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KATRIN (Karlsruhe Tritium Neutrino Experiment) is an experiment to measure the mass of the electron antineutrino with sub-eV precision by examining the spectrum of electrons emitted from the beta decay of tritium. The core of the apparatus is a 200 ton spectrometer, which has been built by MAN DWE GmbH in Deggendorf and was shipped to Karlsruhe via a 8600 km route involving the Black Sea, the Mediterranean Sea, the Atlantic Ocean and the Rhine. The construction is proceeding well with several of the major components being already on-site. The main spectrometer test program will start in 2011 and the complete system integration is planned for 2012. The experiment is located at the former Forschungszentrum Karlsruhe, now Campus Nord of the Karlsruhe Institute of Technology.
The beta decay of tritium is one of the least energetic beta decays. The electron and the neutrino which are emitted share only 18.6 keV of energy between them. KATRIN is designed to produce a very accurate spectrum of the numbers of electrons emitted with energies very close to this total energy (only a few eV away), which correspond to very low energy neutrinos. If the neutrino is a massless particle, there is no lower bound to the energy the neutrino can carry, so the electron energy spectrum should extend all the way to the 18.6 keV limit. On the other hand, if the neutrino has mass, then it must always carry away at least the amount of energy equivalent to its mass by E = mc2, and the electron spectrum should drop off short of the total energy limit and have a different shape.
In most beta decay events, the electron and the neutrino carry away roughly equal amounts of energy. The events of interest to KATRIN, in which the electron takes almost all the energy and the neutrino almost none, are very rare, occurring roughly once in a trillion decays. In order to filter out the common events so the detector is not overwhelmed, the electrons must pass through an electric potential that stops all electrons below a certain threshold, which is set a few eV below the total energy limit. Only electrons that have enough energy to pass through the potential are counted.
The precise mass of the neutrino is important not only for particle physics, but also for cosmology, because it determines whether hot dark matter can be explained as neutrinos. The observation of neutrino oscillation is strong evidence in favor of massive neutrinos, but gives only a weak lower bound, which furthermore depends on whether the neutrino is its own antiparticle or not, i.e., whether it has Majorana mass or Dirac mass.
Along with the possible observation of neutrinoless double beta decay, KATRIN is one of the neutrino experiments most likely to yield significant results in the near future.
^ KATRIN Main Spectrometer Accessed 26 September 2006