3 polarized beam components

• Polarizer cell and solenoid
• Analyzer cell and solenoid
• Larmor-precession spin flippers
• Longitudinal guide fields
• Vertical guide fields
• Sample guide fields

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He-3 polarizer

• 25cm long magnetically shielded solenoid space
  limited.
• 9 cm diameter entrance and exit holes to
  accommodate BT7 incident beam.
• 20cm diameter solenoid shield to accommodate 10cm
  diameter glass cell.
• ?B/B ? 10-4
• 1/e lifetime intrinsic 300 hours. inline 130
  hours

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He-3 analyzer

• Borrowed from reflectometer so not ideal for BT7.
• 46cm long magnetically shielded solenoid. This
  will be reduced to save beam line space without
  much loss in lifetime.
• 9 cm diameter entrance and exit holes to
  accommodate BT7 exit beam.
• 28cm diameter solenoid shield to accommodate 10cm
  diameter glass cell.
• ?B/B ? 10-5
• 1/e lifetime intrinsic 450 hours. inline 350
  hours

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Spin Flippers

• 30 year old flippers need replacing with larger
  beam area and more heavily anodized Al wire.
• Calculation suggests that even with Dl/l0.02 and
  Dq0.02 flipping ratio still gt 100

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Longitudinal Guide

• Simple short solenoid serves as adiabatic guide
  field or transition from vertical to longitudinal
  guide field.

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Vertical Guide

• Vertical guide sections which are too short cant
  transport from He-3 polarizing solenoid unless
  they physically touch the magnetic shield.

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Flipping Ratio
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Transport Tests

• We have made careful transport checks for all of
  our components on a 3.7meV polarized beam
  reflectometer beam line with intrinsic flipping
  ratio of 90.
• Only found problems with precession
  spin-flippers, and short vertical guide sections
  near He-3 magnetic shield.

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Handling Time Dependence

• He-3 polarization decay is highly predictable
  unless there are problems.
• Simplest experimental check is to monitor the
  flipping ratio and compare to calculated.
• With additional beam monitors it is also possible
  to monitor He-3 polarizer transmission. This may
  require handling higher orders in the beam, and
  difficult to implement for analyzer were
  intensity may be low.
• Spread sheet tool

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BT7 He-3 polarizer performance
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BT7 He-3 analyzer
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Conclusion

• He-3 is a flexible competitive solution to
  crystal polarizers.
• BT7 triple-axis polarized beam setup works OK but
  needs work on spin transport. All of the
  components tested OK except for spin-precession
  flippers.
• We plan to have a He-3 cell polarization lab
  setup in the next 4-6 months.