Metal-Organic Liquids Detect Neutrinos

Chemists have developed new metal-organic liquids to help study the chargeless elementary particles known as neutrinos, which are formed by radioactive decay and may provide insight into the fundamental nature of matter and the origin of the universe. Richard L. Hahn, a senior chemist at Brookhaven National Laboratory, described the solutions in a presentation before the Division of Nuclear Chemistry & Technology.

When neutrinos enter a liquid and interact with protons or neutrons, a flash of light is produced. Researchers have found that organic compounds such as linear alkyl benzenes are particularly effective at interacting with the neutrino and emitting the light, allowing for greater experimental sensitivity. Adding metals such as gadolinium to these liquids can enhance the detection of neutrinos, Hahn reported.

Just as electrons have an antimatter counterpart in the positron, so neutrinos have antineutrinos. When an antineutrino interacts with a proton, a positron and a neutron are produced. The positron produces one light flash. Gadolinium can absorb the neutron and emit gamma rays, which produce a second flash.

"Then you have two separate signals occurring within a specific time interval that characterize the reaction," giving researchers more confidence the antineutrino reaction has occurred, Hahn told C&EN. He and colleagues have drawn inspiration from separations chemistry, nuclear chemistry, and radiochemistry to get solutions to incorporate as much as several percent of gadolinium carboxylate complexes and to have the mixtures remain stable for an experimental time frame of at least three years.

Hahn is part of an international collaboration that will use the gadolinium-spiked organic liquids to study antineutrinos produced from nuclear fission in the Daya Bay Nuclear Power Plant in China. Neutrinos oscillate between three "flavors," and the Daya Bay experiment is designed to better characterize one of those flavors.

Another experiment, to be done at the Sudbury Neutrino Observatory, in Ontario, will use a neodymium-containing organic liquid developed by Hahn's team. Neodynium-150 undergoes a nuclear transition known as double decay, which emits two particles and two neutrinos. Researchers are looking for double  decay events that do not emit neutrinos; such events could provide clues as to whether neutrinos and antineutrinos are actually the same particles and insight into why the universe is composed of matter rather than antimatter.

Neutrino experiments have had a major impact on astrophysics, particularly in confirming theories about how the sun works, notes Joseph B. Natowitz, a chemistry professor at Texas A&M University. "The evolution into different detector liquids, all of which is chemistry driven, has expanded the field. It's really a very exciting area."