Boffins slow neutrons to 15mph

Handy, if you're looking for supersymmetry

Boffins at CalTech have found a way of getting free neutrons to slow down to just 15 miles per hour. That is practically stationary, considering they generally zip around at decent fractions of the speed of light.

Researchers will be able to study the way neutrons decay in much more detail, which could provide an insight into some of the most fundamental theories of physics: behaviour that differs from the predictions of the standard model may be evidence for supersymmetry, for example.

Neutrons are unstable beasties. Outside a nucleus, they decay rapidly into a proton, and electron, and an anti-neutrino. Traditionally, researchers try to get hold of free neutrons would slow them down as they came out of a nuclear reactor. They could be slowed quite a lot, but would still be moving at a few feet per second.

The new technique slows the neutrons down even further, and researchers have managed to collect the ultra cold particles in concentrations of 140 per cubic centimeter.

To get a neutron moving this slowly, you have to make it lose a lot of energy. The researchers at the Los Alamos laboratory began by with smashing protons from a nuclear accelerator into a solid material like tungsten. This knocks the neutrons out of the tungsten nuclei.

At this point, they are still moving around pretty fast, so they are bounced around in a nearby plastic material. Some will bounce into a cake sized block of solid deuterium (heavy hydrogen) that has been cooled down to a temperature a few degrees above absolute zero. This slows them down a lot.

In fact, in the deuterium crystal lattice, a fast neutron can lose enough of its energy that it can't get through the walls of the experimental apparatus, and instead bounce along its nickel walls. Once trapped, they can be used for other experiments.

Filipone plans to use the neutrons to investigate gravitational attraction at very small distances, using "an inherant quantum limit" of the neutrons. The team will go back to Los Alamos later this year to see what else they can do with their super-cold, super-slow particles. ®

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