Neutron Trapping Demonstrated for First Time at NIST

Scientists have demonstrated for the first time that they can confine neutrons, one of the basic particles of matter, in a three-dimensional magnetic trap, an achievement that will help expand our knowledge of the creation of matter during the Big Bang.

A team of researchers from the Commerce Department's National Institute of Standards and Technology, Los Alamos National Laboratory, the Hahn-Meitner Institute in Germany and Harvard University (lead institute) report this discovery in the Jan. 6, 2000, issue of the journal Nature.

"Our experiment demonstrates that it is possible to load low-energy neutrons into a magnetic trap and detect their decays," says NIST physicist Paul R. Huffman. "We're excited about this because it gives us a new way to make a much more accurate measurement of the neutron lifetime and to answer other questions fundamental to physics."

Although the neutron lifetime is known to be about 15 minutes, astrophysicists and cosmologists would like a more exact and reliable value for refining models of the early formation of the universe. In the moments after the Big Bang, fundamental particles coalesced into neutrons and protons. These particles in turn started to combine to form helium and other light elements. Meanwhile, free neutrons began to decay. The time it took for the free neutrons to completely decay, in other words, the neutron lifetime, has helped researchers determine the initial ratio and concentration of light elements in the universe.

While physicists have been able to trap atoms in magnetic bottles for almost two decades, neutrons have only been confined magnetically in two-dimensions. The difficulties are that the neutron, one of the basic particles that make up atoms, is neutral, having no electrical charge and it does not interact strongly with surrounding materials.

Scientists overcame this problem by constructing a three-dimensional magnetic trap which takes advantage of the interaction between the neutrons's magnetic moment and a magnetic field to confine them. The neutrons are held in a long, narrow trap with a hollow center made from magnets positioned at right angles to each other. The magnetic fields hold the neutrons in the center of the trap where they remain until they decay.

The neutrons, produced at the NIST Center for Neutron Research in Gaithersburg, Md., are guided through a beamline to the neutron trap. The trap is filled with helium atoms chilled to 0.2 Kelvin (minus 273.4 degrees Celsius or minus 460 degrees Fahrenheit). Neutrons that scatter in the helium are cooled to about a thousandth of a degree above absolute zero. Only a fraction of the billions of neutrons that enter the trap scatter and remain confined by the magnetic field.

When part of an atom, neutrons are very stable, but when they exist as single particles, they eventually decay into an electron, a proton and an anti-neutrino. Scientists use the interactions of the emitted electrons with the helium atoms to detect and measure the neutrons confined in the trap.

In addition, scientists expect that improved measurements of the neutron lifetime will help improve their understanding of the weak force. The weak force, one of four forces that order our universe, governs radioactive decay. The other three forces are gravity, electromagnetism and the strong force.

The techniques developed for neutron trapping could also enable better measurement of the neutron electric dipole moment and improve tests of the Standard Model of Physics, which describes all particles and their interactions.

As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST strengthens the U.S. economy and improves the quality of life by working with industry to develop and apply technology, measurements and standards through four partnerships: the Measurement and Standards Laboratories, the Advanced Technology Program, the Manufacturing Extension Partnership and the Baldrige National Quality Program.