Only 15 minutes of life, no fame, for lone neutrons

Once freed from its home inside the nucleus of an atom, a neutron lives on average 886.8 seconds (about 14.8 minutes), plus or minus 3.4 seconds, according to recent measurements performed at the National Institute of Standards and Technology.

This result, published in the Oct. 10 issue of Physical Review Letters, is the most precise ever achieved using beams of neutrons and is the culmination of almost 10 years of work. The new neutron lifetime value is consistent with physicists’ current theories about the particles and forces of nature. It also will help scientists better understand the creation of matter immediately after the birth of the universe, an important factor in determining what the universe is made of today.

Scientists have been measuring the lifetime of the neutron since the early 1950s. While slightly less precise than a measurement made in 2000 by a different research group using a different method, the in-beam technique provides a strong, independent check on the neutron lifetime and reduces the overall uncertainty in the recommended value.

As neutrons die, they disintegrate into other particles, including protons. The NIST-led group simultaneously counted both the number of neutrons and the number of protons formed as the neutrons fell apart. A beam of slow moving neutrons was passed through a vacuum system. As the neutrons decayed, protons—which have a positive charge—formed and were captured in a powerful electromagnetic trap. Periodically the trap was opened and the protons were counted as they crashed into a semiconductor detector, producing an electrical signal.

The research team included participants from NIST, Tulane University, Indiana University, University of Tennessee/Oak Ridge National Laboratory, and the European Commission’s Joint Research Centre (Institute for Reference Materials and Measurements) in Belgium.

The research was funded by NIST, the U.S. Department of Energy and the National Science Foundation.