Earlier this year, a panicked U.S. congressional panel traded barbs about who was at fault for a sudden and surprising shortage of helium-3. The stable isotope is crucial in MRI lung research, low-temperature experimental physics, and—at the heart of the congressional dustup—in neutron detectors that can reveal smuggled nuclear materials. The United States has historically been the biggest global supplier of He-3, so the shortage there is affecting the entire world. In many countries, authorities are scrambling to find ways to procure more of the gas, stretch their remaining supplies, and find alternatives. But for some users, there are no substitutes.
He-3 is one neutron short of the two neutrons and two protons that make up its heavier cousin, the helium-4 of party balloons and silly voices. The lighter isotope is rare in nature, but it is a by-product of the decay of tritium (hydrogen-3) in thermonuclear weapons.
The unique structure of He-3 makes it ideal for refrigerating science experiments and other machinery to below 1 kelvin. That's critical at major laboratories such as the European Organization for Nuclear Research (known as CERN), in Geneva, where exotic experiments demand extreme cold. He-4, which cools the superconducting magnets inside CERN's Large Hadron Collider, can push temperatures only to about 1.8 K, says Johan Bremer, a physicist who designs and builds cryogenic installations for CERN's experiments. He-3 pushes that number to 0.01 K.
The isotope has also brought about a recent revolution in pulmonary science and medicine, says Dr. Jason Woods, an assistant professor of radiology at Washington University, in St. Louis, who testified before Congress on the shortage. The development of a new kind of He-3 diffusion MRI, he says, has allowed "our scientific knowledge of lung physiology and pathology [to accelerate] exponentially over the past few years."
The problem started after the terrorist attacks in New York City and Washington on 11 September 2001. U.S. government agencies dedicated to national security commissioned large quantities of neutron detectors. Now, many thousands of such scanners are used to watch for plutonium at airports, shipyards, and border crossings. These scanners all rely on He-3 and consumed 80 percent of the gas used in the United States from 2005 to 2009.
But the annual production of He-3 in the United States has fallen to less than 8000 liters, according to John Pantaleo, who directs the U.S. interagency committee in charge of disseminating the gas. Most He-3 is extracted from decaying tritium in thermonuclear warheads—about every five years, when that tritium is replaced. Because the United States has been reducing its nuclear weapons stockpile, the amount of He-3 produced has plummeted over the last two decades. And because demand grew rapidly after 9/11, the U.S. He-3 stockpile has dropped from more than 200 000 L in 2001 to less than 50 000 L today.
According to Steve Fetter, assistant director at large with the White House Office of Science and Technology Policy, global demand is about 40 000 L in 2010. But Pantaleo's group will dole out just 12 000 L. Programs like Woods's, which rely on the unique physical structure of the gas, get first priority, neutron detectors second, and everything else comes third. Woods expects about 1800 L to be distributed to medical imaging researchers this year.
But as much as the shortage may constrict U.S. research, it has a bigger impact on researchers in other countries. Since Russia stopped selling its He-3 supply abroad in 2008, the world is dependent on U.S. gas. The result is an out-of-balance international market. Physics Today reported in June that Netherlands-based Leiden Cryogenics, which makes dilution refrigerators for low-temperature physics, paid US $2150 for one liter of He-3 in early 2010, an amount that just one year earlier would have cost $100.
Scientists feeling the He-3 pinch have three choices: Get He-3 from expensive and unpredictable sources, make more He-3, or find a suitable substitute. The first option is prohibitive. AEGIS, a project Bremer is working on at CERN that measures the effects of gravity on antimatter, needs He-3, but at today's price he estimates that 12 grams of the very light gas would cost $200 000.
The second choice—making more—is not a short-term option. The half life of the tritium in nuclear weapons is 12.4 years. Tritium can also be separated from heavy-water reactor by-products, an option that Pantaleo says is under investigation. However, that option requires diplomatic footwork as well as time.
That leaves the third option: Come up with a substitute. Rebecca Nikolic and her group of engineers at Lawrence Livermore National Laboratory's Center for Micro- and Nanotechnology may have found one alternative to today's neutron sensors, which are the biggest He-3 gluttons. The team has replaced the traditional He-3 gas with solid sensors made of silicon and boron.
For a long-term supply, Pantaleo says the U.S. government is looking into separating the isotope from the U.S. helium-4 reserve near Amarillo, Texas. There, several tens of billions of liters of He-4 could provide several hundred thousand liters of He-3, he says.
Unfortunately, the He-4 reserve may not be an inexhaustible resource, either. According to the United States' National Research Council, that supply is on track to be depleted within the next 40 years.