What is solar neutrino problem?
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Answer:
Theory predicted that the sun produced a lot more neutrinos than we were able to detect. The resolution of the problem came when we figured out neutrino oscillation--the process by which one flavor of neutrino can change to another and thereby avoid detection possibly.
solar neutrino problem
Shortfall in the expected number of neutrinos produced by the Sun that are detected on Earth. Long-running experiments begun in the 1960s found only one-third to two-thirds the number of neutrinos predicted by theory to arrive from the Sun, where they are emitted as a result of nuclear fusion in the solar core. The discrepancy implied either that the theory of solar energy production was wrong or that neutrinos transformed en route to Earth in a way that made some of them seem to vanish. By the early 2000s strong evidence had been gathered in support of the latter explanation — that neutrinos "oscillate" in flight among their different types, not all of which could be detected in the experiments. For this to occur, neutrinos must have at least a tiny mass, though the specific values remained to be determined.
The Sun generates its tremendous energy through nuclear fusion, combining hydrogen atoms to form helium. This process releases energy and also generates an additional, nearly massless particle called a neutrino. First predicted to exist by theorists in 1930, it took scientists a quarter-century to discover them, in large part because neutrinos can pass through matter and rarely interact with it.
The first efforts to measure the neutrino flux from the Sun started in the late 1960s. Physicist Ray Davis filled a tank with 600 tons of dry cleaning fluid, composed primarily of chlorine, and set up equipment to measure the amount of argon created in those rare cases when a neutrino collided with a chlorine atom. The tank was placed deep in an old mine in South Dakota to shield it from cosmic rays.
The results were surprising: the experiment detected only about one-third the neutrinos expected based on models of nuclear fusion in the Sun. Later experiments conducted elsewhere also found far fewer neutrinos than predicted. This meant that either the models for nuclear fusion in the Sun were wrong, or that something was happening to the neutrinos between their creations deep in the Sun and their arrival at the Earth.
To study this, a consortium of Canadian, American, and British universities constructed the Sudbury Neutrino Observatory. Located two kilometers below the surface in a nickel mine in Sudbury, Ontario, the observatory uses heavy water -- where the two atoms of hydrogen in each molecule are replaced with deuterium, a heavy isotope of hydrogen -- as a detector fluid. When neutrinos interact with heavy water, an electron is ejected from the molecule at a speed greater than the speed of light in the water itself, generating a flash of light known as Cerenkov radiation. By measuring those flashes scientists can measure the number of neutrinos and compare those figures with models.
Unlike past experiments, the SNO detector is sensitive to not only the neutrinos generated by the nuclear fusion process, known as electron neutrinos, but two other types, called mu and tau neutrinos. The SNO data showed that the total number of neutrinos detected was equal to the number of electron neutrinos predicted to come from the Sun. Thus, some of the neutrinos changed, or oscillated, to the other neutrino types during transit from the Sun to the Earth.
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