A snake-in-the-ring keeps spins aligned.

A snake-in-the-ring keeps spins aligned

Using a Soviet-conceived device known as a Siberian snake, U.S. physicists have wormed their way out of a tricky technical problem encountered in accelerating elementary particles.

The problem arises in trying to keep the spin of subatomic particles aligned, or polarized, as they whiz around an accelerator. Particles that maintain such alignment during high-energy collisions give scientists a window on the strength of the spin-dependent portion of the strong force, which holds nuclear particles together. Although the magnetic field that keeps a charged particle circling through the accelerator changes the direction of spin with each lap, at most energies these effects tend to cancel each other out after successive laps around the ring.

But at certain energies, called depolarizing resonances, the changes in spin direction become additive, throwing the spin orientation of such particles as protons out of alignment. While misalignment at a particular energy can be overcome electromagnetically, says physicist Alan D. Krisch of the University of Michigan at Ann Arbor, high-energy accelerators have thousands of resonances. Correcting for each misalignment then becomes impractical, if not impossible.

Enter the Siberian snake.

Although the device was first proposed in 1974 by Soviet theorists Yaroslav S. Derbenev and Anatoly M. Kondratenko of the Novosibirsk Laboratory, researchers were unable to build it because none of the world's existing high-energy accelerators had enough room in their beam pipes for the 19-foot-long electromagnet. The snake produces a magnetic field that reverses the spin of every proton in a particle beam each time the particle travels around the accelerator ring. As a result of the spin flip, unwanted magnetic disturbance that redirects a proton's spin after one lap has the apposite effect the next time around. Thus, the two effects cancel each other out and the particle beam remains polarized.

"It's a cute idea and it works for all resonances at once," says Krisch, who collaborated with scientists from the University of Michigan, Indiana University in Bloomington and the Brookhaven National Laboratory in Upton, N.Y., to build and insert the device inside the coolerring accelerator of Indiana University's cyclotron.

During the weekend of Aug. 5, the researchers put the snake through its paces. First, with the Siberian snake switched off, they accelerated protons in the cooler ring to an energy of 108.45 million electron-volts -- one of two depolarazing resonances at the Indiana facility. A polarimeter measured the fraction of protons spinning in each direction. At resonance, the fraction of protons with aligned spins decreased from 70 to 20 percent. But after the investigators turned on the snake electromagnet, spin alignment remained at 70 percent despite the resonance. Krisch says he sent a telegram to the Novosibirsk Laboratory informing the director of the results.

Perhaps fittingly, Krisch says, one of the first high-energy accelerators to exploit the snake concept will be UNK, the Soviet Union's 3-trillon-electron-volt accelerator, located about 70 miles south of Moscow.