Physicists trap ultracold neutrons

Free neutrons are usually pretty speedy customers, buzzing along at a significant fraction of the speed of light. But physicists have created a new process to slow neutrons down to about 15 miles per hour—the pace of a world-class mile runner—which could lead to breakthroughs in understanding the physical universe at its most fundamental level.

According to Caltech professor of physics Brad Filippone, he and colleagues from Caltech and several other institutions recently succeeded in collecting record-breaking numbers of ultracold neutrons at the Los Alamos Neutron Science Center. The new technique resulted in the capture of about 140 neutrons per cubic centimeter, a number that could be raised to five times higher.

“Our principal interest is in making precision measurements of fundamental neutron properties,” says Filippone, explaining that neutrons have a half-life of only 15 minutes. In other words, if a thousand neutrons are trapped, 500 will have broken down after 15 minutes into a proton, electron, and antineutrino.

Neutrons normally exist in nature in a stable state within the nuclei of atoms, joining the positively charged protons to make up most of the atom’s mass. Neutrons become quite unstable if they are stripped from the nucleus, but the very fact that they decay so quickly can make them useful for various experiments.

The traditional way physicists obtained free neutrons was by trying to slow them down as they emerged from a nuclear reactor, making them bounce around in material to burn up their energy. This procedure worked fine for slowing down neutrons to a few feet per second, but that’s still pretty fast.

The new technique at Los Alamos involves a second stage of slowdown that is impractical near a nuclear reactor but that works well at a nuclear accelerator where the event producing the neutrons is abrupt rather than ongoing. The process begins with smashing protons from the accelerator into a solid material like tungsten, which results in neutrons being knocked out of their nuclei.

The neutrons are then slowed down as they bounce around in a nearby plastic material, and then some of them are slowed much further if they happen to enter a birthday-cake-sized block of solid deuterium (or “heavy hydrogen”) that has been cooled down to a temperature
a few degrees above absolute zero.

When the neutrons enter the crystal latticework of the deuterium block, they can lose virtually all their energy, and emerge from the block at speeds so slow they can no longer zip right through the walls of the apparatus. The trapped ultracold neutrons bounce along the nickel walls of the apparatus and eventually emerge, where they can be collected for use in a separate experiment.

According to Filippone, the extremely slow speeds of the neutrons are important in studying their decay at a minute level of detail. The fundamental theory of particle physics known as the Standard Model predicts a specific pattern in the neutron’s decay, but if the ultracold neutron experiments were to reveal slightly different behavior, then physicists would have evidence of a new type of physics, such as supersymmetry.

Future experiments could also exploit an inherent quantum limit of the ultracold neutrons so that they bounce lower and lower but no lower than about 15 microns on a flat surface—or about a fifth the width of a human hair. With a well-designed experiment, Filippone says, this limit could lead to better knowledge of gravitational interactions at very small distances.

The next step for the experimenters is to return to Los Alamos in October, when they will use the ultracold neutrons to study the neutrons themselves.

The research was supported by about $1 million in funding from Caltech and the National Science Foundation.
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