Nuclear Magnetic Voodoo Exorcized
Magnetically aligned (“hyperpolarized”) helium-3 is a gas made of a precious rare isotope in a precarious state, and it begins to lose its alignment even as physicists prepare it. “We always say use it or lose it,” says Brian Saam of the University of Utah in Salt Lake City. Saam provides hyperpolarized helium-3 to medical scientists, who use it somewhat like a dye in the lungs’ air space. The gas lights up in a magnetic resonance imaging (MRI) scan and is safe to inhale. The US Food and Drug Administration is considering it as a standard tool for diagnosis, while other uses from collider experiments to materials analysis continue to emerge.
Scientists prepare and store the gas under pressure in spherical glass ampules two to three inches in diameter. The sphere minimizes the surface with which the gas interacts, says Saam, yet it’s no guarantee of a good “cell,” as users call the vessels. Until recently, he thought that what made a cell good was “voodoo,” but that impression changed with an observation his graduate student made. At first, when the student told him that a strong magnet had corrupted a new cell, Saam thought plenty of other explanations were more likely. “But when he repeated the experiment the third or fourth time,” says Saam, “we knew we had something.”
With other colleagues, the researchers found they could restore wrecked cells by spinning them in the magnet as they turned down its strength–a standard recipe for demagnetization, similar to “degaussing” a computer monitor with an oscillating field. This result implied that the vessels themselves were magnets. Subsequent tests suggested that the cells magnetized only when they contained some rubidium metal inside, a by-product of the helium alignment procedure. The team hypothesizes that rubidium chemically reacts with microscopic iron impurities in the glass to make them into magnets. The orientation of helium nuclei can be disrupted as atoms whiz past these micromagnets, they argue. Based on diffusion theory, they calculated the magnetic particles in the glass to be about 1000 atoms across.
“I think it’s going to take more experimental work to really clarify the physics of all this,” says helium polarization pioneer Will Happer of Princeton University, “but they’ve obviously identified something really interesting–it’s a real discovery.”
“This has always been a squishy field full of lore and black magic,” says Bas Driehuys of Nycomed Amersham, a company in Durham, NC, that sells hyperpolarized helium-3 gas for MRI use. “But this discovery appears to be a foothold from which we can develop a real understanding of what’s going on.”
–Oliver Baker
Oliver Baker is a freelance science writer based in Davis, CA.