Magnetic Levitation of Ultracold Neutrons Yields New Measurement of the Neutron Lifetime

Storing extremely slow neutrons in a novel trap enables precise measurement of a basic property of particle physics.

  • newswise-fullscreen Magnetic Levitation of Ultracold Neutrons Yields New Measurement of the Neutron Lifetime

    Credit: Los Alamos National Laboratory

    View of the ultracold neutron trap, comprising 5,139 permanent magnets arranged in an asymmetric bowl-shaped array, during magnetic field mapping with a robotic arm. The strong magnetic field near the trap surface repels ultracold neutrons, preventing the neutrons from contacting the surface, with gravity preventing the extremely low energy neutrons from bouncing out of the trap.

The Science

When liberated from the atomic nuclei of everyday matter, neutrons spontaneously decay. They decay into other subatomic particles after an average lifetime of about 15 minutes. Pinning the lifetime down to a more precise number is tough. Now, a new approach has pinned this value down to an unprecedented level. The value does not need the corrections typically required in this class of experiment.

The Impact

How long a free neutron lasts is sensitive to the “rules” in the standard model of particle physics. The standard model is the extremely successful theory that explains the interactions of matter and energy in the universe at very small length scales. There is ample evidence that the standard model is incomplete. Improving our knowledge of the neutron lifetime may help reveal insights that give us a new, more complete standard model of particle physics.


To measure the neutron lifetime, the experiment used ultracold neutrons from the Los Alamos National Laboratory ultracold neutron source, currently the most intense in the world. Ultracold neutrons have such low energy that they can only bounce a few meters high in Earth’s gravity, can be completely repelled by easily achieved magnetic fields, and cannot penetrate into many common materials, allowing them to be held in material bottles like a gas. The team measured the lifetime by storing neutrons in a trap and counting survivors after set time periods. Instead of material walls to confine the neutrons in the trap as in previous ultracold-neutron-based experiments, an array of strong permanent magnets repelled the neutrons from the wall, thus avoiding losses at the wall that scientists had to correct for in previous experiments. The experiment also employed a novel technique to count surviving neutrons; a large-area ultracold neutron detector, based on technology developed at Los Alamos National Laboratory for this experiment, was lowered into the trap to soak up and detect the neutrons. The result was a very clean measurement of the neutron lifetime with precision matching the world average of previous experiments but without the need for systematic corrections. The experiment shows promise for improving our knowledge of the neutron lifetime by another factor of four through continued data collection and analysis.


This work was developed by the Los Alamos Laboratory Directed Research and Development (LDRD), the LDRD Program of Oak Ridge National Laboratory, the National Science Foundation, a National Institute of Standards and Technology’s Precision Measurement Grant, Indiana University Center for Space Time Symmetries, the Los Alamos Neutron Science Center Rosen Scholarship program, and the final physics result by the Department of Energy, Office of Science, Office of Nuclear Physics.


R.W. Pattie, N.B. Callahan, et al., (UCNtau Collaboration), “Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection.” Science 360(6389), 627 (2018). [DOI: 10.1126/science.aan8895]

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