Solar Neutrinos

What about those funny neutrino things? What is a neutrino?

• It's very light (perhaps massless, perhaps not) atomic particle.
• lots of them -- >500 million per cubic meter.
• Solar flux: 6x10¹⁴ per m² per sec.
• It can come in a variety of energies.
• It can come in a variety of types ("flavors"):
• electron neutrino
• muon neutrino
• tau neutrino
• It has a very small interaction cross section:

We can calculate the mean free path of a neutrino inside the Sun to be

Once created, they nearly all immediately escape from the Sun. If we can see them, we can see into the very center of the Sun.

How do we detect these neutrinos?

• Take a lot (100,000 gallons) of cleaning fluid: 
• Bury it 1.5 km underground (in the Homestake Mine, SD)
• Note the following reaction:
• Argon is a gas; collect the gas, sweep out the argon.
• Count Argon atoms. (one, two, three... are we having fun yet?)

We don't see enough neutrinos! This is the Solar Neutrino Problem.

What's the solution?

• Experiment is wrong.
Other detectors, using different techniques, have verified the result.

• We don't understand the conditions in the solar interior.
Perhaps the core is a lower temperature? Different composition? All these changes would lead to new problems with the solar model.


• We don't understand neutrinos.
Perhaps neutrinos oscillate between types (electron, muon, tau). Until very recently detectors only sensitive to electron neutrinos. If the neutrinos change type, we may be missing many of them.

Most recent recents from the SuperKamiokande experiment and the Sudbury Neutrino Observatory detect these kinds of neutrino oscillations. This also argues that neutrinos have very small, but non-zero, mass.

Here is a good description of neutrinos and neutrino oscillations from some of the folks involved in Super Kamiokande. And information from the Sudbury Neutrino Observatory, too.