Polarized Neutron Techniques Development at ORNL

Hal Lee, Dennis Rich

Instrument Development Group,

Neutron Facilities Development Division

Oak Ridge National Laboratory

Oak Ridge, TN 37831, USA

• A recent test of SESAME on polarized neutron reflectometer Asterix, LANSCE

• Spin Contrast Imaging

• Polarized 3He Neutron Spin Filter

– Overview of the past & present

– News on commercial SEOP base polarized gas production system

SESAME Test on Asterix

Mike Fitzsimmons1, Wai Tung Hal Lee2, Roger Pynn3, Paul Stonaha3, Shah Valloppilly3 and Adam Washington3

1 Los Alamos National Laboratory, Los Alamos NM 87544

2 Oak Ridge National Laboratory, Oak Ridge TN 37831

3 Indiana University, Bloomington, IN 47408

SESAME Test on Asterix, LANSCE

• The technique measures a real-space correlation function

• Z is approximately equal to the separation of rays by the magnetic field – the difference between them is second order in the scattering angle and third order in the birefringence.

• Spin angle

( R. Gähler, R. Golub, K. Habicht, T. Keller, J. Felber, Physica B 229 (1996) 1-17. )

)0()(

0P

P GZGe

)cos(),,0(1

4)(

2

2

zzy

zy Qd

QQddQdQ

AZG

cot2

2BLh

mZ nn = wavelength, mn = mass, n = gyromagnetic ratio)

L

Q

+B -Bsample

• The main practical issue is how to produce the parallelogram field region. Our approach is to use triangular coils.

• Simulations show fields are uniform within numerical accuracy

• For our coil geometry

• Our samples are diluted polystyrene particles suspended in D2O. Polarization as a function of suspended solid sphere was given in W.G.Bouwman & M.Th. Rekveldt, Physica B, 276-278 (2000) 126. For 100 nm polystyrene particles in 10-20% concentration, we expect the polarization to drop to a minimum at ~11 Å.

SESAME Test on Asterix, LANSCE

+ - - + + - - +

/2 /2200 mm

neutron

45 m, 14.0 T, 006.0B nm, 1for nm 125

nm cot147000 2

LZ

BLZ

SESAME Test on Asterix, LANSCE

VV

V

V

T/TT/T

T/TT/T S

T/T = triangular coil pair

V = v-coil (/2 “flipper”)

= flipper

S = sample

SESAME Test on Asterix, LANSCE

Wavelength (Å)

Pol

ariz

atio

n no

rmal

ized

to

be

am p

ola

rizat

ion

This one shows the polarization vs wavelength when all triangular coils are in place and with the beam at 1x1 cm.

(1) We have spin echo of the incident beam.

(2) The drop in polarization shows the coils are not perfectly aligned but sufficient to do an experiment.

(3) There is a high-frequency oscillation which our evaluation showed it is possibly due to the -flipper not perfectly aligned.

SESAME Test on Asterix, LANSCE

This sample was D2O in a glass capsule. This is the baseline measurement to calibrate the samples which are polystyrene particles suspended in D2O

This sample has 20% volume fraction of polystyrene particles suspended in D2O. The particles have an average size of 100 nm. Our calculation expect the polarization to drop to zero at about 10 Å.

Wavelength (Å) Wavelength (Å)

SESAME Test on Asterix, LANSCE

Comparision of a 10% sample (green) with a 20% sample (white).

And comparison of a 40 nm sample (white) with a 100 nm sample (green).

Wavelength (Å) Wavelength (Å)

Neutron Spin-Contrast Imaging -

A direct 3-dimensional imaging method

Spin Contrast imaging

“Neutron spin-contrast imaging” is based on 2 characteristics of neutron:

(1) Neutron magnetic moment precesses in a magnetic field with a precession angle given by

n = gyromagnetic ratio = -1.8 x 108 Hz/Tesla

(2) The velocity of a neutron in a media is given by v = n v0

v0 = neutron velocity in vacuum

n = Index of refraction

It will take longer for a neutrons of the same

wavelength to go through a material with

smaller index of refraction

21

2Nbn

2

1

)(T

T

n dttB

nA

nB

B

n

(A. I. Frank et. al., Physics of Atomic Nuclei, 65, 2009 (2002) )

Spin Contrast imaging

What does it take to reach z=1 mm resolution?

Suppose we can only resolve to 10º. Typical scattering length density

Nb ~ 10-6 Å-2 with a variation between 10-6 Å-2 down to 10-7 Å-2.

)])([(2

3 zNbBh

m nn

Wavelength (A)

10 20 30 40 50 60 70 80 90 100

B (

Tes

la)

0.01

0.1

1

10

Modeling using Nb ~ 10-6 Å-2, =30Å, B=1T and an ideal polarization analyzer.

Spin Contrast imaging

Use pulsed radio-frequency coil to start the precession while the neutrons are passing the materials. Neutrons at different depth will “map” different path lengths. Using Time-Of-Flight technique to back-track the location of the neutrons when the precession begins, the difference in Nb along the neutron flight path can be calculated by taking the difference in the precession angle between different TOF, i.e. a 3D mapping.

Uniform B

Z1

Z2

Pulsed R.F.

thickness= 4mm

Nb ~ 10-6 Å2

=30Å, B=1T

Time between frames = 5 sec

After subtracting the precession angles (angle variation <0.02º)

In principle it can work, in practice difficult to realize.

Polarized 3He Neutron Spin Filter

Polarized 3He Neutron Spin Filter

• Past– A roll-on/Roll-off polarizer.

– On-beam continuous optical pumping to keep 3He polarization stable.

– Adiabatic fast passage switching of 3He polarization – spin-filter + spin-flipper. Useful for confined space & short wavelengths.

– Experiment on Single Crystal Diffractometer, Intense Pulsed Neutron Source

– Developed an analyzer for the Magnetism Reflectometer at the SNS.

• Present & near future– Building a gas filling station

– Experimenting on wide-angle analyzer cells

– Exploring polarized gas production system

– Tests of online polarizers/analyzers at HFIR & SNS

Polarized 3He Gas Production

The following description of the XeMed system was provided by the company. The information is on the web.

• Laser assembly leverages Xemed capabilities developed for polarizing medical gases; all components in-house & tested.

• Pressure vessel encases pumping cell. Vessel and feed-throughs tested at 160psi.

• Field uniformity yields in situ NMR free-induction decay signal of 2x10-5.

• Pyrex prototype cell completed, Aluminosilicate cell being finalized.

• Two-zone thermal bath regulated by flowing silicone oil and heat spreaders.

• Suitable for use in medical imaging and nuclear/neutron physics.

• Performance projected to equal MEOP at ~1/5 price• First tests November 2007• Orders/deposits accepted in 2008 for 2009 delivery.• Also available with capability doubled.

• Company: XeMed (www.xemed.com)• CEO: Bill Hersman, XeMed & University of New Hampshire

2 m

Polarized 3He Gas Production

The following description of the XeMed system was provided by the company. The information is on the web.

• Theoretical framework includes spectral dependence of laser absorption, variable alkali ratio, temperature, pressure variables

• Calculates alkali polarization as function of alkali thickness, obtains 3He spin-up rate

• Can optimize a defined figure-of-merit• Allows comparison of narrow vs broad lasers,

optimal hybrid K:Rb alkali ratio vs temperature.• For laser power of 2 kW, spectral output 2nm,

divergence 2mrx2mr, optimal parameters are 10cm x 130cm, 8.3 liters, 6 bar (cold, 9 bar hot), 250C, alkali K:Rb 10:1 liquid (3.9:1 vapor)

• Spin-up time is 4 hours, 50 standard liters.• Asymptotic polarization depends on cell lifetime

and X-factor• 20 hr T1 and X=0.2 yields 72% polarization

Large diameter GE180 Glass Tubes

General Electric Glass Plant has made another batch of 1”/ 25.4 mm diameter GE180 glass tubes for us. They arrived a few days ago.

Dimensions: ø1” / 25.4 mm, 60” / 1.5 m long1.4 mm wall

Next run of GE180 will be the end of 2009. Small cells can use the standard ø15 mm tubes.

We will share the tubing free of charge. To avoid too much overseas shipment, a batch will be sent to Ken Anderson next week to be distributed on request. To be sure the supply last till the next GE180 run, we’d suggest a shipment of 3-4 tubes on each request for a start.

We are hiring

R&D efforts in 2008

• Polarized neutron instrumentation R&D – – SESAME techniques – Polarized 3He neutron spin filter

• Polarized nuclei – Polarized Hydrogen – sample polarization– Explore the possibility of a polarized hydrogen based polarizer – Extending to heavier nuclei

Positions in 2008

• Instrument development fellow – www.sns.gov

•Technician – now

• Scientific Associate – March 2008

• Scientific Associate – September 2008