1. Polarized Beams Using He-3 at the NCNR Triple-Axis Spectrometers Ross Erwin Tom Gentile Wangchun Chen Sarah McKenney

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

3. 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.

4. BT7 specifications Contact: Ying Chen (301-975-6442) Ying.Chen@nist.gov or Jeff Lynn (301-975-6246) Jeff.Lynn@nist.gov Ying.Chen@nist.gov Jeff.Lynn@nist.gov Monochromators: doubly focusing (DFM) interchangeable 20x20cm 2 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=1.8x10 8 n/cm 2 /s Background: 1 ct/min at 14.7 elastic PG filter (open-50’) See www.ncnr.nist.gov/instruments/bt7_new/www.ncnr.nist.gov/instruments/bt7_new/

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

6. BT7 with the BNL magnet

7. BT7 3-axis Spectrometer with He-3 Polarized Beam

8. 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.

9. 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

10. 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.

11. 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

12. 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

13. 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

14. 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

15. Longitudinal Guide Simple short solenoid serves as adiabatic guide field or transition from vertical to longitudinal guide field.

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

17. Flipping Ratio

18. 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.

19. 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

20. BT7 He-3 polarizer performance

21. BT7 He-3 analyzer

22. 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.