Neutrinos: Tools to Study the Sun’s Interior


By Emily LevesqueUniversity of Washington

How can we know anything about the stars’ densities or temperatures? One way is through something called asteroseismology. A pulsing star is sending waves propagating through the star, and with detailed observations, we can identify those waves and use them to discern details. However, for the Sun in particular we have a second tool available for studying its interior—neutrinos.

Diagram of the Sun interior
Physicists detected neutrinos to study the Sun’s interior. (Image: Kelvinsong/Public domain)

What Are Neutrinos?

Neutrinos are pretty unusual subatomic particles. They have no electric charge and appear to be incredibly tiny. Unlike photons that could take hundreds of thousands of years to pinball their way out from the center of the Sun, neutrinos are so small that they pass straight through those same dense layers that act like an obstacle course for light. Neutrinos from the Sun are streaming to and even through the Earth (and through you) right now!

However, they interact with atoms so rarely that for years it was incredibly difficult to study them, and we didn’t know some of the most basic facts about them, like what their mass was or whether they even had a mass at all. Still, they’re pretty valuable messengers coming from the center of the Sun, so physicists recognized decades ago that studying neutrinos could hold the key to understanding how the Sun and other stars work.

Experiment to Detect Neutrinos

Detecting neutrinos may sound pretty impossible if they’re so non-interactive, but it is possible, and in the late 1960s astrophysicists John Bahcall and Raymond Davis, Jr., designed an experiment specifically aimed at studying neutrinos from the Sun.

Bahcall had carried out theoretical predictions for astronomers’ standard model of how the Sun should work, including precise estimates for how many neutrinos should be detected coming from the Sun’s core. Davis set about building an innovative detector that could test these predictions using a 100,000-gallon tank of tetrachloroethylene, the same chemical compound commonly used today in dry cleaning! 

Davis’s philosophy was simple: the tank should, occasionally, catch a neutrino from the Sun that would interact with one of the chlorine atoms in the tank and transform it into an argon atom. If Davis simply set up the tank and waited, argon atoms should slowly begin appearing in the tank.

However, on the surface of the Earth, the experiment would prove too messy, with other subatomic particles zipping around that could also potentially interact with the contents of the tank.

Successful Experiment

The solution was to bury the tank deep under the Earth’s surface—about a mile deep—in a laboratory attached to the Homestake gold mine in South Dakota, where only neutrinos coming from the Sun should be able to cause reactions.

The experiment worked wonderfully, and argon atoms began appearing in the tank! Davis purged the tank and counted up the argon atoms and came up with the wrong number.

John Bahcall had predicted that the tank should gain a little more than one argon atom per day from neutrino interactions, but Davis was finding only one argon atom every couple of days; it came out to a number of neutrinos that was only about one-third of the number predicted by astronomers’ model for the Sun.

Davis and Bahcall both checked and re-checked their work: the experiment was fine, and the math was correct. Other experiments began getting the same results, including an immense detector in Japan named Kamiokande.

This article comes directly from content in the video series Great Heroes and Discoveries of AstronomyWatch it now, on Wondrium.

A Glitch

Together they’d definitively proved that the Sun was producing neutrinos, but the puzzle of why we weren’t finding enough neutrinos remained. What was going wrong? Was there something fundamentally wrong with our model of the Sun?

Three types of neutrinos falvor from the Sun.
Muon, tau, and electron are three neutrinos flavors from the Sun. (Image: Fabsanhvasq/Public domain)

As it turned out, there was actually something fundamentally quirky about neutrinos! Neutrinos come in three different types, referred to by particle physicists as flavors: there are electron neutrinos, muon neutrinos, and tau neutrinos.

The three flavors refer to subtle differences in the chemical reactions that produce these neutrinos and the chemical reactions in which they can participate. 

The Solution? 

Bahcall and Davis’s detector could only detect electron neutrinos, and it wasn’t finding enough of them. Had they oscillated, as subatomic particles sometimes do, into other flavors, becoming muon or tau neutrinos that would get missed by the neutrino detectors of the day?

The Sudbury Neutrino Observatory provided the answer in 2001. Like Raymond Davis’s experiment, it was buried in a mine, 1.3 miles below the Earth’s surface inside a nickel mine in Ontario, Canada. The observatory consisted of an enormous sphere that could hold 1,000 metric tons of a compound known as heavy water. Heavy water is made of an oxygen atom and two deuterium atoms—that is, hydrogen atoms with neutrons added to their cores. 

That deuterium was crucial: it would react with electron neutrinos, just like the chlorine atoms in Davis’s experiment, but it would also react with muon and tau neutrinos. Each of these reactions would give off a brief burst of light, so the sphere was lined with detectors that could report the tell-tale flash of a neutrino interaction. 

Nobel Prize

When this experiment began operating, it detected all three flavors of neutrino, and the results were spectacular. They matched John Bahcall’s predictions and directly demonstrated that astronomers’ model of the Sun’s core was correct!

The observations also proved a crucial property of neutrinos; they were oscillating in flavor. The research led to Nobel Prizes in Physics in 2002 and 2015, recognizing Raymond Davis as well as Arthur McDonald of the Sudbury Observatory, and Masatoshi Koshiba and Takaaki Kajita from Kamiokande. Astrophysicists had disentangled a long-standing mystery of particle physics and tested a fundamental model for how stars work—all thanks to the Sun!

Common Questions about Neutrinos as Tools to Study the Sun’s Interior

Q: Why are neutrinos great tools for studying the core of the Sun?

They are incredibly tiny and can pass through the Sun’s dense layers and stream towards and even through the Earth. That’s why they are great tools for studying the core of the Sun.

Q: What was John Bahcall and Raymond Davis’s experiment to detect neutrinos?

To detect neutrinos, Bahcall and Davis built an innovative detector. They used a 100,000-gallon tank of tetrachloroethylene and buried it deep in the ground. Neutrinos could reach the tank and interact with chlorine atoms to produce argon; Their prediction went well, and the experiment succeeded.

Q: What are the different flavors of neutrinos?

Neutrinos exist in three distinct types, referred to as flavors. These flavors include muon neutrinos, tau neutrinos, and electron neutrinos.

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