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 3 He Neutron Spin Filter –Overview of the past & present –News on commercial SEOP base polarized gas production system

SESAME Test on Asterix Mike Fitzsimmons 1, Wai Tung Hal Lee 2, Roger Pynn 3, Paul Stonaha 3, Shah Valloppilly 3 and Adam Washington 3 1 Los Alamos National Laboratory, Los Alamos NM Oak Ridge National Laboratory, Oak Ridge TN 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) )  = wavelength, m n = mass,  n = gyromagnetic ratio) L +B -B sample 

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 D 2 O. Polarization as a function of suspended solid sphere was given in W.G.Bouwman & M.Th. Rekveldt, Physica B, (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  200 mm neutron

SESAME Test on Asterix, LANSCE V V V V T/T  S T/T = triangular coil pair V = v-coil (  /2 “flipper”)  =  flipper S = sample

SESAME Test on Asterix, LANSCE Wavelength (Å) Polarization normalized to beam polarization 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 D 2 O in a glass capsule. This is the baseline measurement to calibrate the samples which are polystyrene particles suspended in D 2 O This sample has 20% volume fraction of polystyrene particles suspended in D 2 O. The particles have an average size of 100 nm. Our calculation expect the polarization to drop to zero at about 10 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)

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 10 8 Hz/Tesla (2)The velocity of a neutron in a media is given by v = n v 0 v 0 = 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 nAnA nBnB B n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 ~ Å -2 with a variation between Å -2 down to Å -2. Modeling using Nb ~ 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 Z1Z1 Z2Z2 Pulsed R.F. thickness= 4mm Nb ~ Å 2 =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 3 He Neutron Spin Filter

Past –A roll-on/Roll-off polarizer. –On-beam continuous optical pumping to keep 3 He polarization stable. –Adiabatic fast passage switching of 3 He 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 3 He 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 2x 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 ( CEO: Bill Hersman, XeMed & University of New Hampshire 2 m

Polarized 3 He 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), 250  C, 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 long 1.4 mm wall Next run of GE180 will be the end of 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 3 He 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 – Technician – now Scientific Associate – March 2008 Scientific Associate – September 2008