Flipping Spin May Boost Neutron Beams

Researchers have devised a way to beef up neutron beams that are used to study everything from the innards of new materials to fundamental particle physics interactions. The technique could potentially double the intensity of these costly beams, in which every particle must spin in the same direction.

Neutrons spin like tops, and when they travel in a beam, some will spin in one direction and some in the opposite. For research purposes, scientists often need a beam in which all the spins point the same way. The tried-and-true method to achieve this is to weed out any neutrons spinning in the wrong direction. But that means researchers must discard half their pricey beam, which can only be made with a nuclear reactor or a specialized particle accelerator. Hartmut Abele, Ulrich Schmidt, and their colleagues at the University of Heidelberg in Germany thought they could do better by flipping the wrong-way spins around.

The researchers passed a beam of very low-energy neutrons through a long electrical coil that produced a steady magnetic field pointing in the direction of the beam. When the neutrons entered the coil, those spinning in the same direction as the field slowed down, while those spinning in the opposite direction sped up. Meanwhile, another smaller coil inside the cylinder created another field, perpendicular to the beam, that flipped the spins back and forth. By tuning the strength of this oscillating field, the researchers could make the forward-pointing spins flip one more time during their passage through the cylinder than the backward-pointing ones. In theory, all the spins would end up pointing the same way as they passed out of the cylinder.

In practice, the new method yielded roughly a 2-to-1 mixture of spins, the researchers report in the 10 April issue of Physical Review Letters, because the steady magnetic field varied from place to place in the large coil, making some neutrons flip too few or too many times. The researchers say they will achieve a mixture of 9-to-1, however, just by using a better coil.

The new method should have plenty of applications, says physicist Paul Huffman of the National Institute of Standards and Technology in Gaithersburg, Maryland, especially if it will work with the higher energy beams commonly used to study materials. "If you can polarize [neutrons] without throwing away half your beam," he says, "you always win."