MAJORANA, the search for the most elusive neutrino of all

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The helicity, or handedness, of neutrinos has only been observed in two states, left-handed neutrinos and right-handed antineutrinos. Whether these are really the only two neutrino handedness states depends on whether neutrinos are their own antiparticles

In a cavern almost a mile underground in the Black Hills, an experiment called the MAJORANA DEMONSTRATOR, 40 kilograms of pure germanium crystals enclosed in deep-freeze cryostat modules, will soon set out to answer one of the most persistent and momentous questions in physics: are neutrinos their own antiparticles? If the answer is yes, it will require rewriting the Standard Model of Particles and Interactions, our basic understanding of the physical world.
The best way to learn whether neutrinos are their own antiparticles would be to observe a certain kind of radioactive decay, called neutrinoless double-beta decay. It has never been detected conclusively, and if it occurs at all, it’s exceedingly rare,” says Alan Poon of the Nuclear Science Division at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
Poon is the current executive-committee chair of the MAJORANA Collaboration, which is comprised of more than 100 researchers from 19 institutions in the United States, Canada, Russia, and Japan, and whose efforts are focused on the experiment now under construction at the Davis Campus of the Sanford Underground Research Facility (SURF) in Lead, South Dakota.
Beta decay, single and double
Ordinary beta decay is a common kind of radioactivity: an atomic nucleus changes into a different kind of element, a neighbor on the periodic table with lower mass, by emitting a beta particle – an electron or positron – plus a neutrino or an antineutrino. For example, carbon-14 transforms to nitrogen-14 when one of its neutrons turns into a proton, emitting an electron and an antineutrino. It was beta decay that led to the proposal that there must be a particle like the neutrino, since an electron alone could not account for all the energy lost in the decay.
“Double-beta decay is also possible and has been observed in a dozen different isotopes since 1986,” says Poon. “But it happens at a really low rate, and not too many nuclei can do it.”
The only conclusive double-beta decays seen so far involve two neutrons that change into two protons while emitting two electrons and two antineutrinos. It’s an uncommon situation, in which single beta decay is blocked because the decaying isotope’s immediate neighbor has a nucleus that’s too heavy, but the nucleus of the neighbor two places away on the periodic table does have lower mass – even though its atomic number is two places higher. Getting there requires double-beta decay….
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Written by physicsgg

May 17, 2012 at 1:54 pm

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