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

SpectrometersRoss ErwinTom Gentile

Wangchun ChenSarah McKenney

Outline

• NCNR triple-axis

• BT7 specs

• He-3 pros and cons

• Setup and components

• Transport tests

• Performance

• Monitoring tool

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.

BT7 specifications

• Contact: Ying Chen (301-975-6442) Ying.Chen@nist.gov or Jeff Lynn (301-975-6246) Jeff.Lynn@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’) flux=1.8x108 n/cm2/s

• Background: 1 ct/min at 14.7 elastic PG filter (open-50’)• See www.ncnr.nist.gov/instruments/bt7_new/

DFM Contact: Stephen Smee (410-516-7097) Instrument Development Group at Johns Hopkins Smee@pha.jhu.edu

BT7 with the BNL magnet

BT7 3-axis Spectrometer with He-3 Polarized Beam

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.

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

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.

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

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.• ΔB/B 10-4

• 1/e lifetime: intrinsic 300 hours. inline 130 hours

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.• ΔB/B 10-5

• 1/e lifetime: intrinsic 450 hours. inline 350 hours

Spin Flippers

• 30 year old flippers need replacing with larger beam area and more heavily anodized Al wire.

• Calculation suggests that even with and flipping ratio still > 100

Longitudinal Guide

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

Vertical Guide

• Vertical guide sections which are too short can’t transport from He-3 polarizing solenoid unless they physically touch the magnetic shield.

Flipping Ratio

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.

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

BT7 He-3 polarizer performance

BT7 He-3 analyzer

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.