Neutrinos are tiny, possibly massless, neutral elementary particles which interact with matter via the weak nuclear force. The weakness of the weak force gives neutrinos the property that matter is almost transparent to them. The sun, and all other stars, produce neutrinos copiously due to nuclear fusion and decay processes within the core. Since they rarely interact, these neutrinos pass through the sun and the earth (and you) unhindered. Other sources of neutrinos include exploding stars (supernovae), relic neutrinos (from the birth of the universe) and nuclear power plants (in fact a lot of the fuel's energy is taken away by neutrinos). For example, the sun produces over two hundred trillion trillion trillion neutrinos every second, and a supernova blast can unleash 1000 times more neutrinos than our sun will produce in its 10-billion year lifetime. Billions of neutrinos stream through your body every second, yet only one or two of the higher energy neutrinos will scatter from you in your lifetime.
In recent years, theoretical models of the sun have permitted detailed calculations of the number (or flux) of neutrinos released from the sun. Several neutrino experiments have detected solar neutrinos and found the flux was too low. It appears that far too few neutrinos are detected than can be consistent with the known energy output of the sun. This is known as the "Solar Neutrino Problem" (SNP).
The neutrino was proposed by Wolfgang Pauli in 1930; but it would be 26 years from then before the neutrino was actually detected. Pauli proposed the existence of the neutrino as a solution to a frustrating problem in a nuclear process called beta decay. It seemed that examination of the reaction products always indicated that some variable amount of energy was missing. Pauli concluded that the products must include a third particle, but one which didn't interact strongly enough for it to be detected. Enrico Fermi called this particle the neutrino which meant "little neutral one". In 1956 Reines and Cowan found evidence of neutrino interactions by monitoring a volume of cadmium chloride with scintillating liquid near to a nuclear reactor. Fred Reines was jointly award the Nobel Prize in physics in 1995 in part for this revolutionary work.
We know that the mass of the neutrino is approximately zero, but we are unsure if it is exactly zero due to the difficulty in detecting neutrinos. This is important because neutrinos are by far the most numerous entity in the universe; thus the neutrinos will dominate the total mass of the universe even if the individual mass is tiny. So the destiny of the universe depends on the mass of the neutrino.
Spectrum of Solar Neutrinos (Bahcall SSM)
Detecting neutrinos from the sun is very valuable as a way of seeing the sun's interior. The neutrinos produced in the core of the sun escape unhindered and a very small number may be detected with suitable apparatus on earth. This is why SNO is often referred to as the "window on the sun", since the neutrinos act as a probe on the mechanisms in the solar core. Now since neutrinos only weakly interact with matter, SNO requires a large detector volume to compensate (1000 tonnes of heavy water).
The flux of solar neutrinos has been measured by previous experiments; but the results have been inconsistent and perplexing. The pioneering experiment is Ray Davis's 600 tonne chlorine tank (actually dry cleaning fluid) in the Homestake mine, South Dakota. His radio-chemistry assay, begun in 1967, finds evidence for only one third of the expected number of neutrino events. A light water Cerenkov experiment at Kamioka, Japan, upgraded to detect solar neutrinos in 1986, finds one half of the expected events for the part of the neutrino spectrum for which they are sensitive. Two recent gallium detectors (SAGE and GALLEX), which have lower energy thresholds, find about 60-70% of the expected rate. The clear trend is that the measured flux is found to be dramatically less than is possible for our present understanding of the reaction processes in the sun (see the John Bahcall's page on the Standard Solar Model for details). Furthermore, the neutrino deficit appears to depend on the energy of the neutrino. The sun produces neutrinos with a range of energies (see the figure above), and the different detectors are sensitive to different energy ranges. The options are:
You could argue that all the experiments are simply wrong, but this is highly unlikely. The different experiments all use diverse detection techniques, overseen by large collaborations, and have been calibrated with a variety of sources.
Now is a good time to introduce another fact about neutrinos; there are actually three types of neutrinos (six types if you count the anti-neutrinos). The three types (called flavours) are the electron-neutrino, the muon-neutrino and the tau-neutrino; they correspond to the three known "generations" of particles that make up the known roster of elementary particles. Normal "earthly" matter is made from first generation particles, protons, neutrons and electrons. The higher generation particles can be created in particle accelerators (that is how they were discovered), but they rapidly decay back to the first generation due to their larger mass.
Now the sun only produces electron neutrinos, and, to date, detectors on earth have only been sensitive to electron neutrinos. So if the neutrinos were undergoing a "flavour" oscillation then the probability of detection would be reduced. There is a proposed scenario, called the MSW effect, where the large mass densities in the sun could greatly enhance this oscillation effect. This turns out to be a very attractive possibility for the solution to the solar neutrino problem.
With the heavy water, the Sudbury Neutrino Observatory (SNO) can detect all three flavours of neutrinos. So the SNO detector will be able to observe separately the number of electron neutrinos and the number of all neutrinos. This allows a determination of the probability for these flavour oscillations to occur. From the neutrino flux and shape of the energy spectrum SNO will be able to determine how strongly the neutrino flavours mix together, and determine information about the neutrino masses. SNO started collecting the first data in April 1999, and this will be a very exciting time for neutrino physics.
N.B. N. Hata and P. Langacker's page "Implications of Solar Neutrino Experiments" provides a good up to date source of current neutrino data and their agreement with various models.
Courtesy of the SNO Home Page at Queen's University, Kingston, Ontario
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