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High Energy Physics Division

Neutrinoless Double Beta Decay

The world’s first neutrino observation in a hydrogen bubble chamber. It was found Nov. 13, 1970, on this photograph from the Zero Gradient Synchrotron’s 12-foot bubble chamber.

Is the neutrino its own antiparticle? Physicists have been searching for neutrino-less double beta decay,” which can answer this fundamental question. 

In the well-understood process of beta decay, a nucleus emits an electron and a neutrino, while the nucleus itself is transmuted (it loses a neutron and gains one positive electric charge). Double beta decay is more exotic and occurs in just a few naturally occurring radioactive isotopes. Here, two electrons and two neutrinos are emitted by the isotope. For example, in the naturally occurring isotope of xenon denoted by the symbol 136Xe, the nucleus is transmuted to 136Ba (the nucleus gains two positive charges) with the release of two electrons and two neutrinos. The outgoing electrons and neutrinos share the decay energy but the neutrinos are almost impossible to detect. The only evidence of this rare decay are the two electrons, as no experiment yet has been able to detect the appearance of the 136Ba isotope. 

If the neutrino is its own antiparticle, it is possible for the two (virtual) neutrinos in the above example to annihilate each other, as particles and antiparticles can do when they are in close proximity. In that case the decay produces just two electrons and the 136 Ba nucleus. This, then, is the golden signature of neutrinoless double beta decay: 136Ba plus two electrons whose energy adds up to exactly 2458 MeV, since there is no energy carried away by undetectable neutrinos. Until now, no such decays have been observed. Background signals that can be mistaken for the actual signal can be reduced significantly with the detection of the barium isotope.

Argonne is engaged in an international program to develop detectors capable of the most precise energy measurement of the electrons as well as the identification of the 136Ba isotope to provide unmistakable evidence of neutrino-less double beta decay. The technique is to use high pressure Xe gas, enriched with 136Xe in a TPC detector with extraordinary energy resolution and capability to detect both decay electrons and the transmuted 136Ba isotope.