A Whole New Angle on Neutron Spin Analysis

For most Americans, neutron spin-polarization filter cells are a relatively rare topic of conversation. Yet these exotic devices are essential to instruments that reveal the nature of certain kinds of magnetic materials – information that may play a major role in future technologies.

Now a team of investigators from PML's Radiation and Biomolecular Physics Division, the NIST Center for Neutron Research (NCNR), and NIST's glass/optical shop have devised an innovative, improved filter with the potential to accelerate progress in this field dramatically.

"The next generation of spin filters will require producing cells with more complex geometries while still maintaining or improving the achievable polarization values and storage times," says Thomas Gentile of the Neutron Physics Group. "Recently we have been able to make substantial advances toward this goal."

Materials scientists employ neutron beams in a variety of ways to examine the interior structure and other characteristics of materials. If a thin-film or bulk material has magnetic properties of interest, however, irradiating it with a beam of spin-polarized* neutrons can expose otherwise undiscoverable aspects of its magnetic structure and associated features.

"In general, neutrons can either scatter by interacting with the nuclei in material or with the magnetism in material," Gentile says. "Because a neutron has a magnetic moment, it interacts with magnetic moments within the material. The interaction affects the angle at which the scattered neutron leaves the sample, and may 'flip' the neutron's spin orientation."

So by measuring the exit angle and the polarization of scattered neutrons, scientists can uncover the arrangement and strength of magnetic regions in the material. That process, of course, requires some way to filter neutrons by their spin, much as sunglasses filter light rays by their angular orientation, and to record their final direction of flight over sufficiently large angles.

Filtering is accomplished by filling a transparent chamber or cell with helium-3 atoms, nearly all of which have been manipulated into the same spin orientation. (Helium-4, with a net spin of zero, cannot be polarized.) A neutron entering the cell will pass through only if it has the same polarization as the helium atoms. All other neutrons are absorbed by the gas.

Making such cells is a daunting endeavor. They are filled with helium-3 at near atmospheric pressure (1 to 3 bar), along with a small amount of nitrogen and two alkali metals: rubidium and potassium. To polarize the helium-3 gas, the cells are first heated to about 210 °C, which produces a fine vapor of metal atoms at a density of about 10¹⁴ per cubic centimeter.

The mixture is then exposed to tuned laser beams which, through a process called optical pumping, place all the alkali atoms in the same spin orientation. Over time, the metal atoms transfer their spins to the surrounding helium-3. The entire process is thus known as spin-exchange optical pumping (SEOP).** "It takes quite a while," Gentile says. "We typically let the process run all night or longer."

In conventional SEOP filter cells, about 75% to 80% of the helium is polarized. But with a new method for narrowing the bandwidth of its lasers, the NIST team was able to raise the polarization as high as 85% -- an apparent world record.

To date, one of the principal constraints on the application of spin filter to neutron scattering has been the angular range over which measurements can be taken. Because of the short angular span of conventional filter cells, most work has been done at comparatively small angles, or with a small angular range about a larger angle. However, several neutron scattering instruments can measure scattering over a large range of angles, and the ability to analyze neutron polarization over a wider arc offers a considerable improvement in measurement efficiency.

So the NIST team set out to see if large cells, covering an arc of about 120 degrees and about 1 liter in volume, could be fabricated.

One immediate problem is that any glass used for the filter cell has to allow maximum transmission of neutrons. That eliminates, for example, familiar borosilicate Pyrex glass. (Boron has a high neutron absorption cross section, and is used in control rods for fission reactors.) But aluminum, another Group IIIA element, is nearly transparent to neutrons. And there is an available boron-free aluminosilicate glass, General Electric's GE180, which also has low helium permeability and is resistant to alkali metals. 

"However, GE180 is only available in small-diameter tubing, and it is a difficult glass with which to work," Gentile says. "So for a while, because of the fabrication issues with GE180, we made a significant effort to employ fused quartz. Despite some success, this approach was ultimately unproductive due to unexpected temperature-dependent relaxation mechanisms.

"After 'going back to drawing board,' NIST's glass/optical shop staff developed a method to fabricate wide-angle cells from GE180. These cells are made from three blown sections that are cut and fused together to cover a 120 degree angular range. That is a remarkable achievement, without which the PML and NCNR work could not have progressed. And we are pleased that we have been able to obtain relaxation times of a few hundred hours and polarization comparable to that achievable in our typical cylindrical cells."

"Cells made from fully blown glass can often yield such long relaxation times, albeit with an arduous preparation process", says Qiang Ye, a guest researcher from Oak Ridge National Laboratory who has been the key person in preparing and testing the new cells. "However, we were concerned about the possible effects of the more complex fabrication process on the achievable relaxation time.  Although we were successful, sometimes we had to run through the entire preparation process twice to yield the best results."

Aaron Kirchhoff, a NIST optical and scientific glass expert who works on the special cells, says that "the inherent physical properties of GE180 causes the fabrication of neutron spin filter cells to be immensely challenging work. The high working temperature, narrow window of malleability, and propensity to boil makes this particular glass quite daunting. GE 180 is at once familiar yet innately unfriendly, much like an undomesticated breed of dog. No matter how cute he looks in the sweater vest your aunt crocheted, when you make eye contact you know that he would much rather eat you for dinner and be left alone in the wilderness."

Kirchhoff is part of a very old tradition that is critically important today. "There are some fabrication processes that have proven their usefulness throughout history and continue to do so, despite being viewed sometimes as archaic," he says. "Glass working is one of those processes. Perhaps unexpectedly, it remains inseparable from scientific research and social progress. Of course, it has not been left unchanged through history. On the contrary, the specific techniques used for developing new instruments must be adapted. Progress absolutely demands technical adaptations in fabrication."

Gentile and colleagues have assembled the new wide-angle cells into a test system. It is surrounded by a cylindrical solenoid (using aluminum wire to avoid neutron absorption) that is 37 cm in diameter and maintains a homogenous 25 gauss field that preserves the orientation of the neutrons. Neutrons pass through the solenoid and into an initial polarization cell. From there, the spin-polarized neutron beam travels to the sample, which is surrounded on both sides at a distance of 7 cm by the new wide-angle cells, each of which is 8 cm in cross-sectional diameter. If this setup performs as anticipated, Gentile and coworkers want to use it on a NCNR instrument called the Multi-Axis Crystal Spectrometer (MACS), which has recently been upgraded and is expected to be re-commissioned in a few months.

"Early on in the development of MACS, we reluctantly abandoned polarized neutrons to focus on high intensity," says John Hopkins physics professor Collin Broholm, a NCNR guest researcher who is responsible for the design, construction, operation and scientific program on MACS.

"We were surprised and delighted when - through the work of Tom [Gentile] and his group - it could be retrofitted to provide both high intensity and full polarization analysis. We expect many nice scientific results from the use of high intensity polarized neutrons on MACS."

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* Spin is a quantum-mechanical form of angular momentum that occurs in elementary and composite particles, and in whole atoms. It manifests itself as if the particle or atom were a solid charged object spinning around a central axis, and thus generating a magnetic dipole. Electromagnetic radiation of exactly the right energy can exert a sort of torque on the object, reversing or "flipping" its spin orientation. Spin-flipping lies at the heart of magnetic resonance imaging (MRI) and many other investigational techniques.

** An alternative polarization method, called Metastability Exchange Optical Pumping, is used at the leading European facility for polarized neutron research – the Institut Laue–Langevin (ILL) in Grenoble, France. ILL is also engaged in studying wide-angle polarization analyzers.