Michael Moe Honored for the Discovery of Double Beta Decay at UC Irvine

Michael Moe was honored by the American Physical Society with the Tom W. Bonner Prize of 2013 for Nuclear Physics. This is well described at the APS and the UCI School of Physical Sciences.

The experiment was carried out by Michael Moe, Steve Elliott (now at Los Alamos National Lab), and Alan Hahn(now at Fermilab) at a lab in the basement of what is now Rowland Hall. There is a plaque on the outside of that lab. Yesterday, Michael Moe gave the physics colloquium, and explained the process of two neutron double beta decay, and how they did the experiment. The lifetime they measured was exceedingly long, at 10^20 years. Compare this to the lifetime of the universe at 1.37×10^10 years.

Normal beta decay is where a free neutron or one in a nucleus decays to a proton, plus an electron and an electron anti-neutrino. This balances charges since the beginning neutron is neutral, the proton is positive, and the electron is negative. It also increases the number of protons in the nucleus by one (or changes Z to (Z+1)), and decreases the neutron number by one. For some nuclei, this is impossible since the nucleus with these changes has more mass than the starting nucleus. Yet in some cases, if another beta decay occurs at the same time, leading to the Z+2 nucleus with less mass than the starting nucleus, the double beta decay is energetically allowed. Since the weak process of beta decay takes a long time, two simultaneous beta decays into exactly the required energy difference between nuclear masses takes a very long time. There is a good wikipedia article on this.

Today, Steve Elliott gave a talk on an experiment to measure double beta decay with no neutrino emission. If it can occur, it will be a much longer lifetime, greater than 10^25 years. Their preliminary experiment to demonstrate a very low level of background events is called Majorana Demonstrator.  Neutrinoless double beta decay can only occur if the neutral neutrino is its own anti-particle. In that case, the anti-neutrino which is emitted from the first beta decay, is really also the incoming neutrino to hit a neutron and produce another electron and a proton. This process has two neutrons converting to two protons and emitting two electrons, but not emitting any neutrinos.  Neutrinoless double beta decay is on the right, and the observed normal double beta decay on the left.

neutrinoless-beta-decay

If a neutrino is also its own anti-particle, it is called a Majorana neutrino, after the Italian physicist Ettore Majorana, who hypothesized them in 1937. By the weak interactions, the emitted anti-neutrino has to be right handed, but the absorbed neutrino it becomes has to be left handed. This conversion only occurs proportional to the neutrino mass squared. In the neutrino-less double beta decay, two electrons are created, which means that electron number is not conserved. Usually a created electron has to come from an electron neutrino, which has the same electron number as the electron. Only if both conditions are physical will the process occur. The eventual experiment with a ton of enriched Germanium will be be sensitive to electron neutrino masses of 50 milli-electron Volts.

For this rare visit of these experimenters to UCI, I took them down to the lab to get their picture, together with the plaque. Mike Moe is on the left with the Tom W. Bonner prize, Steve Elliott is on the right, and the plaque is in the middle.

image

Here is a similar shot taken outside of Frederick Reines Hall.  Reines encouraged the research on his DOE grant.

Moe outside

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For a decade I have been active in learning about energy and the environment, and in lecturing and attending classes at the Osher Lifelong Learning Institute (OLLI) at UC Irvine.
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