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Ultra-cold neutron source at Los Alamos confirmed as world’s most intense


Some slow, cold visitors stopped by Los Alamos National Laboratory last week, and their arrival could prove a godsend to physicists seeking a better theory of everything.

Researchers working at the University of California’s Los Alamos Neutron Science Center and eight other member institutions of an international collaboration took a giant step toward their goal of constructing the most intense source of ultra-cold neutrons in the world, measuring ultra-cold neutron production in their new source for the first time.

"Ultimately, we want to be able to bottle ultra-cold neutrons and watch them decay, giving us new insights into particle physics," said Tom Bowles of Los Alamos’ Physics Division, who leads the team.

Neutrons are at once enigmatic and fundamental to all matter. Ultra-cold neutrons are even more elusive, with wavelengths greater than 500 angstroms and temperatures of 0.001 degrees Kelvin above absolute zero (460 degrees below zero Fahrenheit). They move at velocities slower than 25 feet a second and can only rise about 10 feet in height against the pull of gravity.

Physicists need ultra-cold neutrons because they can be confined in physical or magnetic bottles where they decay with a characteristic lifetime of about 15 minutes. After trapping them, researchers can measure such basic neutron properties as lifetime and decay correlations and search for possible new properties such as an electric dipole moment. Such data can lead to accurate measurements of fundamental constants of nature, advances in the quest for new particles predicted by unified field theories, and new insights into how matter began in the Big Bang.

Preliminary measurements over the past week demonstrated that the source will provide the highest density of UCNs in the world, enabling the team to begin a major research program at Los Alamos.

"Our initial results are very encouraging and we expect by this fall to complete commissioning of what will be the most intense source of UCN in the world. Coupled with the unique properties of UCN, this will provide a new window at Los Alamos through which we can work to understand some of the most puzzling issues facing modern physics," Bowles said. "We also will be pursing the exciting prospect of studying the potential application of UCN to a wider range of research that may benefit studies of microscopic surface properties of materials and structures of large macro-molecules."

The key to Los Alamos’ success dates back to 1994 and a collaboration to develop a solid deuterium source for ultra-cold neutrons with the Petersburg (Russia) Nuclear Physics Group, another member of the team. Four years later, design work was completed and the team built a prototype super-thermal source at the Weapons Neutron Resource, part of the Los Alamos Neutron Science Center.

The 800-million-electron-volt LANSCE proton beam strikes a tungsten target; each proton that hits produces about 14 neutrons at energies of a few million electron volts, which are reduced to typical cold neutron temperatures of 40 Kelvin by scattering in polyethylene moderators.

As they interact with the solid deuterium inside a guide tube coated with nickel-58, the cold neutrons give up their energy and become ultra cold. In effect, the weak crystal structure of the solid deuterium trap creates a one-way pipe that scatters the cold neutrons’ energy away and won’t let them regain energy above this so-called ground state. The UCNs then travel through a guide tube and are detected by a helium-three detector.

Previous sources, built at nuclear reactors, couldn’t produce enough ultra-cold neutrons for key experiments, such as those under way at Los Alamos to measure neutron beta decay with sufficient precision to tell whether massive subatomic particles exist that influence the so-called electroweak force, one of the fundamental forces of nature. Because the UCNs are produced at a neutron spallation source instead of a reactor, researchers can acquire data without a continual beam of protons striking the source, so sensitive experiments can be performed with much smaller backgrounds that might throw off measurements.

LANSCE is uniquely suited to the production of UCNs and fundamental research with neutrons, Bowles said.

"The proton accelerator, a well-equipped experimental hall built up during 30 years of nuclear physics studies, and a pool of highly skilled and experienced personnel - all these factors make it possible to bring a major new research tool online cost-effectively in a short period of time," he said. "Ultimately, we expect this new capability to develop into a national UCN user facility serving a wide range of researchers from around the world."

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA’s Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos develops and applies science and technology to ensure the safety and reliability of the U.S. nuclear deterrent; reduce the threat of weapons of mass destruction, proliferation and terrorism; and solve national problems in defense, energy, environment and infrastructure.

Jim Danneskiold | LANL
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