Hannah Burrall

Optimization of 3He Neutron Spin 

Filters for the Neutron Spin Echo 

Spectrometer
Presentation Overview

◦ Motivation

◦ Background

◦ What is a neutron spin filter (NSF)?

◦ Why use NSF?

◦ Developing 3He polarizer and analyzer

◦ Optimization techniques

◦ Applications on CHRNS Neutron Spin Echo (NSE) Spectrometer

◦ Closing Remarks

2
Project Motivation

Neutron Spin Echo 

Spectrometer

3
Project Motivation

Neutron Spin Echo 

Spectrometer

4
Project Motivation

Neutron Spin Echo 

Spectrometer

5
Polarized Neutron Scattering

Moon et al, Phy. Rev.181, 920 (1969)

Cross Sections:

◦ Study magnetic 

systems

◦ Measure four cross 

sections

◦ Example:

◦ Polarize spin-up

◦ Collect spin-down

◦ Separate magnetic 

scattering from 

nuclear scattering

(Polarization analysis)

Slide courtesy of Joelle Baer

6
Neutron Spin Filter (NSF)

K.P. Coulter et al, NIM A 288, 463 (1990)

polarized 

outgoing 

neutrons

unpolarized

incoming 

neutrons

polarized 

3He

3He Isotope

◦ Neutron Spin Filters:

1. Polarize broad wavelength 

band of neutrons

2. Polarize larger & widely 

divergent neutron beams

3. Integrate polarizer and flipper 

capabilities

◦ Strong spin-dependent 

neutron absorption cross 

section

H

7
What is a 3He Cell? 

Blush

Reliance

Mozart

◦ Boron-free aluminosilicate 

GE180 glass container

◦ Filled with pressurized 3He

◦ Combination of Rb/K 

distilled metals

◦ Variety of shapes & sizes

8
Polarizing the Cell Spin-Exchange Optical 

Pumping (SEOP)

1. Polarize rubidium (Rb)/potassium (K) 

electrons

2. Spin exchange between Rb/K and 3He 

nuclei

◦ Heat 3He cell to vaporize Rb/K

◦ Fully illuminate near infrared 

laser light on cell

◦ Photons excite electrons in the 

metal 

Diagram of SEOP Station

Schematic from Wangchun Chen

9
Polarizing the Cell

◦ Hyperfine 

interaction between 

Rb/K and 3He

◦ Cool cell before 

moving to beamline

◦ 1-2 days to prep for 

experiment

3He SEOP Setup

10
Magnetically Shielded Solenoid

Diagram from M. T. Hassan

◦ Optimize 3He cell lifetime

◦ T1

d-d - Dipole-dipole

◦ T1

wall - Cell wall

◦ T1

fg - Field gradient

Diagram of Magnetically Shielded Solenoid

11
Magnetically Shielded Solenoid

Side View 

RF 

Sleeve

NMR 

Coil

Front View 

3He Cell

12
◦ Computer controlled 

mapping system

◦ 3-axis Hall probe & 

Fluxgate magnetometer

◦ Axial & vertical 

translation

Field Mapping Setup 

Z

Y

Optimization of End Compensation

13
◦ Scan length of device plus 3 cm 

on either side (0-24 cm)

◦ Take B-Field measurement 

every 0.2 cm

Optimization of End Compensation

14
Optimization of End Compensation

◦ Fit scan with a 

superposition of three 

Gaussian functions

15
Optimization of End Compensation

◦ Scan multiple end 

compensation winding 

configurations

◦ Vary number of turns

◦ Calculate field gradient 

(8 – 16 cm)

Best Configuration

Front 6, Back 7

16

Cell Length
Optimization of End Compensation

◦ Manual maps limit 

noise from DAQ

◦ Gradient: ∂Bz/∂x

B

◦ Scan: 5.6 x 10-4 cm-1

◦ Manual: 4.8 x 10-4 cm-1

17
Testing the Cell Lifetime

◦ Characterize field 

gradients on PHADES 

neutron beamline

◦ Cell lifetime: 

107.4 ± 0.2 hours

◦ Confirms previous 

optimization results

◦ Front 6, Back 7

Plot of 3He polarization

PHe,

PHe,

Enlarged plot 

from left

4 – 10 hours

3He Polarization

3He Polarization

18
Flipping the 3He Cell Polarization 

◦ 3He magnetic moment (μ) 

precess about B0

◦ Larmor Frequency: ω=γB0

◦ Oscillating B1 orthogonal to B0

causes μ to precess about B1

◦ Adiabatic Fast Passage

(AFP) Condition

◦ Rotation rate (ω′)<< Larmor 

Frequency (ω)

3He

B0

μ

3He Precession Rotating 

Frame

B0

B1

μ

3He

He 3

end

3He

start

He 3

ω′ < γB0

Flipping 3He Polarization

Schematic from Joelle Baer

19
Building the RF Sleeve

◦ Sheet of Teflon rolled into 

a cylinder

◦ 8 cm2 hole for cell tip-off

◦ 2.5 cm on either end for 

interlocking tabs

◦ ~0.5 cm slits in sinusoidal 

distribution

2 cm

4 cm

14 cm

36.4 cm

31.4 cm

Schematic from Wangchun Chen

20
Building the RF Sleeve

◦ 48 slits wound 

with 22 AWG Cu 

wire

◦ Unique sinusoidal 

winding pattern

◦ Right Hand Rule

◦ Creates B Field 

(B1) orthogonal to 

main field (B0)

B1 B1

B0

21
Testing the RF Sleeve

◦ Free Induction Decay 

(FID) Signal from NMR 

pick-up coil 

◦ 180° phase shift after 

AFP inversion

◦ 4 x 10-4 loss in 

polarization

180°

22
Application on NSE Spectrometer

Neutron Spin Echo 

Spectrometer

23
Application on NSE Spectrometer

Neutron Spin Echo 

Spectrometer

NSF 

Device

◦ New measurement capability 

◦ Intensity Modulated Neutron 

Spin Echo Spectroscopy

24
Acknowledgements Special thanks to

Wangchun Chen, Shannon 

Watson & Taufique Hassan

and

Gordon Jones & Joelle Baer

Scott Slifer, NCNR Mechanical Technician Group

National Science Foundation

Center for High Resolution Neutron Scattering

Julie Borchers, Joe Dura & Brandi Toliver

NIST SURF Program

Fellow SURFers!!

25
Making a 3He Cell 

Rb

Retort

K 

Retort

Cell

26

Photos from M. T. Hassan