First Light Shines in Instrument Designed to Solve the Neutrino Controversy

KATRIN project achieves transmission of electrons through completed apparatus, opening new doors to understanding the universe.

Neutrino Controversy
Electrons striking the 10-centimeter-diameter, 148-pixel silicon detector at the focal plane of KATRIN. The colors indicate the rate on each pixel. The electrons are produced by a photoelectron source at the other end of the KATRIN system, 70 miles away. Image courtesy of G. Drexlin

The Science

Neutrinos, elusive elementary particles created in the big bang and in stars such as the sun, have been shown to have mass, an advance recognized by the 2015 Nobel Prize in Physics. The actual size of the mass remains unknown. A large international experiment, called KATRIN, is under construction in Germany to measure that mass. Nuclear physicists in the United States play a leading scientific role in the experiment. Also, they have contributed instrumentation to the effort.

The Impact

Neutrino mass was predicted by the highly successful Standard Model of Particle Physics to be zero. However, experiments observing neutrinos produced by cosmic rays in the upper atmosphere and by reactions in the core of the sun show the mass is not zero. This contradiction is a definitive clue to new physics that lies beyond the standard model, probably arising at extremely high energies. If large enough, neutrino mass may play a major role in the formation of the largest clusters of galaxies in the universe. A laboratory measurement of the mass is therefore of great scientific importance.

Summary

The KArlsruhe TRItium Neutrino mass (KATRIN) project can measure neutrino mass by a very careful determination of the electron spectrum emitted by tritium, a radioactive isotope of hydrogen. When the tritium nucleus decays, a neutrino and an electron share the energy available. KATRIN begins with a long tube containing flowing tritium gas and surrounded by superconducting magnets. More magnets guide the electrons into a retarding-field analyzer that allows only the most energetic electrons to pass. Those are detected and counted in a multi-pixel silicon detector (see picture). A neutrino mass two million times smaller than the electron mass can be detected by KATRIN. Following commissioning, scientists expect to introduce tritium into KATRIN by the end of 2017 to conduct the first tests. The total measurement time is 5 years to generate a result.