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Sun-Earth Day 2008: Space Weather Around the World

Sun-Earth Day 2008: Space Weather Around the World

The core of the sun fuses hydrogen to helium. A by-product of this is a slippery particle called the neutrino, so why can't scientists find them?

ISSUE #59: WHERE DID ALL THE NEUTRINOS GO?

Figure 1:   A model of the solar interior  based on data from a solar seismometer .The hot core is false-colored blue to turquoise to highlight its changing properties. (Courtesy - SoHO)

Figure 1: A model of the solar interior based on data from a solar seismometer .The hot core is false-colored blue to turquoise to highlight its changing properties. (Courtesy - SoHO).

Statement of the Problem:

For a long time, theoretical models have predicted that core of the sun has a temperature of 15.7 million K. At this temperature, hydrogen atoms fuse into helium to produce the light and heat energy that makes our sun a star. But nuclear physics predicts that this fusion process should also produce sub-atomic particles called neutrinos. Neutrinos do not interact with matter very strongly, so they should stream out of the core of the sun at the speed of light, and escape into space. Sensitive instruments designed to detect them were only able to account for 1/3 of the predicted neutrinos, so 2/3 of the expected neutrinos were missing! What was happening to them, or could it be possible that our theory of solar nuclear fusion is all wrong to begin with?

History:

Neutrinos are a special type of elementary particle that travel at the speed of light, but interact very weakly with matter. Light (photons) can be absorbed by a single sheet of paper, but neutrinos are so slippery that, under certain conditions, they can pass through billions of kilometers of dense matter without being diminished.

Figure 2:  Nuclear reactions that produce four different energies of neutrinos in the sun. They are predicted to have energies of 0.86, 1.44 and 14 million electron volts (MeV). By comparison, photons in the visible spectrum carry only about 1 electron volt of energy.

Figure 2: Nuclear reactions that produce four different energies of neutrinos in the sun. They are predicted to have energies of 0.86, 1.44 and 14 million electron volts (MeV). By comparison, photons in the visible spectrum carry only about 1 electron volt of energy..

According to well-established theories in nuclear physics, neutrinos are produced in many kinds of nuclear reactions whenever, as a part of the reaction, one particle decays into another. Neutrons are common ingredients in these reactions, and when one of them decays into a proton and an electron, a neutrino is produced. There are three different types of neutrinos; the electron neutrino, the muon neutrino and the tauon neutrino. These are produced whenever electrons, muons or tauons are produced in nuclear reactions from decay processes. Neutrinos also carry energy - the amount depends on the kidn of reaction that produced them. In the sun's core, the reactions that fuse hydrogen into helium produce electron neutrinos with energies of 0.86, 1.44 and 14 million electron volts (MeV). By comparison, photons in the visible spectrum carry only about 1 electron volt of energy.

The obvious thing for astronomers to do is to look for the sun's electron neutrinos to confirm that the sun's core really is 'fusing' the way that physicists expect it to.

In 1967, physicist Ray Davis created one of the first neutrino detectors designed to look for neutrinos coming from the sun. Deep underground, in the abandoned Homestake Gold Mine in South Dakota, a huge tank filled with 600 tons of dry cleaning fluid was built. Out of the trillions of neutrinos passing through the tank each day, occasionally one of them would interact with a chlorine-37 nucleus in the cleaning fluid and cause it to become an argon-37 nucleus! Over the course of many years, however, the number of neutrinos from the sun that he detected was far less than what the models of the sun's core predicted. In fact, he only detected about 1/3 of the neutrinos that should have been there!

In 1986, a different experiment was set up to detect the faint flashes of light that neutrinos should leave behind as they speeded through a large tank of pure water. This 'Cherenkov' experiment at Kamioka, Japan, was only able to find one half of the expected neutrinos to which this experiment was sensitive.

More recent neutrino detection experiments called SAGE and GALLEX were able to detect very low energy electron neutrinos, but found only about 60-70% of the expected rate.

Explanations:

Figure 3 :  The Sudbury Neutrino telescope  in Canada. The 12-meter sphere contains  12,000 detectors that watch for the light from neutrinos that streak through the water filling the interior of the sphere. (Courtesy - Stanford Solar Center)

Figure 3 : The Sudbury Neutrino telescope in Canada. The 12-meter sphere contains 12,000 detectors that watch for the light from neutrinos that streak through the water filling the interior of the sphere. (Courtesy - Stanford Solar Center).

For some reason, the sun seemed to be producing fewer neutrinos that could be expected from theoretical models, and the rush was on to uncover what was happening. One clue seemed to be that the neutrino 'deficit' seemed to depend on the energy of the neutrino. The sun produces neutrinos with a range of energies and the different detectors are sensitive to different energy ranges. But could this explain the whole problem?

The core is switched-off

One way to solve the solar neutrino problem is to lower the central temperature of the Sun by a few percent. This will mean fewer high-energy nuclear reactions occurring in the solar core and thus, fewer neutrinos being produced and hence detected. There are a number of ways to lower the central solar temperature. Mixing will cause fresh fuel to be brought into the core, and thus a lower temperature will be needed to maintain equilibrium.

Neutrino Oscillations

As early as 1969, Bruno Pontecorvo proposed that neutrinos might oscillate between the electron and muon flavor states (the only ones known then). Oscillations can occur if the physical neutrinos are actually particles with different masses. The Mikheyev-Smirnov-Wolfenstein (MSW) effect claims that electron neutrinos may transform or oscillate into either muon or tauon neutrinos. That means that, somewhere between the Sun and Earth, electron neutrinos may be changing into muon (or tauon!) neutrinos and vice versa. If your instruments on Earth are only designed to detect electron neutrinos, you will miss count the number of actual neutrinos that escaped the sun!

Figure 4 - An image of the neutrinos from the sun created by combining data from  the Super-Kamiokande neutrino detector.  (Credit: R. Svoboda and K. Gordan (LSU))

Figure 4 - An image of the neutrinos from the sun created by combining data from the Super-Kamiokande neutrino detector. (Credit: R. Svoboda and K. Gordan (LSU)).

New kinds of Neutrinos

For decades, theories that account for the diversity of elementary particles have predicted other kinds of particles that have not been confirmed. The familiar neutrinos physicists have been studying since the 1960's are called 'left-handed' because of the way they spin relative to their directuion of motion. But some theories also predict there should be right-handed neutrinos too. The neutrino experiments currently running on Earth only detect left-handed electron neutrinos, so it could be possible that some of the left-handed 'normal' neutrinos might be changing into right-handed neutrinos somewhere between the Sun and Earth, and escaping detection.

And the answer is...

The first strong evidence for neutrino oscillation came in 1998 from the Super-Kamiokande collaboration in Japan. More direct evidence came in 2002 from the Sudbury Neutrino Observatory (SNO) in Canada. SNO's gigantic apparatus consists of 1000 tons of heavy water (worth $300 million Canadian) held in an acrylic vessel surrounded by a galaxy of phototubes, the whole device resides 2 km beneath the Earth's surface in an Ontario mine, the better to filter out distracting background interactions. SNO looks at a particular reaction in the sun: the decay of boron-8 that produces beryllium-8 plus a positron and an electron neutrino (See Figure 2).

SNO detected all types of neutrinos coming from the sun, and was able to distinguish between electron-neutrinos and the other two types. About 35% of the arriving solar neutrinos are electron-neutrinos, with the others being muon- or tau-neutrinos. The total number of detected neutrinos agrees quite well with the earlier predictions from nuclear physics, based on the fusion reactions inside the sun.

GALLERY

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