A method for reversing the velocity spread in neutron beams should boost the accuracy of precision experiments

Neutrons offer a combination of properties that make them exquisitely sensitive and versatile sensors. They are charge neutral, which means they do not interact with electric fields, and they possess a magnetic momentum, making them perceptive to magnetic fields. To achieve the highest sensitivity in neutron-based experiments, researchers aim to produce very dense neutron beams. But they also have to ensure that the density does not decrease as the neutrons are transported from source to target. Addressing this latter issue, an international research team including Yoshichika Seki of the RIKEN Nishina Center for Accelerator-Based Science, Wako, Japan, has demonstrated a method for refocusing a neutron beam that has lost its initial density.

As neutrons exit their source, they behave in a similar way to marathon runners. Initially they are densely packed but gradually they spread out over the course. For neutrons, this decrease in density is difficult to prevent. Conventional focusing techniques fail, precisely because neutrons carry no charge and so cannot be controlled with electric fields. Seki and colleagues have now found another way. Consider the marathon runners: if after some time into the race all athletes are asked to return to the starting line, then the advantage of the faster runners will become a disadvantage. By the time the runners are back at the starting point, they will all be bunched together again.

Seki and his colleagues use a similar trick to undo the velocity spread in a neutron beam. Their 'runners' move at a velocity of a few meters per second through a magnetic field. As they do so, their magnetic moments can be flipped using radiofrequency fields, causing a change in energy—and thus in velocity. When the strength of the magnetic field changes within the region through which the neutrons pass, the flipping can be done in a way to 'punish' the fast neutrons that have already established an advantage, while 'rewarding' the slow ones (Fig. 1). Eventually, the neutrons regain their initial density.

The new method will help in experiments to search for the neutron's so-called electric dipole moment, which in turn could provide a clue about the origin of matter in the Universe. The researchers expect the technique to also be useful in practical applications. "Nowadays neutron beams are widely employed in material science and in the medical field, so our method should have a very broad impact," says Seki.