Neutrinos fascinate me. Their history is particularly interesting: they were one of the first particles hypothesized by theory and then found experimentally. They were once thought to be massless, but now we know they have a very, very tiny mass. They have antiparticle counterparts (which at first blush is confusing, considering they are electrically neutral). And they “oscillate” from one flavor to another as they travel across space! I’m pretty familiar with solar neutrino detection experiments, but last week I learned about a whole new use for these mysterious particles: discovering what’s happening beneath Earth’s crust.

A young Julia in open-mouth-awe while visiting SnoLab, a solar neutrino detector in Canada.

I attended a Caltech Seminar talk (via Zoom, yay!) by Hiroko Wantanabe, titled Neutrino Geoscience: measuring the Earth’s neutrino flux and constraining its composition (slides here). I assumed Earth’s neutrino flux meant the flux of solar-originating neutrinos felt on Earth’s surface, but Wantanabe was actually describing neutrinos coming from inside the Earth itself. These neutrinos (well, technically they’re antineutrinos) come from the beta decay of Uranium, Thorium, and Potassium in the Earth’s mantle.

You’ll notice the Potassium decay only produces one electron and one antineutrino. This decay also only happens 90% of the time. K-40’s other decay is an electron-capture, which produces a normal electron-neutrino. Wantanabe said that current experiments cannot detect the K-40 decay.

The U-238 and Th-232 decays are much more complicated than the drawings shown here (U-238 to Pb-206 has something like 26 steps, which you can explore here), but the key thing to focus on is those lovely electron-antineutrinos. By measuring these geo-neutrinos, we can uncover:

  • how much of the Earth’s heat is produced by radioactivity
  • how much this radioactivity contributes to mantle convection and plate tectonics
  • and how the mantle is composed.

My first big takeaway from this talk (besides realizing that neutrinos don’t just have to come from space) was that the majority of Earth’s heat comes from the mantle. I thought it was from that “hot iron core” we hear about in elementary school.

My second takeaway was that these neutrino physicists are the picture of patience: in one of the detectors, they only detect a geo-neutrino event about once every three days, and in another detector it’s once in three weeks! Sometimes good things are worth waiting for!

Excited to see a whole new side to neutrinos,


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