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

Jan 18, 2013
An ensemble of spread-out neutrons (left) can be regrouped by passing the beam through a spatially varying magnetic field and flipping the neutrons at suitable times with a radiofrequency (RF) field. Eventually the neutrons will be bunched up again (right). Credit: 2012 Masaaki Kitaguchi, Kyoto University

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 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 : 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 . Their 'runners' move at a velocity of a few meters per second through a magnetic field. As they do so, their can be flipped using radiofrequency fields, causing a change in energy—and thus in . When the strength of the 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.

Explore further: 3-D imaging reveals hidden forces behind clogs, jams, avalanches, earthquakes

More information: Arimoto, Y., et al. Demonstration of focusing by a neutron accelerator. Physical Review A 86, 023843 (2012). pra.aps.org/abstract/PRA/v86/i2/e023843%20

add to favorites email to friend print save as pdf

Related Stories

Shielding for ambitious neutron experiment

Jul 24, 2008

In science fiction stories it is either the inexhaustible energy source of the future or a superweapon of galactic magnitude: antimaterial. In fact, antimaterial can neither be found on Earth nor in space, is extremely complex ...

Neutrons escaping to a parallel world?

Jun 15, 2012

In a paper recently published in European Physical Journal C, researchers hypothesised the existence of mirror particles to explain the anomalous loss of neutrons observed experimentally. The existence of such mirror matter ...

Are pulsars giant permanent magnets?

Nov 22, 2011

Some of the most bizarre phenomenon in the universe are neutron stars. Very few things in our universe can rival the density in these remnants of supernova explosions. Neutron stars emit intense radiation ...

Recommended for you

Argonne research expanding from injectors to inhalers

18 hours ago

There is a world of difference between tailpipes and windpipes, but researchers at the Department of Energy's Argonne National Laboratory have managed to link the two with groundbreaking research that could ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.