| In 1984, the late Professor Herb Chen from the University of
California at Irvine published an article describing his idea for
resolving the discrepency between the theoretical calculations of
expected numbers of neutrinos being produced in the nuclear fusion
reactions of the sun and the measurements of these neutrinos in
experiments conducted in underground labs on the earth. The
theoretical/experimental discrepency, which is widely known as
the Solar Neutrino Problem, was that the experiments reported
observations of neutrinos amounting to only about one third of
the number predicted by the models. The question of what has
happened to the other two thirds of the neutrinos has had
physicists scratching their heads since the late 1960's.
Many proposed solutions to the question of the missing neutrinos came out of all of the mental effort put toward the problem. The most favored solution was that the neutrinos switched their nature and became "invisible" to the experiments. The idea of Professor Chen was to directly test this hypothesis by building a detector which would be able to see the "invisble" neutrinos. The title and abstract of his article is reproduced here: |
| The solar model predicts the number of electron-type neutrinos produced in fusion reactions the sun. These neutrinos are also what was being detected in the experiments. If, after being produced in the fusion processes, the neutrinos change to another type (muon or tau neutrinos), the experiments which had been conducted would not have been able to see them. The experiment proposed by Professor Chen would be able to provide a measure the number of electron neutrinos as well as all of the neutrinos arriving at the detector. As noted in the reproduced abstract, an electron neutrino can interact with a deuteron in heavy water through the charged-current reaction. On the other hand, neutrinos of all types can interact with a deuteron through the neutral-current reaction. Thus if these two processes are observed, one can directly test if the neutrinos which had gone missing in the past experiments were present, but invisible. |
| Charged-current reaction (CC): an electron neutrino interacts with the deuteron and transforms the neutron into a proton and an electron. The outgoing electron carries off most of the energy of the incoming neutrino and so will create Cherenkov light while it travels faster than the speed of light in the water. |
| Neutral-current reaction (NC): a neutrino of any type interacts with the deuteron to break it apart so that there is an outgoing neutrino plus a free neutron and proton. The neutron wander within the detector and will eventually get captured by another nucleus. If the capturing nucleus is for deuterium, a 6.25 MeV gamma-ray is produced; the gamma-ray will scatter off of an atomic electron (Compton scatter) and the scattered electron will create Cherenkov light. If the capturing nucleus is 35Cl, a series of gamma-rays with a total energy of 8.6 MeV is produced; some of these gamma-rays will Compton scatter and cause Cherenkov light. |
| Elastic scattering reaction (ES): a neutrino scatters off of an atomic electron and the outgoing electron produces Cherenkov light. A neutrino of any type can participate in this reaction, but the electron neutrinos are six times more likely to do so. The direction of the outgoing electron is highly correlated with the direction of the incoming neutrino, so it is said that these events point back to the neutrino source. |
Find out more about the SNO experiment at the LBNL Neutrino Astrophysics home.