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KATRIN (for Karlsruhe Tritium Neutrino Experiment) is a German experiment to measure the mass of a neutrino, i.e., to reduce the known bounds on the mass, or more specifically, to narrow the probability density function (PDF) of possible neutrino masses. It consists of some tritium (3H), which beta decays into helium-3, ejecting an electron and an electron-antineutrino. By carefully measuring the kinetic energy of the electrons, the energy remaining for the neutrino's kinetic energy can be determined. Much of the experiment consists of the mass spectrometer precise enough to do this. Such bounds on the mass of a neutrino affect interpretation and analysis of neutrino detections from the Sun and more distant sources.
Katrin's spectrometer technology consists of a MAC-E filter (magnetic adiabatic collimation with an electrostatic filter), which can be adjusted to allow only electrons of a narrow range of kinetic energies through. By viewing many electrons from the beta decays setting the filter to allow different kinetic energy ranges, a PDF of electron kinetic energies can be discerned and through analysis, bounds on the neutrino mass can be determined.